National Academies Press: OpenBook

Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards (2015)

Chapter: 7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems

« Previous: 6 Review of X-Ray Backscatter Advanced Imaging Technology Studies
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

7

Measurements, Dose
Calculations, and System Design
for X-Ray Backscatter Advanced
Imaging Technology Systems

This chapter contains the measurement procedures developed by the committee, including half-value layer (HVL) measurements, percent depth dose (PDD) measurements, air kerma measurements, and determination of the kerma per screening outside the inspection area. After the section describing the measurement procedures there follows the measurement results acquired by the National Research Council (NRC)1 subcontractor for the Rapiscan Secure 1000 advanced imaging technology (AIT) system and the AS&E SmartCheck AIT system and the committee’s review of each system’s design. The next section makes use of the measured data for the dose computations. The dose computations include descriptions of the X-ray source term; features of the reference AIT system used; validation of Monte Carlo sampling procedures; description of the passenger irradiation geometry, including phantoms; and dosimetry results for standard screening conditions. Additional information extracted from the dose computations include variations in X-ray tube voltage, dose to radiosensitive cells in the skin, and failure mode analysis. The chapter concludes with findings and recommendations.

Evaluation of effective dose from an unknown X-ray source, such as an AIT scanner, can be accomplished by measuring HVL, depth dose, and air kerma and then using the HVL and depth dose data to calculate the photon spectrum. With the photon spectrum and a suitable mathematical model of the person being scanned,

_______________

1 Effective July 1, 2015, the institution is called the National Academies of Sciences, Engineering, and Medicine. References in this report to the National Research Council are used in an historic context identifying programs prior to July 1.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Monte Carlo calculations can be used to determine the organ dose per incident photon, and the effective dose per photon can be calculated using organ weighting factors. Finally, the measured air kerma can be used to determine the number of incident photons and, therefore, the effective dose. The ANSI reference effective dose (EREF) is determined through a simple mathematical relationship2 based on measurements of the HVL and air kerma.

MEASUREMENT PROCEDURES

The National Research Council (NRC) subcontractor David Hintenland, Advanced Laboratory for Radiation Dosimetry Studies, J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, performed field measurements of HVL, PDD, air kerma, and dose outside the scanning area for two AIT systems: (1) the AS&E SmartCheck-HT dual-pose system being evaluated at the Transportation Security Administration (TSA) Systems Integration Facility (TSIF) and (2) a Rapiscan Secure 1000 single-pose system located at the National Institute of Standards and Technology (NIST). Both systems operate with an applied voltage of 50 kV, although they use different X-ray tubes, anode currents, and scan mechanisms and scan rates.

Measuring Half-Value Layer

Background

The HVL in aluminum provides a common description for characterizing the spectrum of an X-ray beam. With the addition of aluminum, the lower-energy components of the X-ray spectrum are preferentially attenuated and create a difference between the first and second HVLs. Measured values of the first and second HVLs were utilized to characterize the energy spectrum of the AIT systems. The attenuation curve in aluminum was evaluated in increments as small as 0.05 mm of aluminum (mm Al) across the expected range. This not only permitted the first and second HVLs to be extracted from the data but also provided a continuous curve of the attenuation in aluminum against which the X-ray spectrum of the simulated X-ray source term could be precisely matched.

_______________

2 Formulas for calculating EREF are provided in Section 6.1.3 of American National Standards Institute/Health Physics Society (ANSI/HPS), “Radiation Safety for Personnel Security Screening Systems Using X-Ray or Gamma Radiation,” ANSI/HPS N43.17-2009, 2009, http://hps.org/hpssc/index.html.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Experimental Approach

HVL measurements were made utilizing a Keithley model 96035b 15 cc parallel-plate ion chamber interfaced with a Univision electrometer from PTW.3 The ion chamber has two windows. The thinner window, 32 mg/cm2 or 320 µm tissue equivalent (mammography focus), was utilized for characterizing the HVLs of the expected, relatively soft X-ray spectrum generated by the AIT systems. The determination of HVL requires a series of measurements of beam intensity performed with increasing aluminum attenuator thicknesses relative to the unattenuated beam intensity. An absolute calibration of the chamber is not critical to the measurement results. The chamber was, however, calibrated to a molybdenum/molybdenum target/filter mammography X-ray standard at 28 kV.

To obtain improved signal-to-noise ratio for this set of measurements, both types of AIT systems were operated in a partially fixed scanning mode; in this mode, only one AIT system module (either the anterior or the posterior) is operating for a series of measurements. The mode is partially fixed, meaning that the vertical movement of the X-ray source is stopped but the horizontal is not; hence, only a line, and not an entire area, is scanned by the pencil beam. The service/engineering mode was utilized to perform measurements with the X-ray tube raised to a fixed vertical location while the beam continued to scan in the horizontal direction. Although the X-ray source and associated beam-forming equipment generates a pencil beam, the continual horizontal scanning at a fixed vertical location results in irradiating a vertical line or band across the face of the ion chamber. Although only a portion of the chamber is exposed, the same portion is exposed for each attenuator thickness, and all measurements are referenced to the same geometry exposure with no attenuator present. A series of at least three replicate measurements were made for each attenuator thickness under these conditions. Six replicate measurements were made for an aluminum attenuator thicknesses of less than 0.6 mm, where the relative change in exposure is expected to be the greatest due to the fact that the thin attenuators affect the low-energy portions of the X-ray spectrum.

The coordinate system for describing measurements on the AIT systems is set up along the central medial (longitudinal) axis of a subject undergoing the scanning process, which was defined as being collocated with the geometric central axis of the AIT system. This position is designated in the Cartesian coordinate system as x = 0 cm, y = 0 cm, where x is the dimension toward or away from the anterior X-ray source (−x is closer to the source), and y is left or right as the screening subject faces the anterior source (−y is to the left, +y is to the right) (Figure 7.1).

For the HVL measurements, the 15 cc parallel-plate ion chamber (approximately 4 cm in diameter) was centered in the AIT system halfway between the

_______________

3 See the PTW Freiburg GmbH website at http://www.ptw.de, accessed January 13, 2015.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 7.1 The coordinate system for describing measurements on the AIT systems. X is the dimension toward or away from the anterior (dark gray) X-ray source (−x is closer to the source), and y is left or right as the screening subject faces the anterior source (−y is to the left, +y is to the right), +z is pointing upward, with z = 0 at the base of the AIT system modules.

entrance and exit portals, at x = 0 cm, and centered between the anterior and posterior4 (or transmission detector), at y = 0 cm. The vertical position of the chamber was adjusted by trial and error to correspond with the position of the X-ray beam and correspondingly provides the greatest signal response by having the maximum exposed chamber area. The beam was further collimated by utilizing a sheet of lead that was custom fitted to the chamber and located behind the aluminum attenuators to reduce scatter contributions from the aluminum attenuators and to provide narrow beam geometry.

_______________

4 This is the case for the Rapiscan Secure 1000; for the AS&E AIT system, the manufacturer’s suggested position (footprints) were used.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Measuring Percent Depth Dose

Background

Percent depth dose (PDD) profiles are a relative measurement of dose as a function of depth into tissue.5 PDD is defined as a function of depth in tissue maintaining a constant source-skin distance (SSD), although it is more practical to directly measure the tissue-maximum ratio (TMR), where the reference depth remains constant, and subsequently transform the acquired data to the PDD. One objective for the PDD measurements is to elucidate the skin dose that may be delivered to radiobiologically sensitive layers of the skin such as the epithelial layer. Another is to provide relative PDD depth profiles that can be used to validate the computationally determined radiation doses to the radiobiologically sensitive regions of the skin. The shallow skin layers for which these measurements are desired present technical challenges due to the low energy of the X-ray sources of interest, requiring that thin layers of tissue-equivalent materials be produced and that measurements be made using an ion chamber with a very thin window in order to minimize secondary electron production prior to the X-ray beam’s reaching the sensitive volume of the chamber. To minimize the effects of dose buildup at these relatively low energies, it is important to utilize an ion chamber that has a very thin (and therefore fragile) entrance window. Such chambers have small volumes (a few cubic centimeters), however, so that the entrance window does not break from the detector being moved and handled, with resulting low sensitivity. The NRC subcontractor selected a Capintec PS-033 parallel-plate ion chamber to perform these measurements. This chamber provides a thin mylar window (0.5 mg/cm2 or 5 µm tissue-equivalent thickness) and a relatively large sensitive volume (4.9 cm3) for a chamber of this type with a diameter of 2.5 cm and a total thickness of 1 cm. The PS-033 parallel-plate ion chamber was integrated with a PTW Unidose electrometer for this set of measurements. Because the PDD measurement is a relative measurement, the chamber was not calibrated ahead of time, although the response at 50 kV was well characterized by the NRC subcontractor and compared to other parallel-plate ion chambers used in this study.

To develop the PDD curves, it is necessary to have a tissue-equivalent material of appropriate thicknesses. For a 50 kV X-ray source with little filtration, quite thin layers of tissue-equivalent material are required. The NRC subcontractor, therefore, fabricated custom-made layers of tissue-equivalent materials and a tissue-equivalent phantom-block in which the thin-window ion chamber was embedded. The lateral extent of the phantom-block is 10 cm × 10 cm with an additional 5 cm of tissue-equivalent material behind the chamber in order

_______________

5 The dose values are divided by the maximum dose.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

to capture the scatter contributions to the PDD. The subcontractor fabricated a series of tissue-equivalent material thicknesses as thin as 25 µm and up to 10 mm thick.6 The precise thickness of the tissue-equivalent slabs were individually determined by measurements performed with an analog dial indicator with 25 µm increments.7 The tissue-equivalent materials were evaluated against several other commonly used phantom materials, including BR12 (a breast tissue-equivalent material having a lower density) and acrylic (a plastic having a higher density but lower attenuation coefficient), at 50 kV in the NRC subcontractor’s laboratory. The materials produced performed as expected relative to these benchmarks and were determined to have a density of 1.04 g/cm3, matching that of skin and soft tissue. These materials permitted the development of a PDD from 0 to over 60 mm with high spatial resolution.

Experimental Approach

The general procedure utilized for performing the PDD measurements closely followed the procedure used for performing the HVL measurements where the tissue-equivalent material was used in place of aluminum sheets and with no lead collimation for the ion chamber. In order to maximize the signal-to-noise ratio for this set of measurements, the AIT system was operated in a partially fixed scanning mode (described in detail under the section “Half-Value Layer Measurements”). This was particularly important for this set of measurements because of the small physical dimensions of the thin-window chamber. A series of at least three replicate measurements were made for each attenuator thickness under these conditions.

An additional set of PDD measurements was made using the Keithley model 96035b 15 cc parallel-plate ion chamber with the low-energy (32 mg/cm2) window oriented toward the beam for comparison with the PS-033 chamber. While the 15 cc chamber has a relatively thin window at 32 mg/cm2, it is significantly thicker than the 0.5 mg/cm2 window of the PS-033 chamber.

The thin-window parallel-plate ion chamber (approximately 2.5 cm in inner diameter) was centered in a 10 cm × 10 cm block of phantom material with 5 cm of backscatter material behind it. The center of the chamber was positioned at x = 0 cm and y = 0 cm. The vertical position of the chamber was adjusted by trial and error to correspond with the position of the X-ray beam that correspondingly

_______________

6 The committee is not familiar with any previous study that has successfully measured PDD curves at these low energies, in part because they call for very thin layers of tissue-equivalent materials to be fabricated.

7 The University of Florida has extensive experience with the development and fabrication of tissue-equivalent materials and developed these materials specifically for this application and in order to provide empirical data that could be used for the verification of the parallel computational effort.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

provided the greatest signal response by having the maximum exposed chamber area. This was determined to occur for the center of the chamber at z = 19.5 cm.

In order to measure the exposure as a function of tissue depth, the desired thickness of tissue was placed directly in front of the ion chamber. A 1 cm air gap was maintained between the tissue-equivalent materials and the chamber to prevent inadvertent contact and potential damage to the thin window of the ion chamber. Note that the geometry selected here differs from the usual approach to measure PDD because of experimental practicality. To assure accuracy, the detector block was not moved back with each additional layer of tissue but was kept at a fixed location. The data were then transformed to the PDD by the Mayneord factor,8 which in this case simplifies to a correction using the inverse square law; because the beam is broad, distances are large compared to the tissue thicknesses, and the scatter phantom area is fixed.

Measuring Air Kerma

Background

Air kerma measurements provide an absolute measure of the air kerma (in Gy) at discrete locations in the AIT system X-ray field. These measurements provide an absolute reference against which any computational efforts can calibrate their calculations in order to predict dose to the scanned subject. In contrast to the measurements performed for HVL and PDD, the measurement of air kerma is performed with the AIT system operated in its normal scanning mode. In this mode of operation, the X-ray exposure is quite low, and it is most appropriate to use a large-volume parallel-plate ion chamber to obtain the best possible signal-to-noise ratio. Corrections for temperature and pressure, relative to the chamber calibration conditions, are applied to ensure the precision of these measurements.

Experimental Approach

Air kerma measurements were performed utilizing a Keithley model 96020C 150 cc parallel-plate ion chamber in conjunction with a PTW Unidose electrometer. The ion chamber was previously calibrated using an H60 spectrum.9 Due to the time limitations of this study, it was not possible to perform a chamber calibration

_______________

8 W.V. Mayneord and L.F. Lamerton, A survey of depth dose data, British Journal of Radiology 14:255, 1941.

9 The H60 spectrum, formed at NIST by filtration using 4 mm Al and 0.61 mm of copper, was used for air kerma calibrations.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

using an M5010 spectrum until after all the measurements were done on both X-ray backscatter AIT systems. However, the post calibration allowed for a correction of all affected data so that no incorrect values were used or included in this report. The M50 is expected to be most representative of the beam for both the AS&E SmartCheck and the Rapiscan Secure 1000 AIT systems that were tested.

Measurements for the air kerma were performed with the electrometer operated in integrating mode over the course of a normal screening subject scan. The total charge (picocoulombs) was recorded for each case. The charge was subsequently converted to air kerma using the appropriate chamber calibration factor and making air temperature and pressure corrections relative to the chamber’s calibration conditions.

For each of the air kerma measurements, the 150 cc parallel-plate ion chamber was centered at x = 0 cm, y = 0 cm, with the vertical dimension (z) referenced to z = 0 cm at floor level. Air kerma was measured at four vertical locations: z = 32 cm, z = 120 cm, z = 150 cm, and z = 202 cm. The measurement locations were designed to roughly evaluate any variations in beam intensity as the beam scans vertically. Because of the low intensity of the scanning beam, a series of 10 individual measurements were recorded at each vertical location.11

Determination of the Kerma per Screening Outside the Inspection Area

Background

In view of the unique conditions of low X-ray energy and exposure times on the order of a few seconds, typical low-dose-rate survey instruments are limited in their ability to accurately respond to these fields. In order to accurately determine the exposure outside of the inspection area, the NRC subcontractor utilized a large-area parallel-plate ion chamber (Keithley model 96020C, 150 cc) interfaced with the PTW Unidose electrometer.

In order to accurately account for the scatter contributions from the passenger being screened, a scatter medium that simulates the presence of the human as a scatter source must be included. A variety of scatter sources were considered. The University of Florida has previously constructed a series of anthropomorphic phantoms representing a variety of human anatomies. The full human-sized phantom is constructed of tissue-equivalent materials designed to mimic the response of

_______________

10 The M50 spectrum, formed at NIST by filtration using 1.07 mm Al, was used for air kerma calibrations.

11 Note that there is a platform that can be inserted into the screening system that screening subjects would normally stand on that is 14 cm tall at its midpoint. Thus, the bottom of subjects’ feet would be at z = 14 cm when the platform is used. The platform was removed for the measurements performed in this study.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

tissues in the low-energy X-ray range and includes a realistic internal anatomy. The phantom development and details are available in the published literature.12 These phantoms have been utilized extensively for radiation dosimetry studies of clinical X-ray systems. They are fabricated as physical analogs to selected phantoms in the Computational Phantom Library, accurately reproducing the anatomy and incorporating appropriate tissue-equivalent materials.

Dose outside the screening area may arise from two sources: radiation scattered from scanned subjects to the area outside the scanning region and X-ray leakage from the X-ray tube housing area.

Experimental Approach

For each set of measurements, a large-area Keithley model 96020C 150 cc parallel-plate ion chamber interfaced with a PTW Unidose electrometer was used. The electrometer was utilized in integration mode13 for each of these measurements. A Fluke 451B survey ion chamber was also positioned in close proximity to the parallel-plate ion chamber and was operated in integration mode utilizing the thin mylar window.

Measurements for the air kerma were performed with the electrometer operated in integrating mode over a repeated series of normal screening subject scans. Measurements were performed at several locations outside of the subject inspection area to individually quantify the contributions from each of these sources, as described below. The measurements integrated over at least 40 scans performed at each of the measurement positions. The total charge (picocoulombs) was recorded for each case and divided by 40; in this way, smaller charges could be measured. The charge was subsequently converted to air kerma using the appropriate chamber calibration factor and including air temperature and pressure corrections relative to the chamber’s calibration conditions.

A measurement location on the backside of the scanning unit was selected to evaluate leakage radiation that may be exiting the unit in that direction. The 150 cc parallel-plate ion chamber was positioned against the rear exterior of the AIT system assembly at a height of z = 150 cm, centered on the path that the X-ray source travels while vertically scanning.

Again, a series of 40 normal subject-screening scans were performed while the detection systems continuously integrated. Here, an anthropomorphic phantom

_______________

12 J.F. Winslow, D.E. Hyer, R.F. Fisher, C.J. Tien, and D.E. Hintenlang, Construction of anthropomorphic phantoms for use in dosimetry studies, Journal of Applied Clinical Medical Physics 10(3):195-204, 2009.

13 Integration mode accumulates charge over a specified time and can be more sensitive, albeit slower, than the rate mode, which detects the current.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

representative of an adult male was used for this purpose. The region from the bottom of the pelvis to the top of the head was used as the scatter source. The phantom was positioned on a stool such that the base of the pelvis was located at z = 74 cm and the top of the head was z = 170 cm. The phantom was positioned such that the center of the longitudinal axis of the phantom was centered in the AIT system at x = 0 cm, y = 0 cm.

The parallel-plate ion chamber was positioned at the two positions along the edge of the AIT system exit on the side toward the operator position. The detector was centered at a height z = 120 cm for each scatter measurement, essentially even with the midpoint of the vertical extent of the phantom. The lateral positions were characterized by x = 0 cm, y = −66 cm, and x = −27 cm, y = −66 cm.

AS&E SMARTCHECK SYSTEM MEASUREMENT RESULTS AND SYSTEM DESIGN

Under the direction of the committee, the NRC subcontractor made measurements on the AS&E SmartCheck (Serial No. 1004) AIT system using the same protocol used for the Rapiscan Secure 1000. The AS&E SmartCheck AIT system has not been approved by TSA for airport deployment yet; thus, the system inspected by the committee may not be the final version that may be deployed in airports in the future.

Half-Value Layer Results

Measurements of HVL were made at a fixed height using a SmartCheck software command that caused the tube head to come up to a fixed height and operate for the 3 seconds required for a normal scan. This mode of operation requires a password that would be available only to maintenance personnel. The vertical position of this beam was z = 19.5 cm (with x = 0 cm and y = 0 cm). HVL measurements provided highly reproducible data and high resolution of spectral hardening with increasing thicknesses of aluminum attenuators. The data permit not only the extraction of a first and second HVL but also a detailed attenuation curve as a function of aluminum filter thickness that can be used to refine estimates of the incident X-ray spectrum for computational dose determination. Aggregate measurements of the HVL and associated uncertainties are tabulated in Table 7.1 and

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.1 Half-Value Layer Measurements for the AS&E SmartCheck

Aluminum Thickness (mm) Exposure Measurement (pC) Uncertainty (pC)
0.0 5.52 0.06
0.1 5.15 0.08
0.2 4.70 0.13
0.3 4.47 0.06
0.4 4.16 0.10
0.5 3.92 0.04
0.6 3.65 0.03
0.7 3.39 0.04
0.8 3.19 0.04
0.9 3.04 0.04
1.0 2.90 0.02
1.5 2.31 0.02
2.0 1.87 0.03
2.5 1.53 0.03
3.0 1.28 0.02
3.5 1.11 0.02
4.0 0.97 0.01
4.5 0.82 0.01
5.0 0.71 0.02

NOTE: Each measurement is the average of at least three measurement scans. The uncertainty represents the observed standard deviation in the replicate measurements for each aluminum thickness. pC, picocoulomb.

graphically illustrated in Figure 7.2. The specific values that are identified as the first and second HVLs are 1.1 and 1.7 mm Al, respectively.14

Percent Depth Dose Measurements

The PDD measurements are tabulated in Table 7.2 for the PS-033 thin-window (0.5 mg/cm2) 4.9 cm3 chamber and illustrated graphically in Figure 7.3.

Again, a second set of PDD data was collected using the larger 15 cc parallel-plate ion chamber with a 32 mg/cm2 window thickness. A comparison of the response for the two chambers is illustrated in Figure 7.4.

For this AIT, standard deviations for each set of exposure measurements were

_______________

14 The high spatial resolution of the HVL data for thin layers of aluminum will permit the low-energy components of the X-ray beam to be incorporated into any computational candidate spectrum. Thus, the complete aluminum attenuation curve can provide improved fitting of the X-ray spectrum compared to previous models that incorporated simply the first, and sometimes second, HVL. This process and the effects on the PDD simulation process are discussed in more detail in the “Measuring Percent Depth Dose” section.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 7.2 High-value layer measurements as a function of aluminum thickness for the AS&E SmartCheck. Error bars are not shown since they are approximately the size of or smaller than the symbols.

calculated based on the replicate data set and averaged less than 2 percent of the mean value, the largest being 4.2 percent for the thin-window (0.5 mg/cm2) 4.9 cm3 chamber. For the 15 cc chamber, the standard deviation for each set of exposure measurements averaged less than 1 percent of the mean value with the largest being 1.5 percent.15

It is important to point out here why extra efforts were taken to acquire extremely high-resolution PDD data. Because concerns have been expressed that the short range for dose buildup at low energies generated by an X ray tube, with a voltage of 50 kV applied, delivers a peak dose to radio-sensitive portions of the skin and that this may be great enough to produce significant risk of skin cancer. Accurate PDD data are needed in order to investigate this claim. While PDD profiles are commonly characterized for radiation therapy beams operated at much higher energies (in the ~ 10 MV energy range), they are not commonly characterized for energies at which AIT systems operate. This presents a unique challenge because any peak dose for 50 kV systems is expected to occur at very shallow tissue depths and requires a specialized ion chamber having a very thin entrance window, which

_______________

15 Uncertainties in the tissue depth are substantially smaller than the size of the symbols in the accompanying figures. Hence, you might not see them even though they are there.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.2 Percent Depth Dose for the AS&E SmartCheck Using the PS-033 Thin-Window (0.5 mg/cm2) 4.9 cm3 Ion Chamber

Tissue Depth (mm) PDD (%) Standard Deviation (%)
0.0 92.2 3.6
0.0 91.7 1.6
0.2 93.5 1.6
0.3 89.8 3.2
0.3 92.6 1.6
0.5 99.0 1.6
0.7 100.0 1.6
1.0 92.4 4.2
2.0 92.2 4.2
3.1 88.3 1.6
4.2 79.9 1.6
5.9 77.7 3.2
8.0 67.4 1.6
9.6 56.4 1.6
15.6 44.1 1.6
20.4 35.8 1.6
26.2 28.3 1.6
31.0 20.4 1.6
40.7 14.9 1.6
51.4 8.90 1.6
62.2 7.88 1.6

minimizes production of secondary electrons as the beam enters the chamber, and specialized phantom materials to represent tissue attenuation and the thin thicknesses of interest.

Phantom materials were successfully produced in thin layers, as small as 25 µm. A series of thicknesses were fabricated in 10 cm × 10 cm cross sections so that these could be assembled in various combinations to develop the data for the PDD curves. The PDD data demonstrated a peak dose occurring at a depth around 0.5-0.7 mm. The peak, however, is not significantly greater than is the entrance skin dose—it exceeds the entrance skin dose by approximately 8 percent. Consequently, there should not be concern that the shallow dose is preferentially deposited to radio-sensitive layers of the skin.

The remainder of the PDD curve demonstrates the attenuation of the 50 kV X-ray beam for thicknesses up to 62 mm. A first HVL of 13 mm and a second HVL of 15 mm in tissue is observed for this beam. About 10 percent of the entrance dose is delivered at a depth of 62 mm.

A second set of PDD data was collected using the larger-volume (15 cc), thicker-window ion chamber. This data set provided enhanced measurement accuracy over all but the smallest range of tissue depths. When normalized to the

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 7.3 Percent depth dose collected for the AS&E SmartCheck using the PS-033 thin-window (0.5 mg/cm2) 4.9 cm3 ion chamber.

maximum, the PDD curves from the two ion chambers match within the measurement uncertainties for tissue depths greater than 1 mm. At depths less than 1 mm, the larger-volume chamber does not resolve the dose peak due to the thicker window that generates secondary electrons prior to the X-ray beam entering the sensitive volume of the ion chamber. While this effect must also occur to some degree in the thin-window PS-033 chamber, the curves demonstrate the need to utilize a thin-window ion chamber to more accurately resolve the details of the PDD curve at this low energy and this tissue depth.

The PDD data were also used as a validation tool for the computational simulations. Good agreement (see Figure 7.13) was obtained between the computational and empirical evaluations of the PDD curves.

The initial simulations were not particularly good matches to the empirical PDD curves. Through a series of discussions and a review of both the HVL and the PDD data, it was recognized that the candidate X-ray spectrum that was being utilized based on previously collected data was overly hard (i.e., excessively filtered, low-energy beam components). By developing a softer candidate spectrum (reducing the filtration to include more low-energy components), a good match was obtained for both HVL and PDD curves. This demonstrates the value of obtain-

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 7.4 Comparison of percent depth dose data collected for the AS&E SmartCheck using the thin-window (0.5 mg/cm2) 4.9 cm3 chamber (green triangles) and the 32 mg/cm2 window 15 cc chamber (blue circles). The inset shows the region from 0 to 8 mm enlarged, indicating a peak around 0.5 to 0.7 mm.

ing high-spatial-resolution measurements for HVL and PDD curves to accurately characterize the X-ray spectrum for AIT systems.

Air Kerma Results

Person Being Screened

A series of 10 individual air kerma measurements were made at each measurement position. Each measurement was made with the ion chamber positioned as previously described, and a normal subject scan was made along the central axis (x = 0 cm, y = 0 cm) at several vertical positions (z). The two z = 32 cm results represent two separate sets of 10 individual scans performed under the same conditions. The average and standard deviations for the air kerma at each vertical position are provided in Table 7.3.

The measurements of air kerma provided good reproducibility at each location. A slight variation of air kerma was observed as a function of z. The air kerma aver-

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.3 Air Kerma Measured Along the Central Axis (x = 0 cm, y = 0 cm) for the AS&E SmartCheck at Transportation Security Administration’s Systems Integration Facility

Vertical Position-z (cm) Air Kerma per Scan (nGy) Standard Deviation (nGy)
32 113 4
32 113 4
120 108 5
150 105 4
202 111 3

aged 110 nGy per scan and was observed to be a few percent higher at the upper and lower regions of the scan. Because the system utilizes a fixed, horizontally oriented tube geometry that is translated vertically (there is no tube angulation), it would be expected that the air kerma should be nearly constant along the vertical extent of the scan. The small variations of air kerma observed may result from the time required for tube acceleration and deceleration at the beginning and end of the scan, respectively. Uncertainty in the absolute value of the air kerma is also subject to uncertainty in the chamber calibration. Comparisons among various chamber calibrations performed on a 50 kV beam (2.08 mm HVL) performed at the University of Florida indicates that the uncertainty in the 150 cc chamber calibration performed for the L60 beam should be less than 4 percent different from the spectral response of this 50 kV beam.

Bystander (Outside the Screening Volume)

Leakage Radiation Geometry

The low levels of radiation encountered during the bystander set of measurements required continuous integration over a period of 4-5 minutes. The Fluke 451B was operated in integration mode and never recorded any values above 0 nGy. Results presented are based on the Keithley model 96020C 150 cc ion chamber with charge collected by the PTW Unidose electrometer. At low exposure levels, the background and electrometer/chamber leakage can produce a significant contribution to the measured result. In order to reduce this contribution, a baseline measure of background electrometer/chamber leakage rate was made. An estimate of this total contribution was made based on the resulting integration time and subtracted from the gross charge accumulated over the period. The net accumulated charge was subsequently converted to air kerma using the appropriate chamber calibration factor and making air temperature and pressure corrections relative to the chamber’s calibration conditions. The integrated results were then divided

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.4 Air Kerma per Scan Outside the Inspection Area for the AS&E SmartCheck

Position Air Kerma per Scan (nGy) Standard Deviation (nGy)
Leakage—Rear exterior 0.23 0.83
x = −100 cm, y = 0 cm, z = 150 cm
Scatter—Center of exit 1.7 0.45
x = 0 cm, y = −66 cm, z = 120 cm
Scatter—Operator side of exit 2.8 0.25
x = −27 cm, y = −66 cm, z = 120 cm

by the number of scans included for each measurement position to provide the average air kerma per scan at each measurement position, illustrated in Table 7.4.

Standard deviation is relatively large at the very low exposure levels for each of these measurements. The standard deviation observed between multiple sets of measurements average around 0.5 nGy per scan. Consequently, the reported values of the leakage radiation are so low as to be statistically indistinguishable from the background radiation. This should not be surprising because the X rays produced by a 50 kV beam are readily attenuated by quite modest thicknesses of lead that may be expected to be incorporated into the scanning system.

Scatter Radiation Geometry

Measurement of the scatter radiation fields was performed at a very conservative position (the very edge of the AIT system) and produced small but measurable radiation exposures. The reported values suggest that there may be slightly more scatter intensity in the direction toward the X-ray source. This may be expected because a greater surface area of the phantom is available to scatter into this direction, and the generation of scatter within the phantom is expected to be mostly isotropic at these energies. Measurements were thus not made at the operator position, only at the edge of the AIT system. It should also be noted that the operator position can be altered from side-to-side (i.e., it can be located on either the entrance or the exit side) on this system, but it is confined to the side housing the anterior X-ray source. Radiation fields are expected to be symmetrical in the +/– y directions based on the system geometry and measurements performed on a previous generation AS&E AIT system by NIST.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Committee Review of the AS&E SmartCheck System Design

The committee did not have the same level of access to the AS&E system that it had to the Rapiscan Secure 1000 system at NIST. This difference in access is due to the AS&E system being subjected to TSA qualification tests that impose restrictions on how it can be handled, but an AS&E representative pointed out and described the various interlocks and safety features. From these descriptions, the committee developed a summary of the committee’s findings, given in Table 7.5. As with the Rapiscan Secure 1000 installed at NIST, the committee was able to confirm that the AS&E AIT system installed at TSIF met the same 6 of 14 requirements for all radiation-emitting devices in ANSI/HPS N43.17 (Section 7.2.1) and the same 5 of the 6 requirements for general-use radiation-emitting devices (Section 7.2.2). An additional 3 requirements (Sections 7.2.1 j and l, and 7.2.2 e) could not be confirmed because the operational AIT system cannot be forced to perform the action. The interlocks, safety, and control requirements in ANSI/HPS N43.17-2009 that could be verified for the AIT system are listed in Box 7.1.

Key Finding: Based on the committee’s inspection of the AS&E SmartCheck system with the AS&E representative present, the committee was unable to identify any circumstances where an accidental failure or deliberate reconfiguration of the AIT system could result in either a person being screened or the operator receiving a larger X-ray dose than the normal screening dose.

Because there is no failure mechanism that would give more than a normal screening dose, as stated in the key finding, scanning time is the only factor left. The AIT system would have to operate for more than 16 hours to exceed the dose limit of 250,000 nSv, as stated in Table 7.5 (row 7.2.1 m); it is unreasonable to expect that a person being scanned would be exposed for that amount of time.

THE RAPISCAN SECURE 1000 SYSTEM MEASUREMENT RESULTS AND SYSTEM DESIGN

A Rapiscan Secure 1000 (serial number S51023005) became available for measurements in July 2014. This AIT system had been in service at LaGuardia Airport and was later transferred to NIST, where it was installed and calibrated by the manufacturer’s technicians. This AIT system can thus provide information indicative of the performance of the AIT systems that were installed previously in airports. For the purposes of the measurements done by the NRC subcontractors, the software used by TSA and NIST were identical. During some measurements, the engineering mode was used to control scan motion.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.5 Summary of the Committee’s Review of the AS&E SmartCheck Compared to the Requirements in Sections 7.2.1 and 7.2.2 of ANSI/HPS N43.17-2009

Text Committee Comments
ANSI/HPS Section 7.2.1
a. There shall be at least one indicator, clearly visible from any location from which a scan can be initiated, that indicates when a scan is in progress. Confirmed on NIST AIT system.
b. There shall be at least one lighted indicator clearly visible from the inspection zone. For portal systems the indicator shall be visible from any approach to the inspection zone to indicate that a scan is in progress. Confirmed on NIST AIT system.
c. Power to the system shall be controlled by a key switch. The key shall be captured (unable to be removed) whenever it is in a position that allows exposures to be initiated. Confirmed on NIST AIT system.
d. Each system shall have a means for the operator to initiate the emission of radiation other than the function of an interlock or the main power control. Operator initiates scan from computer console. Confirmed on NIST AIT system.
e. Each system shall have a means for the operator to terminate the emission of radiation other than the function of an interlock. “Stop scan” icon included on computer screen. Confirmed on NIST AIT system.
f. Means shall be provided to ensure that operators have a clear view of the scanning area. Operational requirement that is site specific. Not confirmed on NIST AIT system.
g. A ground fault shall not result in the generation of X rays or activate a scan beam from a sealed radioactive source. Committee unable to confirm impact of a ground fault.
h. Failure of any single component of the system shall not cause failure of more than one safety interlock. Committee unable to determine the impact of component failure.
i. A tool or key shall be required to open or remove access panels. Access panels shall have at least one safety interlock. Confirmed on NIST AIT system.
j. For stationary-subject systems, the scanning motion of the X-ray beam relative to the subject shall be interlocked and the exposure shall terminate when the rate of motion of the beam in any direction falls below a preset minimum speed. Not confirmed. Committee believes image quality would prevent operation in these conditions when advanced imaging technology is not used.
k. For portal systems, the minimum walking or driving velocity through the inspection zone shall be determined by the manufacturer. Not applicable to stationary subject AIT systems.
l. Operational interlocks shall terminate the primary beam in the event of any system problem that could result in abnormal or unintended radiation emission. Not confirmed. Committee believes increased radiation exposure would distort image or cause daily test to fail.
m. In the event of a malfunction, the system shall terminate radiation exposure rapidly enough so that no location on the subject’s body shall receive an ambient dose equivalent (H*10) exceeding 250 µSv (250,000 nSv), regardless of the size of the exposed area. In this report it is shown that for a normal scan the computed effective doses are about an order of magnitude lower than the recommended ANSI standard of 250nSv/screen. Comparing the 6 second scan and ~25 nSv/screen versus a 250,000 nSv limit, the exposure would have to last for more than 16 hours.
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Text Committee Comments
n. Following interruption of X-ray production or external gamma emission by the functioning of any safety interlock, resetting the interlock shall not result in the production of X rays or emission of gamma radiation. Confirmed on AS&E prototype AIT system at TSIF.
 
ANSI/HPS Section 7.2.2
a. For any X-ray system that normally keeps high voltage applied to the X-ray tube at times other than during a scan, there shall be at least one lighted “X ray on” indicator at the control console where X rays are initiated indicating when X rays are being produced. Confirmed on AS&E prototype AIT system at TSIF.
b. Technique factors for each mode of operation shall be preset by the manufacturer and shall not be alterable by the system operator. Confirmed on AS&E prototype AIT system that technique factors cannot be changed when AIT system is in operator mode. Verified in discussion with TSA.
c. Each access panel to the X-ray source shall have at least one safety interlock to terminate the X-ray production when opened. Confirmed on AS&E prototype AIT system at TSIF.
d. The following warning label shall be permanently affixed or inscribed on the X-ray system at the location of any controls used to initiate X-ray generation: “CAUTION: X-RAYS PRODUCED WHEN ENERGIZED.” Confirmed on AS&E prototype AIT system at TSIF.
e. X-ray emission shall automatically terminate after a preset time or exposure. Not confirmed. Committee believes this verification is part of the qualification testing under way at TSIF.
f. For portal systems, motion sensors shall monitor the speed of pedestrians or vehicles through the inspection zone (in the forward direction) and the radiation exposure shall terminate when the speed drops below the minimum (as determined according to Section 7.2.1k). Not applicable to stationary subject AIT systems.

Half-Value Layer Results

In order to stop the vertical scan motion, the power to the vertical positioning motor was turned off and the X-ray head was manually moved to the desired height and clamped in place. In this position, the top of the tube housing was horizontal, at approximately z = 85 cm. The engineering software was utilized to override interlocks and allow X-ray production in this configuration. One AIT system unit was turned off while measurements were made for the other one. The results for both the anterior and the posterior unit are shown in Table 7.6 and Figures 7.5, 7.6,

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

BOX 7.1
ANSI/HPS N43.17-2009 Requirements Verified for the AS&E SmartCheck

7.2 Indicators, Controls, and Safety Interlocks

7.2.1 Requirements for All Systems: The requirements of this subsection apply to all the systems regardless of category or type of radiation source. In addition to these requirements systems must comply with the requirements of one of the sections 7.2.2 through 7.2.5 as appropriate.

a. There shall be at least one indicator, clearly visible from any location from which a scan can be initiated, that indicates when a scan is in progress.

b. There shall be at least one lighted indicator clearly visible from the inspection zone. For portal systems the indicator shall be visible from any approach to the inspection zone to indicate that a scan is in progress.

c. Power to the system shall be controlled by a key switch. The key shall be captured (unable to be removed) whenever it is in a position that allows exposures to be initiated. Turning on the key switch shall never result in the external emission of radiation.

i. A tool or key shall be required to open or remove access panels. Access panels shall have at least one safety interlock.

j. For stationary-subject systems, the scanning motion of the X-ray beam relative to the subject shall be interlocked and the exposure shall terminate when the rate of motion of the beam in any direction falls below a preset minimum speed. The minimum speed shall be chosen so that the dose during the exposure period is within the applicable limit.

l. Operational interlocks shall terminate the primary beam in the event of any system problem that could result in abnormal or unintended radiation emission. This shall include, but is not limited to, unintended stoppage of beam motion, abnormal or unintended X-ray source output, computer safety system malfunction, termination malfunction, and shutter or beam stop mechanism malfunction.

m. In the event of a malfunction, the system shall terminate radiation exposure rapidly enough so that no location on the subject’s body shall receive an ambient dose equivalent (H*10) exceeding 250,000 nSv (25 mrem), regardless of the size of the exposed area.

7.2.2 Requirements for General-use Systems Using X-ray Sources: In addition to the requirements of Section 7.2.1, “Requirements for All Systems,” the following requirements apply to general-use systems using X-ray sources:

c. Each access panel to the X-ray source shall have at least one safety interlock to terminate the X-ray production when opened.

d. The following warning label shall be permanently affixed or inscribed on the X-ray system at the location of any controls used to initiate X-ray generation: “CAUTION: X-RAYS PRODUCED WHEN ENERGIZED.”

e. X-ray emission shall automatically terminate after a preset time or exposure.

SOURCE: The ANSI/HPS N43.17-2009 standard, “Radiation Safety for Personnel Security Screening Systems Using X-Ray or Gamma Radiation,” is available at the Health Physics Society website at http://hps.org/hpssc/index.html.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.6 Rapiscan Secure 1000 Half-Value Layer Measurement Results

Aluminum
Thickness (mm)
Normalized
Exposure
Anterior Unit
Uncertainty
Anterior Unit
Normalized
Exposure
Posterior Unit
Uncertainty
Posterior Unit
0.0 1.00 0.005 1.00 0.007
0.05 0.95 0.008 0.94 0.003
0.1 0.90 0.005 0.89 0.004
0.15 0.86 0.018 0.84 0.004
0.2 0.83 0.003 0.81 0.003
0.25 0.79 0.012 0.77 0.004
0.3 0.76 0.005 0.75 0.000
0.35 0.73 0.009 0.71 0.005
0.4 0.71 0.002 0.68 0.008
0.45 0.68 0.004 0.65 0.002
0.5 0.65 0.006 0.63 0.003
0.55 0.63 0.003 0.60 0.003
0.6 0.61 0.007 0.58 0.002
0.65 0.59 0.003 0.56 0.003
0.7 0.58 0.004 0.56 0.005
0.8 0.55 0.010 0.52 0.002
0.9 0.51 0.004 0.48 0.003
1.0 0.48 0.007 0.46 0.005
1.1 0.45 0.000 0.43 0.002
1.2 0.44 0.003 0.42 0.000
1.3 0.41 0.002 0.39 0.000
1.4 0.40 0.002 0.37 0.004
1.5 0.37 0.007 0.35 0.002
2.0 0.30 0.003 0.28 0.000
2.5 0.24 0.002 0.22 0.004
3.0 0.20 0.003 0.19 0.005
3.5 0.17 0.000 0.16 0.003
4.0 0.15 0.004 0.13 0.002
4.5 0.13 0.006 0.12 0.002
5.0 0.11 0.003 0.10 0.003

NOTE: Each measurement result shown is the average of at least three measurement scans. The uncertainty represents the observed standard deviation in the replicate measurements for each aluminum thickness.

and 7.7. Each measurement is the average of at least three measurement scans, as measured in coulombs, and then normalized to the value at zero aluminum thickness. HVL1 and HVL2 were 0.92 and 1.47 mm Al for the anterior unit and 0.85 and 1.42 mm Al for the posterior unit, respectively.

Percent Depth Dose Results

Depth dose measurements were at the same height as the HVL measurements, z = 85 cm, with x = 0 cm and y = 0 cm. The results are tabulated in Table 7.7 and shown in Figures 7.6 and 7.7.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.5 Normalized exposure as a function of aluminum thickness for the Rapiscan Secure 1000 for (a) the anterior unit and (b) the posterior unit. Error bars are not shown since they are approximately the size of or smaller than the symbols.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.6 Percent depth dose (PDD) for the Rapiscan Secure 1000 anterior unit collected using the PS-033 thin-window (0.5 mg/cm2) ion chamber at tissue thicknesses (a) up to 6 mm and (b) less than 10 mm.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.7 Percent depth dose (PDD) for the Rapiscan Secure 1000 posterior unit collected using the PS-033 thin-window (0.5 mg/cm2) ion chamber at tissue thicknesses (a) up to 6 mm and (b) less than 10 mm.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.7 Percent Depth Dose (PDD) for the Rapiscan Secure 1000 Anterior and Posterior Units for Data Collected on the PS-033 0.5 mg/cm2 and 32 mg/cm2 Window Chambers

Tissue Depth
(mm)
Anterior Posterior
PDD (%)
0.5 mg/cm2
chamber
PDD (%)
32 mg/cm2
chamber
PDD (%)
0.5 mg/cm2
chamber
PDD (%)
32 mg/cm2
chamber
  0.00   98.98 100.00 100.00 100.00
  0.02 100.00       —   96.19       —
  0.20   96.87   98.17   96.77   97.96
  0.34   98.36   97.47   98.62   97.69
  0.51   98.30   96.76   94.77   96.48
  0.71   96.70   94.49   90.92   95.49
  0.99 102.36   92.43   85.79   93.32
  2.03   91.69   86.74   81.72   86.25
  3.12   88.31   81.47   75.16   80.43
  4.24   80.39   76.56   73.94   74.37
  5.87   78.45   68.58   69.50   66.98
  7.95   66.29   58.45   57.93   59.68
  9.65   55.12   56.29   56.99   54.78
  15.55   37.99   40.96   36.06   41.12
  20.35   34.52   32.58   31.95   31.96
  26.25   25.40   24.97   24.38   25.34
  31.05   22.23   20.36   20.57   18.68
  40.70   14.82   14.15   10.81   12.91
  51.40   13.03     9.79     9.65     8.96
  62.20     5.67     7.59     9.65     6.45

PDD data were collected using the larger 15 cc parallel-plate ion chamber (32 mg/cm2 window thickness). The larger sensitive volume of this chamber provided improved signal collection at the low exposures produced by the scanning system. A comparison of the response for these two chambers is reported in Figure 7.8. The comparison demonstrates that consistent data were collected from both chambers except at very small values for tissue depth where the dose peak observed using the thin-window (0.5 mg/cm2) 4.9 cm3 chamber was overwhelmed by the thicker (32 mg/cm2) window of the 15 cc chamber.

Standard deviations for each set of exposure measurements were calculated based on the replicate data set and were typically about 3 percent of the mean value, with the largest being 5.7 percent for the thin-window chamber. For the 15 cc chamber, the standard deviation for each set of exposure measurements averaged less than 1 percent of the mean value, with the largest being 1.5 percent. Uncertainties in the tissue depth are substantially smaller than the size of the symbols in the accompanying figures.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.8 Comparison of percent depth dose (PDD) data for the Rapiscan Secure 1000 collected using the thin-window (0.5 mg/cm2) chamber (triangles) and the 32 mg/cm2 window chamber (circles) for (a) the anterior unit and (b) the posterior unit.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Air Kerma Results

Person Being Screened

Air kerma was measured with the 150 cc parallel-plate ion chamber centered at multiple vertical locations (see Table 7.8) for both the anterior and the posterior unit. The measurement locations were selected to evaluate any variations in beam intensity as the beam scans vertically. Because of the low intensity of the scanning beam, a series of 10 individual measurements were recorded and averaged to provide the air kerma at each vertical location.

Two additional sets of measurements were made at positions x = 25.8 cm, y = 0 cm, z = 82.8 cm, and x = −25.8 cm, y = 0 cm, z = 82.8 cm, which specify locations 30 cm from the front surface of the anterior and posterior units, respectively. This provided a reference air kerma that may be used to demonstrate compliance with the applicable ANSI standard and is also identified in Table 7.8.

A series of 10 individual measurements were made at each measurement location. Each measurement was made with the ion chamber positioned as previously described, and a normal, but single-sided, subject scan was performed. The average and standard deviation for the air kerma at each vertical position are provided in Table 7.8.

TABLE 7.8 Air Kerma Measured Along the Central Axis (x = 0 cm, y = 0 cm) at Several Vertical Positions (z) for the Rapiscan Secure 1000

Vertical Position z (cm) Anterior Unit Posterior Unit
Air Kerma
per Scan
(nGy)
Standard
Deviation
(nGy)
Air Kerma
per Scan
(nGy)
Standard
Deviation
(nGy)
  26.8 27 2 26 2
  48.8 28 2 31 1
  82.8 29 2 32 2
122.8 31 2 30 2
138.8 28 2 30 2
161.8 27 2 30 2
184.8 26 1 29 2
184.8 29 2
194.8 33 1
At 30 cm from x-surface
  82.8 48 3 56 4

NOTE: The two z = 184.8 cm results represent two separate sets of 10 individual scans performed under the same conditions.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Bystander (Outside the Screening Volume)

Leakage Radiation Geometry

A measurement location on the backside of the posterior unit was selected to evaluate leakage radiation that may be exiting the unit. The 150 cc parallel-plate ion chamber was positioned against the rear exterior of the posterior AIT system assembly at a height of z = 171 cm, with x = −130 cm, and y = 0 cm, centered on the path that the X-ray source travels while vertically scanning.

Scatter Radiation Geometry

In order to represent the geometry that would cause radiation to be scattered outside of the subject scanning area toward bystanders (e.g., passengers not being screened and transportation security officers), an anthropomorphic phantom representative of an adult male was used as described previously.

Results

The low levels of radiation that were expected to result from this set of measurements required continuous integration over a period of approximately 2 minutes. The Fluke 451B was operated in integration mode and never recorded any values above 0 nGy. Results presented are based on the Keithley model 96020C 150 cc parallel-plate ion chamber with the charge collected by the PTW Unidose electrometer. At low exposure levels, the background and electrometer/chamber leakage can produce a significant contribution to the measured result. In order to reduce this contribution, a baseline measure of background electrometer/chamber leakage rate was made. An estimate of this total contribution was made based on the resulting integration time and subtracted from the gross charge accumulated over the period. The net accumulated charge was subsequently converted to air kerma using the appropriate chamber calibration factor and making air temperature and pressure corrections relative to the chamber’s calibration conditions. The integrated results were then divided by the number of scans included for each measurement position to provide the average air kerma per scan at each measurement position, illustrated in Table 7.9.

Committee Review of the Rapiscan Secure 1000 System Design

The committee did not have access to Rapiscan personnel or AIT system design documentation during the course of this evaluation because Rapiscan could not participate within the set time frame of the study, but the committee did review

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.9 Air Kerma per Scan Outside the Inspection Area of the Rapiscan Secure 1000

Position Air Kerma per Scan (nGy) Standard Deviation (nGy)
Leakage—Rear exterior 4.0 6.0
x = 130 cm, y = 0 cm, z = 171 cm
Scatter—Center of exit 0.0 16
x = 0 cm, y = 72 cm, z = 120 cm
Scatter—Operator side of exit 7.0 4.0
x = −44.8 cm, y = 72 cm, z = 120 cm

the public documentation available, from both public sources and TSA, for the Rapiscan Secure 1000. The committee also inspected the Rapiscan Secure 1000 AIT system that was made available to the committee at NIST.

A few of the interlocks, safety, and control requirements in the ANSI standard could not be verified in the field or without reviewing the machine design or fully disassembling a machine. However, the committee assumes that the Rapiscan Secure 1000 machine design and interlocks were (as required) demonstrated by the vendor to meet ANSI/HPS N43.17-2009 or the version of ANSI/HPS N43.17 in force at the time of qualification testing at TSA prior to deployment at airports.

Documentation for Compliance with ANSI Standards

The factory acceptance test (FAT) and the site acceptance test (SAT) and other operating procedures define the procedures prior to deployment and after installation of any backscatter system. The documents used by TSA and equipment producers in previous years, during deployment of the Rapiscan backscatter AIT systems at airports, refers to Form R-0646, “Radiation Emission Measurement for Secure 1000.” This form, and the accompanying Form R-0685, defines what is inspected and measured from a radiation-safety perspective during FAT and SAT and in the following situations:

  • After system relocation,
  • During maintenance, and
  • Annually.

Forms R-0646 and R-0685 for all fielded AIT systems have been publicly available on the TSA website for several years,16 but they are currently unavailable

_______________

16 Transportation Security Administration, “Surveys of Backscatter Imaging Technology Machines,” http://www.tsa.gov/research-center/surveys-backscatter-imaging-technology-machines, accessed July 23, 2014.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

online; only a list of documents is provided on the website.17 The forms request the following information pertinent to radiation safety:

  • X-ray tube serial number (anterior and posterior unit);
  • The result of an emergency stop button test;
  • If scan in progress lights were operational;
  • If X-ray caution labels were present on the anterior, posterior, and communications units;
  • If X-ray warning labels were present on the X-ray generators for the anterior and posterior;
  • The outcome of a safety interlock test;
  • If the key is removable when the unit is in operation;
  • Settings of the generator (voltage and current);
  • Length of the scan time, in seconds;
  • If the operator instructions are available; and
  • General condition of the AIT system.

It is not specified what the exact safety interlock test referred to above is. The committee notes the following points related to existing interlock systems as they are described in the Rapiscan operator manual:

  • Power to the system is controlled by a key switch. The key switch has three positions “off,” “standby,” and “on.” When the key is placed on “standby,” the system waits for commands from the operator console and will time out if commands are not received.
  • A mechanical sensor at the bottom of the door is depressed when the door is closed. If the door is opened, the sensor is released and the interlock prevents operation of the system. Both anterior and posterior access-panel interlocks must be enabled for a scan to initiate and complete.
  • X-rays will terminate if there is over voltage or over current.
  • Reference detector signal. When a detector is placed in the X-ray beam, the X-ray intensity is monitored for radiation levels out of range.
  • Velocity of vertical motion. A sensor that travels with the X-ray tube will generate electrical impulses to monitor vertical motion. X-rays are terminated if motion stops.
  • Velocity of horizontal motion of the scanned beam. An optical interrupt sensor located on the assembly is used to monitor that the rotational velocity is maintained. X-rays are terminated if speed is out of tolerance limits.

_______________

17 The links to the documents posted on April 11, 2013, May 24, 2011, and March 16, 2011, no longer work.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
  • X-ray tube head temperature. X-rays will terminate if the X-ray tube is out of range.

Safety-related information that was requested by Form R-0646 included the following:

  • Test procedure used (described below),
  • Background radiation reading,
  • Measurement instrument type (and serial number), and
  • Data acquired.

The form indicated that data acquired should include measurements of where a person would stand when being screened—called in beam radiation exposure measurements—which consist of total integrated exposure as averaged over 10 measurements using a Fluke 451P meter positioned 304.8 mm (12 inches) from the center of the scan window and 914.4 mm (36 inches) from the floor of both anterior and posterior units.18 Data acquired should also include radiation leakage measurements, which are similar to in beam measurements but located at four positions at the center of the active units’ external surface, for both the anterior and the posterior, where a potential bystander could be. The final data acquired should include inspection zone boundary radiation dose measurements, done 304.8 mm (12 inches) from the edges of the units scan windows (four measurements), again where a potential bystander could be.

The data on Forms R-0646 and R-0685 were compared to the exposure limits set, and if results were within the administrative integrated exposure limits, the AIT system was considered to meet the ANSI/HPS N43.17-2009 standard with respect to limits for reference effective dose and X-ray leakage. If any value exceeded the limits, that fact would be reported to service program managers prior to placing a system into operation.

Key Finding: Acceptance tests and periodic inspection tests guided by the safety inspection forms previously used during deployment are sufficient to

_______________

18 The Model 451P Ion Chamber Survey Meter is a hand-held, 8 atm pressurized, 230 cc active volume air ionization chamber meter designed to measure gamma and X-ray radiation above 25 keV and beta radiation above 1 MeV. The plastic chamber wall is 200 mg/cm2 thick. The instrument has a ±10 percent accuracy of reading between 10 and 100 percent of full-scale indication on any range. The typical relative energy response is approximately 0.4 for 20 keV, 0.8 for 40 keV, and 1 for 50 keV (Fluke Biomedical, “451P Pressurized µrad Ion Chamber Radiation Survey Meter,” http://www.flukebiomedical.com/biomedical/usen/radiation-safety/Survey-Meters/451P-pressurized-ionchamber-radiation-detector-survey-meter.htm?PID=54793, accessed July 23, 2014).

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

meet the indicators, controls, and safety interlocks requirements of the ANSI/HPS N43.17-2009.

Safety Procedures for Maintaining the Rapiscan Secure 1000 During the Course of Its Use

According to the written safety procedures for the Rapiscan Secure 1000, prior to any startup of a unit, there should be verification that service access doors for each AIT system module are closed and locked before powering up the AIT system modules. In order to ensure that the system is working properly, personnel should run a scan at least once a day and inspect the image of a test piece to ensure that needed safety systems were working properly. On a monthly basis, there should be inspection of all external cables for possible wear or damage as well as inspection of the “Scan in Progress” light to ensure that all words are illuminated. On a semiannual basis, there should be inspection of the functionality of the X-ray power supply, the controller, and the internal control computer, as well as for all warning lightbulbs.

The equipment was to be serviced only by qualified and trained service providers. Operators of the equipment were not to open any cabinets. The ultimate responsibility for the radiation safety of the system, the operators, and the general public rests with the owner,19 which designated individuals responsible for ensuring compliance with the requirements of ANSI/HPS N43.17-2009. The owner was also responsible for ensuring a personnel training program, with refresher training provided at least once every 12 months.

Key Finding: Equipment manufacturers recommend that a test piece be scanned daily to evaluate proper operation of the AIT system because this ensures that many of the needed safety system requirements in ANSI/HPS N43.17-2009 work properly. The committee agrees with this recommendation but was unable to determine if this was being done because of the current lack of X-ray backscatter AITs in the field at commercial airports.

Committee Review of the Interlocks on the Rapiscan Secure 1000 System

The committee considered potential failure mechanisms that could result in X-ray overexposure of the person being screened or bystanders such as the operator. On the Rapiscan Secure 1000 located at NIST, committee members verified that the X-ray source can be activated only with the key in the “on” position with a scan initiated and that the key cannot be removed unless it is in the “off” position.

_______________

19 In this case, the owner would be TSA.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

The committee confirmed that the X-ray source turns off at the end of each scan. Scans could not be initiated if any of the panel doors enclosing the X-ray source and other electronics were ajar or if the emergency-off button had been activated. The committee verified that the X-ray source turned off immediately even though the vertical mechanical scan bar completed full travel to the park position at the top or bottom of the AIT system. These investigations are listed, together with previous studies’ findings, in Tables 6.5 and 6.6 in Chapter 6 for comparison to other researchers’ results and also given in Table 7.10.

The subcontractor’s measurements required that the vertical scan bar be locked at a specific height. To enable this configuration, the AIT system was put into engineering mode, which requires a higher-level password than that used for screening but gives greater access to controlling individual functions. This way, the vertical scan motor was turned off and the scan bar was moved by hand. Table 7.10 summarizes the committee’s review compared to ANSI/HPS N43.17-2009, Section 7.2, “Indicators, Controls, and Safety Interlocks.”

The committee was able to confirm that the Rapiscan Secure 1000 AIT system installed at NIST met 6 of the 14 ANSI/HPS N43.17-2009, Section 7.2.1 requirements for all radiation-emitting devices and 5 of the 6 ANSI/HPS N43.17-2009, Section 7.2.2 requirements for general-use radiation-emitting devices. One requirement (7.2.1 f) is site specific and could not be verified outside of observing an AIT system in operation in an airport, and two of the requirements (7.2.1 g, h) could not be confirmed because they would require the deliberate activation of a potentially destructive fault, which NIST would not allow. Requirements 7.2.1 k and 7.2.2 f do not apply to portals that require the subject being screened to be stationary, such as the Rapiscan Secure 1000. An additional three requirements (7.2.1 j, l and 7.2.2 e) could not be confirmed because the operational AIT system cannot be forced to perform the action, but the committee believes that these faults in the field would distort the image to the extent that the screener could not make a pass/fail determination for that screening, and the screening would not result in overexposure of the person being screened. Once the AIT systems use automatic threat recognition, the operators will not see the image, and this check would no longer apply.

Requirement 7.2.1 m states that no malfunction of the AIT system shall result in an exposure exceeding 250,000 nSv. The committee could identify no failure mechanism that would result in an increase in the X-ray photon emission from the X-ray source (e.g., increasing the X-ray tube voltage or current) so that scanning time is the only malfunction that could result in X-ray exposure exceeding the ANSI standard. In this report, the committee shows that for a standard screening of approximately 6 seconds, the computed effective doses are about an order of magnitude lower than the recommended ANSI standard of 250 nSv/screen. The person being screened would have to stand in the AIT system with the X-ray source

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.10 Summary of the Committee’s Review of the Rapiscan Secure 1000 Compared to the Requirements in Sections 7.2.1 and 7.2.2 of ANSI/HPS N43.17-2009

Text Committee Comments
ANSI/HPS Section 7.2.1
a. There shall be at least one indicator, clearly visible from any location from which a scan can be initiated, that indicates when a scan is in progress. Confirmed on AS&E prototype AIT system at TSIF.
b. There shall be at least one lighted indicator clearly visible from the inspection zone. For portal systems the indicator shall be visible from any approach to the inspection zone to indicate that a scan is in progress. Confirmed on AS&E prototype AIT system at TSIF.
c. Power to the system shall be controlled by a key switch. The key shall be captured (unable to be removed) whenever it is in a position that allows exposures to be initiated. Confirmed on AS&E prototype AIT system at TSIF.
d. Each system shall have a means for the operator to initiate the emission of radiation other than the function of an interlock or the main power control. Operator initiates scan from computer console. Observed on AS&E prototype.
e. Each system shall have a means for the operator to terminate the emission of radiation other than the function of an interlock. “Stop scan” icon included on computer screen. Confirmed on AS&E prototype AIT system at TSIF.
f. Means shall be provided to ensure that operators have a clear view of the scanning area. Operational requirement that is site specific. Not confirmed on AS&E prototype AIT system at TSIF.
g. A ground fault shall not result in the generation of X rays or activate a scan beam from a sealed radioactive source. Committee unable to confirm impact of a ground fault.
h. Failure of any single component of the system shall not cause failure of more than one safety interlock. Committee unable to determine the impact of component failure.
i. A tool or key shall be required to open or remove access panels. Access panels shall have at least one safety interlock. Confirmed on AS&E prototype AIT system at TSIF.
j. For stationary-subject systems, the scanning motion of the X-ray beam relative to the subject shall be interlocked and the exposure shall terminate when the rate of motion of the beam in any direction falls below a preset minimum speed. Not confirmed. Committee believes this verification is part of the qualification testing under way at TSIF.
k. For portal systems, the minimum walking or driving velocity through the inspection zone shall be determined by the manufacturer. Not applicable to stationary subject AIT systems.
l. Operational interlocks shall terminate the primary beam in the event of any system problem that could result in abnormal or unintended radiation emission. Not confirmed. Committee believes this verification is part of the qualification testing under way at TSIF.
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Text Committee Comments
m. In the event of a malfunction, the system shall terminate radiation exposure rapidly enough so that no location on the subject’s body shall receive an ambient dose equivalent (H*10) exceeding 250 µSv (250,000 nSv), regardless of the size of the exposed area. In this report it is shown that the computed effective doses for a normal scan are about an order of magnitude lower than the recommended ANSI standard of 250 nSv/screen. Comparing the 6 second scan and ~25 nSv/screen versus a 250,000 nSv limit, the exposure would have to last for more than 16 hours.
n. Following interruption of X-ray production or external gamma emission by the functioning of any safety interlock, resetting the interlock shall not result in the production of X rays or emission of gamma radiation. Confirmed on NIST AIT system.
 
ANSI/HPS Section 7.2.2
a. For any X-ray system that normally keeps high voltage applied to the X-ray tube at times other than during a scan, there shall be at least one lighted “X ray on” indicator at the control console where X rays are initiated indicating when X rays are being produced. Confirmed on NIST AIT system.
b. Technique factors for each mode of operation shall be preset by the manufacturer and shall not be alterable by the system operator. Confirmed on NIST AIT system that technique factors cannot be changed when AIT system is in operator mode. Verified in discussion with TSA.
c. Each access panel to the X ray source shall have at least one safety interlock to terminate the X-ray production when opened. Confirmed on NIST AIT system.
d. The following warning label shall be permanently affixed or inscribed on the X-ray system at the location of any controls used to initiate X-ray generation: “CAUTION: X-RAYS PRODUCED WHEN ENERGIZED.” Confirmed on NIST AIT system.
e. X-ray emission shall automatically terminate after a preset time or exposure. Not confirmed. Assumed to have been demonstrated at the time of qualification.
f. For portal systems, motion sensors shall monitor the speed of pedestrians or vehicles through the inspection zone (in the forward direction) and the radiation exposure shall terminate when the speed drops below the minimum (as determined according to Section 7.2.1k). Not applicable to stationary subject AIT systems.
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

operating for approximately 10,000 times longer than a single scan (16 hours) to approach the limit set in Section 7.2.1 m, an obviously long time. The committee considers this requirement to be met because the X-ray source is not capable of producing a higher X-ray energy or flux, and the time a person would have to stand in the AIT system to receive such a dose is unrealistically long.

The overall committee assessment is that the Rapiscan Secure 1000 as designed meets the requirements of ANSI/HPS N43.17-2009.

Key Finding: Based on the committee’s review and test of the Rapiscan Secure 1000’s interlocks, the committee was unable to identify any circumstances where an accidental failure or a deliberate reconfiguration of the AIT system could result in either a person being screened or the operator receiving an effective dose larger than that from a normal screening.

Because there is no failure mechanism that would give more than a normal screening dose, as stated in the key finding, scanning time is the only factor left. The AIT system would have to operate for more than 16 hours to exceed the dose limit of 250,000 nSv as stated in Table 7.10 (row 7.2.1 m); it is unreasonable to expect that a person being scanned would be exposed for that amount of time.

In addition, it is worth mentioning that the AIT system inspected at NIST cannot be reconfigured remotely because it is not connected, nor can it be connected, to the Internet or by modem to any other device. Reconfiguration can only be done from the AIT system itself.

SUMMARY OF KEY FINDINGS FOR BOTH MEASURED SYSTEMS

The key parameters for the Rapiscan Secure 1000 and the AS&E SmartCheck systems to be used in the dose computations are summarized in Table 7.11.

TABLE 7.11 Summary Measurement Results for the Rapiscan Secure 1000 and the AS&E SmartCheck

HVL1
(mm Al)
HVL2
(mm Al)
50 PDD
(mm)
Air Kerma
(nGy)
EREF
(nSv)
Rapiscan anterior 0.92 1.47 ~11 30.6 3.5
Rapiscan posterior 0.85 1.42 ~11 29.8 3.2
AS&E 1.1 1.7 ~12.5 113 15.5

NOTE: EREF, reference effective dose; HVL1, first half-value layer; HVL2, second half-value layer; PDD, percent depth dose.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

DOSE COMPUTATIONS

Objectives

The charge to the committee included the task of determining if radiation exposures received during X-ray backscatter AIT screening were in compliance with applicable health and safety standards for the general public and occupationally exposed individuals. It is clear from previous reports (reviewed in Chapter 6) and the measurements that the NRC subcontractor made (see earlier in this chapter), together with the computations below, that the AIT systems evaluated meet the ANSI requirement for EREF below 250 nSv per screening. However, it has been speculated in recent years that it might be possible for some individuals to receive an effective dose, E, during a screening that exceeds the EREF by a large enough margin to result in exposures exceeding ANSI recommendations. Therefore, one of the efforts with this independent study was to go beyond previous evaluations (see studies described in Chapter 6) to include an assessment of absorbed dose and effective dose for adults, children, and pregnant women under routine screening conditions as well as doses that might be received as a result of serious malfunctions.

Introduction

The NRC subcontractor Wesley Bolch, Advanced Laboratory for Radiation Dosimetry Studies, J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, applied state-of-the-art computational techniques to assess the organ-absorbed dose and whole-body effective dose received by adults, children, and the developing fetus of pregnant females scanned by a computationally modeled X-ray backscatter AIT system and by variations on that system that include some current and anticipated engineering designs. Passengers were simulated using a suite of hybrid digitized phantoms. The computational phantoms used in this study were obtained from the University of Florida computational hybrid phantom library.20,21 The phantoms are constructed as a collection of mathematical surfaces based in NURBS (non-uniform rational basis spline) that define the shapes and locations of individual internal organs as well as the body surface contour. From this library, each phantom was adjusted using targeted values for sitting height (i.e., based on the length of the torso, neck, and head) and four body circumferences:

_______________

20 A.M. Geyer, S. O’Reilly, C. Lee, D.J. Long, and W.E. Bolch, The UF/NCI family of hybrid computational phantoms representing the current US population of male and female children, adolescents, and adults—Applications to CT dosimetry, Physics in Medicine and Biology 59(18):5225-5242, 2014.

21 M.R. Maynard, N.S. Long, N.S. Moawad, R.Y. Shifrin, A. Geyer, G. Fong, and W.E. Bolch, The UF family of hybrid phantoms of the pregnant female for computational radiation dosimetry, Physics in Medicine and Biology 59:4325-4343, 2014.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

waist, buttocks, arm, and thigh. Human morphometric data were obtained from the National Health and Nutrition Examination Survey database, from data collected between 1999 and 2006. The phantom library covers the 5th to 95th height and weight percentiles for children and adolescents (age 2 to 20) and adults (age 20 to 85) in the United States.

This family of hybrid computational phantoms was based on a set of reference phantoms representing ages 1, 5, 10, and 15 and adult males and females. These reference phantoms have heights, weights, and masses for internal organs described in International Commission on Radiological Protection (ICRP) Publication 89.22 Scaling was then performed to either increase or decrease the torso height together with the volumes, and hence the mass, of all internal organs. The final stage was to add or subtract subcutaneous fat to create phantoms of different weights, thus forming an array of phantoms of differing height and weight combinations. The assigned body mass index (BMI) for each phantom is not unique because a number of different combinations of height/weight can yield an equivalent value of BMI. This process for phantom library creation does not include the possibility that a given phantom will have its assigned weight based on increases or decreases in skeletal muscle or lean body mass. The implicit assumption is that weight changes at a given phantom height are dictated by proportional changes in subcutaneous fat. It does not imply that these phantoms represent the only or true morphology for that value of BMI for all airline passengers.

These computations were not designed to estimate radiation exposure to specific individuals. The methodology was developed to provide estimates of radiation doses received by a population of passengers with a range of morphologies during routine screening. This was complimented with a series of sensitivity studies to investigate additional variations. The combination of these approaches provides information on the global uncertainties inherent in the computational approach.

The committee chose to use a generic mathematical model (or reference model) of an X-ray backscatter AIT system rather than an exact model of an existing system. The idea is that with such a model system it is possible to compare how differences in design and settings affect dose for not only previous systems but also potential future systems. Thus, a Monte Carlo simulation model of a reference X-ray backscatter AIT system was implemented for the computations. This reference system was based on a compilation of information assembled from the following sources:

_______________

22 International Commission on Radiological Protection, Basic Anatomical and Physiological Data for Use in Radiological Protection Reference Values, ICRP Publication 89, Annals of the ICRP 32(3-4), 2002.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
  1. Previous reports documenting approximate scanning geometry, including vertical transport of the X-ray source, beam size and divergence, and polar angle of the beam as a function of elevation;
  2. Previous reports documenting information on beam intensity (i.e., air kerma) as a function of vertical and horizontal position of the X-ray beam;
  3. Data acquired by the committee from measurements of HVL in aluminum;
  4. Data acquired by the committee from measurements of PDD in tissue-equivalent material; and
  5. The ability to have just an anterior or both an anterior and a posterior system.

The simulated X-ray backscatter AIT system is a reference system composed of information available to the committee at the time it conducted its investigation. The nature of this Monte Carlo approach is such that the results can be scaled to any system with similar geometry and photon energy distribution by multiplying the ratio of the air kerma from any other AIT system to the air kerma of the AIT reference system.

Three populations of air passengers were simulated as follows:

  1. Reference adult males and females at 50th percentile height and at five different weight percentiles;
  2. Children (male and female) at ~105 cm height and at three different weight percentiles; and
  3. The developing fetus of pregnant females at three periods of gestation.

In addition to routine or standardized scanning operations, the following sensitivity studies were performed to evaluate variations that might affect compliance with the radiation protection standards:

  1. The effect of variations in the irradiation geometry of the reference system and the presumption of a simplified broad parallel beam of the same energy distribution23 on organ and effective dose;
  2. The effect of variations in organ and effective dose due to changes in the horizontal location of the screened passenger between the anterior and posterior units;

_______________

23 This study was conducted to enable evaluation of and comparisons to AIT systems that may not include angular variations in the vertical and horizontal scan, which is created by tilting the X-ray source as it moves from the bottom to the top of the scanning region and fixed horizontal position of the tube anode. The study also intends to do comparisons with previous computations that made the parallel beam assumption due to modeling limitations of their Monte Carlo approach.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
  1. Assessment of both organ and effective dose as a function of X-ray tube potential greater than the nominal 50 kV;
  2. Assessment of the radiation absorbed dose averaged across the entire skin of an adult passenger (both dermis and epidermis) during one screen as compared to the associated radiation dose to the radiosensitive region of skin at a depth of 50 to 100 µm from the skin surface, also averaged across the total body; and
  3. Assessment of the radiation absorbed dose per screen to the adult female breast and lens of the eye and a central region of the skin during a presumed worst-case malfunction where the horizontal and vertical sweep remained stationary for the same duration as that of a normal scan.

X-Ray Source Term

Features of the Reference AIT System

To develop the mathematical model of the reference AIT system, some features reported by NIST were adopted, including (1) the relative geometrical configurations of the anterior and posterior scanning units and their X-ray tubes; (2) the relative values of horizontal and vertical air kerma within the scanning region; and (3) the measured air kerma per scan at a reference point (used to normalize the Monte Carlo organ doses). The geometry of the scanning units is discussed in a later section. Items 2 and 3 were incorporated directly into the construction of the X-ray source term used to irradiate the computational phantoms during virtual screening simulations.

The horizontal (parallel to AIT system face) and vertical exposure maps, shown in Figures 7.9 and 7.10, provided an approximate measure of the relative photon fluence at a plane located a fixed distance from the anterior and posterior units. The figures are normalized to 1.0 at the location in the two-dimensional plane having the maximum value of air kerma (i.e., at a vertical distance of 192 cm from the mat). These distributions of relative exposure were implemented for the reference AIT system and formed the basis for particle sampling functions used in the virtual X-ray source term for both the anterior and the posterior unit.

The results of the Monte Carlo process are initially normalized to the absorbed dose per incident photon.24 The objective is to obtain the effective dose per scan, which in turn requires the number of incident photons per scan. This “normaliza-

_______________

24 Because Monte Carlo calculations follow the histories of individual photons and average the results over large numbers of photons, the result is the energy deposited (or absorbed dose) per photon. The absorbed dose at a specified location resulting from exposure to a specific number of photons is the sum of the absorbed doses at that specified location produced by each of the protons.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.9 Horizontal exposure variation of the posterior unit measured at a height of 100 cm from the mat at a distance of 30 cm from the front plane of the AIT system (red, upward scan; black, downward scan). The maximum exposure from the posterior unit was measured at a vertical position of 192 cm and a horizontal position of 0 cm. SOURCE: Figure 10 from NIST Report to DHS, Assessment of the Rapiscan Secure 1000 Single Pose (ATR version) for Conformance with National Radiological Safety Standards, Jack L. Glover, Ronaldo Minniti, Lawrence T. Hudson, and Nicholas Paulter, National Institute of Standards and Technology, Gaithersburg, Md., final report related to IAA No. HSHQDC-11-X-00585, April 19, 2012.

images

FIGURE 7.10 Vertical exposure variation of the posterior unit along the central axis of the person being scanned at a distance of 30 cm from the front plane of the AIT system. The greatest exposure is seen toward the top of the scan. The downward scan (black) shows a maximum at a height of 192 cm. The upward scan (red) shows its maximum at a height of 188 cm. SOURCE: Figure 11 from NIST Report to DHS, Assessment of the Rapiscan Secure 1000 Single Pose (ATR version) for Conformance with National Radiological Safety Standards, Jack L. Glover, Ronaldo Minniti, Lawrence T. Hudson, and Nicholas Paulter, National Institute of Standards and Technology, Gaithersburg, Md., final report related to IAA No. HSHQDC11-X-00585, April 19, 2012.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

tion factor” is obtained from the ratio of measured air kerma per scan to the Monte Carlo-simulated value of air kerma per photon. This yields the number of photons per scan. The results presented in the following sections are normalized to a value of 9.203 × 108 photons per scan, which corresponds to an air kerma of 68 nGy/scan representing a simulated scanning protocol for the reference AIT system. All results relating to organ absorbed dose and effective doses received during passenger screening will scale linearly with the value of air kerma.

Photon Energy Distribution Using HVL and PDD

The energy spectrum of the incident photons is a major component of the input to the Monte Carlo calculations. The MATLAB-based code SPEKTR,25 based on the methods presented by Turner et al.,26 was used to generate a candidate spectrum. It generates a candidate “soft” clinical X-ray spectrum based on entering the high (peak) voltage, in kilovolts (kV), and high-voltage ripple.27 Typically, the user then “hardens” the candidate spectrum by calculating the effect of filtering the spectrum with a selected absorbing material (e.g., aluminum) until the resulting HVLs of this modified spectrum match the measured values. This process yields the representative spectrum emerging from that X-ray tube and inherent filtration.

The first available data from the NRC subcontractor yielded an HVL (HVL1) of 1.18 mm Al obtained from measurements of an AS&E SmartCheck AIT system. However, the candidate spectrum using 50 kV generated by SPEKTR had an HVL1 value of approximately 1.45 mm Al. The candidate spectrum was therefore “softened” by iteratively adding low-energy photons using an inverse process of exponential attenuation as a function of photon energy. The eventual result was a suitably softened spectrum with an HVL1 matching that initially measured for the AS&E SmartCheck. Figure 7.11 visually compares the original candidate spectrum and the softened spectrum matching the AS&E SmartCheck HVL1 values initially measured.28

_______________

25 J.H. Siewerdsen, A.M. Waese, D.J. Moseley, S. Richard, and D.A. Jaffray, Spektr: A computational tool for x-ray spectral analysis and imaging system optimization, Medical Physics 31:3057-3067, 2004.

26 A.C. Turner, D. Zhang, H.J. Kim, J.J. DeMarco, C.H. Cagnon, E. Angel, D.D. Cody, D.M. Stevens, A.N. Primak, C.H. McCollough, and M.F. McNitt-Gray, A method to generate equivalent energy spectra and filtration models based on measurement for multidetector CT Monte Carlo dosimetry simulations, Medical Physics 36:2154-64, 2009.

27 Power supplies of today have very little ripple, and thus the voltage is close to constant and the peak value (Vp) is the same as the average value; in this case, it is enough to report the voltage (V) instead of Vp and the ripple.

28 The initial measurements were used for designing the generic computation model for dose calculations; later, the measurements were refined and are reflected in the reported values of Table 7.11.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.11 Comparison of the initial calculated X-ray spectrum at 50 kV yielding a first half-value layer (HVL1) of 1.45 mm of aluminum (mm Al) and the revised softened spectrum that yields an HVL1 of 1.18 mm Al. SOURCE: Tom Borak.

Energy Sampling

The softened energy spectrum shown in Figure 7.11 was incorporated into the computational X-ray source term. Standard Monte Carlo methods based on random numbers were used to sample photon energies for the emerging scanning beam with probabilities represented by this distribution.

Simulating the Scanning X-Ray Beam Spot Incident Upon the Passenger

The objectives of this process are both to reproduce the intensity of the X-ray beam at locations in the y-z reference plane and to include the direction or vector of the beam as it intercepts the passenger. This depends on the tilt angle of the beam during vertical translation and on the lateral angle of the beam spot as it scans horizontally.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Vertical and Horizontal Intensities of Scanning Beam

The basis for sampling the origin and direction of the photons used for the X-ray source term of the reference AIT system are given in Figures 7.9 and 7.10. The software PlotReaderTM was used to manually capture data points from each figure. NIST reported relative exposures in the reference plane for both upward and downward scanning motions. Upward and downward exposure data were averaged at each point for each figure. The horizontal data (Figure 7.9) were then fitted piece-wise to two Gaussian functions and then normalized. The vertical data (Figure 7.10) were digitized into 1 cm bins using linear interpolation in MATLABTM and then normalized. The horizontal and vertical data sets now represented the relative distribution of photons (as a function of y and z) that should be expected at a reference plane located 30 cm from and parallel to the front plane of a scanning unit. These relative distributions were assumed for both the anterior and the posterior unit. An implicit assumption in this work is that this X-ray energy distribution was constant across all horizontal angles of the beam sweep.

Photon Starting Position and Direction

Randomly sampling the two probability distributions representing Figures 7.9 and 7.10 described above provides the location of a photon incident upon a reference plane located 30 cm from the front of a scanning unit. The starting position of the photon was determined by linearly mapping (ray tracing29) back toward the geometric location of the X-ray source in the reference AIT system, accounting for additional specifications such as vertical translation and tilt angle of the source associated with that vertical position. The ray trace provides a unit vector corresponding to the direction of the emitted photon.

Normalization to Air Kerma

All results provided by the MCNPX code were normalized to the number of starting source photons. The number of starting source photons must be scaled by the total number of X-ray photons per scan to yield doses in absolute units. The user must, therefore, post-process results with some physical quantity (or quantities) to provide an anchoring between simulated and physical dose quantities. All results from a scan simulation obtained in this manner must be multiplied by a “normalization factor” to convert, for example, from dose-to-passenger per starting photon to dose-to-passenger per scan. Once the X-ray source term was constructed

_______________

29Ray tracing is a technique for generating an image by tracing the path of light (or X rays) through the pixels in an image plane and simulating the effects of its encounters with virtual objects such as, in this case, the human phantom or parts of the X-ray backscatter AIT system.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

and compiled, the normalization factor was estimated by simulating a virtual air kerma measurement analogous to that made with a physical ion chamber in an earlier report (68 nGy per scan at a known location).30 A sphere of air with a radius and location equivalent to the Radcal Corporation ion chamber used by NIST was modeled in MCNPX and irradiated using the simulated X-ray source term. The NIST air kerma measurement was divided by the resulting virtual air kerma (nGy per starting photon), yielding an estimate of the total number of photons emitted per scan (9.203 × 108 photons per scan in this study). It was not necessary to model the complex geometry of the NIST ionization chamber because the chamber was calibrated by NIST to provide an air kerma measurement to a similarly sized volume of air as was modeled in MCNPX.

Validation of Monte Carlo Sampling Procedures

The X-ray source term was validated in three ways: (1) by confirming that the simulated virtual scans were generating the desired photon energy fluence and thus the correct HVL; (2) by comparing simulated and analytical normalization factors; and (3) by comparing simulated PDD with measured PDD.

The virtual fluence validation was achieved using the reference plane located 30 cm away from and parallel to the front plane defining the scanning region of the AIT system. The surface was divided into 1-cm bins along the horizontal and vertical directions. Virtual scans were performed using the X-ray source term, and the photon fluence in each of the surface bins was quantified and plotted. As expected, the photon fluence as a function of horizontal and vertical distance (Figure 7.12) mirrors the composite of Figures 7.9 and 7.10, indicating that the distribution of photons produced by the simulated reference AIT system is very similar to that produced by the Rapiscan Secure 1000 AIT system tested at NIST.

The normalization factor derived from MCNPX (9.203 × 108 photons per scan) was validated via comparison to a calculated normalization factor. Using the HVL1matched (1.18 mm Al) 50 kV spectrum, mass energy-absorption coefficients for air (a standard NIST table31) were used to calculate the number of photons required to deliver 68 nGy to a volume of air equal in size to the detector used for air kerma measurements. Additionally, using MCNPX, the fluence into that volume of air

_______________

30 The value of 68 nGy was selected based on an earlier measurement made by NIST because measurements by the NRC subcontractor of air kerma per scan were not available when it was necessary to begin the Monte Carlo computations. All results relating to organ absorbed dose and effective doses received during passenger screening will scale linearly with the value of air kerma. NIST has since updated its report, and, therefore, 68 nGy is considered a generic value.

31 P.J. Lamperti and M. O’Brien, NIST Measurement Services, Calibration of X-Ray and Gamma-Ray Measuring Instruments, NIST Special Publication 250-58, April 2001, http://www.nist.gov/calibrations/upload/sp250-58.pdf.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 7.12 Composite horizontal and vertical relative photon fluence at plane 30 cm from front plane of a scanning unit as simulated in MCNPX. SOURCE: Tom Borak.

from one complete scan was calculated to make the value relative to one scan. A calculated value (8.00 × 108 photons per scan) based on a first collision approximation was within 15 percent of the normalization factor derived from MCNPX.

The validation using PDD was achieved by virtually simulating the pertinent irradiation geometry adopted by the NRC subcontractor during the physical measurements. The beam was modeled simulating a full horizontal scan with a 3 mm vertical collimation. The virtual simulations incorporated a soft tissue phantom with density and elemental compositions comparable to the physical phantom material used in the physical measurements. The detailed geometry of the parallel-plate ionization chamber did not need to be modeled in the virtual simulations because the software was able to estimate tissue absorbed dose directly. Simulated doses were quantified as a function of depth in the virtual tissue phantom and used to generate a PDD curve. The simulated PDD is compared to the measured PDD in Figure 7.13.

Description of the Passenger Irradiation Geometry

A visual representation of the irradiation geometry used for the reference AIT system is provided in Figure 7.14. The NIST coordinate system was adopted (Figure 7.1), where the x-axis is the direction parallel to the faces of the scanning units, the y-axis is the direction orthogonal to the faces of the scanning units, and the z-axis is the vertical direction. The distances between the AIT system’s anterior and posterior surfaces (109 cm) and planes (87 cm) were obtained from the NIST report.32 The distance from the anterior plane to the source (48 cm) was also ob-

_______________

32 Glover et al., Assessment of the Rapiscan Secure 1000 Single Pose (ATR version) for Conformance with National Radiological Safety Standards, 2012.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 7.13 Comparisons between measured and percent depth dose (PDD) in tissue equivalent material. Blue circles are results from MCNP radiation transport simulation, and orange triangles are measurements by the NRC subcontractor on the AS&E SmartCheck system. Results are shown with (a) linear and (b) logarithmic depth scale. SOURCE: Tom Borak.

tained from the same NIST report. A reference point of 30 cm from the anterior plane was used in the irradiation geometry. The horizontal (x-axis) scan width was approximately 90 cm at this point, according to measurements detailed in the NIST report discussed in this section. Based on visual inspection of a Rapiscan Secure 1000 unit at TSIF, the source vertically traverses (z-axis) approximately 118 cm in one scan and reaches a maximum height of approximately 152 cm. The rotational angle of the source at its most vertical point (+45 degrees) was obtained from the original patent application. This configuration yields a maximum vertical scan height of approximately 230 cm. The rotational angle of the source at its lowest vertical point was estimated to be −26 degrees.

With the exception of the passenger-positioning-sensitivity calculations done

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 7.14 Dimensional data used to establish the irradiation geometry for the NRC reference X-ray backscatter AIT system used in the Monte Carlo radiation transport simulations. SOURCE: Thomas B. Borak and Wesley E. Bolch, University of Florida.

by the NRC subcontractor, all phantoms were centered as follows: (1) parallel to the faces of the AIT systems (x-axis), facing the anterior unit, and (2) between the anterior and posterior units (center of phantom at a distance of 43.5 cm from the front plane of the AIT system). All phantoms were arranged in a representative scanning position with arms raised (Figure 7.15). It is noted that because the virtual passengers were centered based on their anatomical extent, the locations of the feet were slightly different as the subject’s body weight increased among the model series.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.15 Graphical images of the computational hybrid phantom, an adult 50th percentile male, with arms raised and positioned within the scanning location between the anterior and posterior units. SOURCE: Wesley E. Bolch, University of Florida.

Adult Reference Phantoms

The adult males and females representing the 5th, 25th, 50th, 75th, and 95th percentiles for weight at their respective 50th percentile for height were utilized. Lateral and anterior-posterior views of the adult male and female phantoms that were used in these simulations are shown in Figures 7.16 and 7.17, respectively. The colored features provide visual distinction of tissues and organs. Gray regions represent variations in additional fat and surface skin. The figures do not include pixilation that quantitatively differentiates skin and subcutaneous fat.

Pediatric Phantoms

Pediatric male and female phantoms were chosen at a height of 105 cm and corresponding to 5th, 50th, and 95th percentiles for weight. The 105 cm height corresponds to an approximate age of a 4.5-year-old child. This height was meant to correspond to the shortest passengers that might be screened. Lateral and anterior-posterior views of the pediatric male and female phantoms that were used for these simulations are shown in Figures 7.18 and 7.19, respectively.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.16 Left lateral (top) and anterior-posterior (bottom) views of the adult male phantoms used for the passenger screening simulations set at 50th percentile for height and varied by body mass index: (A) 5th percentile, (B) 25th percentile, (C) 50th percentile, (D) 75th percentile, and (E) 95th percentile. SOURCE: Wesley E. Bolch, University of Florida.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.17 Lateral (top) and anterior-posterior (bottom) views of the adult female phantoms used for the passenger screening simulations set at 50th percentile for height and varied by body mass index: (A) 5th percentile, (B) 25th percentile, (C) 50th percentile, (D) 75th percentile, and (E) 95th percentile. SOURCE: Wesley E. Bolch, University of Florida.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.18 Left lateral and anterior-posterior views of the pediatric male phantoms used for screening simulations set at 105-cm height and varied by body mass index: (A) 5th percentile, (B) 50th percentile, and (C) 95th percentile. SOURCE: Wesley E. Bolch, University of Florida.

images

FIGURE 7.19 Left lateral and anterior-posterior views of the pediatric female phantoms used for screening simulations set at 105-cm height and varied by body mass index: (A) 5th percentile, (B) 50th percentile, and (C) 95th percentile. SOURCE: Wesley E. Bolch, University of Florida.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Pregnant Female Phantoms

The pregnant female phantom series provides highly detailed anatomical representation for eight fetal ages spanning an entire pregnancy. Three of these phantoms, representing fetal ages (post-conception) of 15 weeks, 25 weeks, and 38 weeks (Figures 7.20 to 7.22), were virtually screened in MCNPX to estimate fetal doses resulting from the simulated X-ray backscatter AIT system irradiation.

Dosimetry Results for Standard Screening Conditions

All phantom simulations presented in this report were performed in MCNPX v2.7 using the custom X-ray source term described earlier and the passenger positioning described above in Figure 7.14. All simulations were performed on the University of Florida HiPerGator supercomputer using particle history specifications that yielded sufficiently low relative errors (majority <1 percent). Anterior

images

FIGURE 7.20 The 15-week pregnant female phantom: a frontal view (A) and magnified right-oblique view (B). SOURCE: Wesley E. Bolch, University of Florida.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 7.21 The 25-week pregnant female phantom: a frontal view (A) and magnified right-oblique view (B). SOURCE: Wesley E. Bolch, University of Florida.

images

FIGURE 7.22 The 38-week pregnant female phantom: a frontal view (A) and magnified right-oblique view (B). SOURCE: Wesley E. Bolch, University of Florida.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 7.23 Diagram showing how the adult male and female phantom data are combined to obtain sex-averaged effective dose results for adult passengers. NOTE: M refers to male, F refers to female, and T refers to a specific tissue or organ. SOURCE: Tom Borak.

and posterior scans were simulated independently and mathematically summed to provide results for a total passenger protocol.

The effective dose is determined by applying tissue-weighting factors to all exposed tissues and organs. However, these weighting factors represent mean values for humans, averaged over both sexes and all ages. In order to facilitate comparisons of these results with the ANSI limit of reference effective dose, the committee combined the results of the male and female phantoms using the process outlined in ICRP Publication 10333 (see Figure 7.23).

Results of the dosimetry calculations for adult passengers are shown in Table 7.12 (and for pediatric passengers in Table 7.13 and for the developing fetus in Table 7.14). The absorbed dose received per screen to pertinent organs is shown separately for males and females. Complete lists of absorbed doses for all male and female organs are provided in Appendix C. The effective dose is the value obtained by averaging the results from both male and female phantoms.

Sensitivity Analysis

Several sensitivity analyses were conducted to explore possible dose variation under different screening scenarios. These analyses include irradiation geometry, passenger position, skin sensitivity, failure modes, and X-ray energy and spectral shape.

_______________

33 ICRP Publication 103, 2007.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.12 Summary per AIT Screening of Critical Organ Absorbed Dose and Effective Dose to Adult Passengers of 50th Height Percentile at Five Different Weight Percentiles

Weight Percentiles, U.S. Adults
5th 25th 50th 75th 95th
Male absorbed dose per screen (nGy)
Thyroid 31 27 24 24 16
Skin 44 43 43 42 42
Eye Lens 44 44 42 42 39
Female absorbed dose per screen (nGy)
Breast 26 23 23 20 18
Thyroid 22 21 17 11 4
Skin 46 45 44 45 46
Eye Lens 46 44 43 37 32
Effective Dose (nSv)
Per anterior scan 12 10 9 7 4
Per posterior scan 3 3 3 2 2
Per screen 15 13 12 9 6

NOTE: For comparison, the ANSI reference effective dose for an HVL1 of 1.18 mm Al and an air kerma of 68 nGy per scan is 20 nSv.

TABLE 7.13 Summary per AIT Screening of Critical Organ Absorbed Dose and Effective Dose to Pediatric Passengers of ~105 cm in Total Height and Three Different Weight Percentiles

Weight Percentiles, U.S. Children
5th 50th 95th
Male absorbed dose per screen (nGy)
Thyroid 47 45 47
Skin 49 49 48
Eye Lens 60 60 60
Female absorbed dose per screen (nGy)
Breast 43 39 32
Thyroid 47 44 48
Skin 49 46 48
Eye Lens 60 54 60
Effective Dose (nSv)
Per anterior scan 20 18 16
Per posterior scan 6 5 5
Per screen 25 23 22

NOTE: For comparison, the ANSI reference effective dose for an HVL1 of 1.18 mm Al and an air kerma of 68 nGy per scan is 20 nSv.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.14 Absorbed Doses per AIT Screening to the Fetus and Four Fetal Organs, Incurred during Anterior Scan, Posterior Scan, and Total Screen for the U.S. Adult Pregnant Female at 15 Weeks, 25 Weeks, and 38 Weeks Post-Conception

Absorbed Dose (nGy)
15 Weeks Post-Conception 25 Weeks Post-Conception 38 Weeks Post-Conception
Anterior Posterior Screen Anterior Posterior Screen Anterior Posterior Screen
Whole body 7.2 1.3 8.5 3.4 0.6 4.0 3.4 0.9 4.3
Brain 3.7 2.3 6.0 0.8 1.4 2.2 0.5 2.7 3.2
Lungs 8.3 0.9 9.2 5.1 0.2 5.3 2.6 0.2 2.7
Thyroid 5.8 1.4 7.2 2.5 0.4 2.9 1.3 0.3 1.6
Active bone marrow 14 2.5 16 6.4 1.8 8.1 8.4 2.7 11
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Standard Geometry versus Plane Parallel Beams

The effect of beam geometry on passenger dose was investigated in this analysis. Two beam geometries were simulated: a broad, uniform parallel beam, and the geometry of the NRC reference AIT system described above. The uniform parallel beam was modeled using the same X-ray spectrum (Figure 7.11) and air kerma (68 nGy per scan) described earlier. However, the horizontal exposure profile in Figure 7.9 as well as the vertical exposure profile in Figure 7.10 were replaced with uniform distributions. Thus, the two-dimensional beam profile for the reference AIT system in Figure 7.12 is represented as a flat surface. The adult male and adult female phantoms (50th percentile height and weight) were scanned using each geometry, and the resulting doses are summarized in Table 7.15.

Passenger Position Within the Unit

Variations in the dose to organs and effective dose received by the passenger due to position variations between the anterior and posterior units was investigated by placing the adult male phantom closer and further from each scanning unit

TABLE 7.15 Absorbed Doses to Critical Organs for Males and Females and Effective Doses for 50th Percentile of U.S. Adults Using the Beam Geometry Developed for the NRC Reference X-Ray Backscatter AIT System and a Plane Parallel Beam Geometry Incident Upon the Passengers

Geometry
Reference AIT Scanner Uniform Parallel Beam Ratio
Male absorbed dose per screen (nGy)
Thyroid 24 49 2.0
Skin 43 78 1.8
Eye Lens 42 65 1.6
Female absorbed dose per screen (nGy)
Breast 23 36 1.6
Thyroid 17 40 2.3
Skin 44 80 1.8
Eye Lens 43 68 1.6
Effective Dose (nSv)
Per anterior scan 9 15 1.7
Per posterior scan 3 4 1.5
Per screen 12 19 1.7

NOTE: The third column shows the ratio between these two geometries. For comparison, the ANSI reference effective dose for an HVL1 of 1.18 mm Al and an air kerma of 68 nGy per screen is 20 nSv.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.16 Changes in Organ Absorbed Dose and Effective Dose per Screen with Shifts Toward the Anterior Unit or Toward the Posterior Unit

Distance Away from Center Scanning Position (cm)
Toward Anterior Unit Toward the Posterior
15 10 5 0a 5 10 15
Male absorbed dose per screen (nGy)
Thyroid 30 28 26 24 22 21 20
Skin 43 43 42 43 43 44 45
Eye Lens 53 49 45 42 38 35 33
Female absorbed dose per screen (nGy)
Breast 30 27 25 23 21 20 18
Thyroid 20 19 18 17 17 16 15
Skin 45 44 44 44 45 46 47
Eye Lens 57 51 46 43 40 36 33
Effective dose (nSv)
Per anterior scan 12 11 10 9 8 8 7
Per posterior scan 2 2 2 3 3 3 3
Per screen 14 13 12 12 11 11 11
Ratio (position to nominal) 1.2 1.1 1.0 1.0 1.0 0.9 0.9

a The zero value refers to the nominal and central scanning position between an anterior and a posterior unit.

NOTE: For comparison, the ANSI reference effective dose for an HVL1 of 1.18 mm Al and an air kerma of 68 nGy per screen is 20 nSv.

in increments of 5 cm. The total translational distance was 30 cm (±15 cm from center). The resulting doses are summarized in Table 7.16.

Variations in X-Ray Tube Voltage

A sensitivity analysis was performed to investigate the effects of varying beam voltage on doses to the adult phantom. Doses resulting from the beam spectrum generated for this report (the beam spectrum used was matched to the HVL1 measured by the NRC subcontractor) were compared to doses resulting from additional spectra as-generated from the SPEKTR code34 (i.e., the HVL1s of these spectra were not modified beyond their inherent values). Each spectrum was incorporated into the AIT system X-ray source term and utilized to virtually screen the adult male and female phantom under the same irradiation geometry described earlier. Table 7.17 provides a summary of the resulting organ absorbed doses and effective dose received by the passenger.

_______________

34 J.H. Siewerdsen, A.M. Waese, D.J. Moseley, S. Richard, and D.A. Jaffray, Spektr: A computational tool for x-ray spectral analysis and imaging system optimization, Medical Physics 31:3057-3067, 2004.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.17 Comparison of Reference X-Ray Backscatter AIT System of the Organ Absorbed Dose and Effective Dose as a Function of X-Ray Tube Potential

Absorbed Dose
X-Ray Tube Potential (kV) 50a 50 60 70 80 90 100
Half-value layer (HVL1) 1.18 1.45 1.74 2.02 2.30 2.58 2.86
Male absorbed dose per screen (nGy)
Thyroid 24 27 33 37 41 45 48
Skin 43 45 47 50 52 54 55
Eye Lens 42 44 45 47 48 50 51
Female absorbed dose per screen (nGy)
Breast 23 26 29 32 35 37 39
Thyroid 17 22 26 31 35 39 42
Skin 44 46 49 52 54 56 57
Eye Lens 43 45 47 49 50 52 53
Effective dose (nSv)
Per anterior scan 9 11 13 15 17 19 21
Per posterior scan 3 3 5 6 8 10 11
Per screen 12 14 18 22 25 29 32
Ratio (to column 1) 1.0 1.2 1.4 1.7 1.9 2.2 2.3
ANSI Reference Effective Dose
20 25 30 34 39 44 49

a Values in the first data column are results from the PDD validated and softened X-ray spectrum at the operational value of 50 kV. All other values are from TASMIP-generated spectra (without HVL1 matching) and assuming an equivalent reference air kerma of 68 nGy per scan. http://www.ncbi.nlm.nih.gov/pubmed/9394272.

Dose to Radiosensitive Cells in the Skin

The skin is divided into two main regions: the dermis and the epidermis. The biological response in the epidermal region occurs soon after an exposure to ionizing radiation whereas the biological response in the dermal region occurs after a latent period following exposure to ionizing radiation.35 The biological targets are basal cells in the epidermis and the fibroblasts/vascular endothelial cells in the dermis. Irradiation of the basal cell layer can lead to desquamation, while irradiation of the fibroblasts and vascular endothelial cells can lead to erythema (skin reddening). The ICRP reference value for the thickness of the epidermal layer is 70 µm for adults.36

_______________

35 E.J. Hall and A.J. Giaccia, Radiobiology for the Radiologist, Lippincott Williams and Wilkins, Philadelphia, Pa., 2006.

36 ICRP, ICRP Publication 89: Basic Anatomical and Physiological Data for Use in Radiological Protection: Reference Values, Annals of the ICRP 32, 2002.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

A sensitivity analysis was performed to compare the radiation dose to the entire skin thickness with the dose to just the radiosensitive epidermal skin layer. To conduct this analysis, a stylized elliptical cylinder torso model was created to represent the 50th percentile adult male using three anterior-posterior and three lateral measurement averages (representing the upper, middle, and lower regions of the trunk). The torso model was composed of soft tissue covered by a 0.158 cm skin layer (corresponding to the assumed standard ICRP skin thickness of an adult male).

The model was centered in the AIT system and doses to the 0.158 cm skin depth region and the dose to the sensitive area (50 to 100 µm) skin depths were simulated. The ratio of the dose in the sensitive region to the total skin dose was calculated. Because this ratio is near unity, the dose to the entire skin thickness was considered a reasonable surrogate for dose to the sensitive layer. Using this ratio, the dose to the sensitive skin layer of the adult male hybrid phantom was estimated from the whole skin dose presented earlier. Table 7.18 summarizes the results.

Failure Mode Analysis

Failure mode analysis of two conditions of equipment failure was performed on the adult female phantom in order to examine possible maximum doses to the lens of the eye, breast tissue, and skin. The first failure mode assumed a stationary vertical beam position and a functional chopper wheel resulting in a horizontally broad and vertically narrow (~3 mm) beam. The second failure mode assumed a stationary vertical beam position and a nonfunctional chopper wheel resulting in a stationary pencil beam. A circular beam cross section was assumed for the second failure mode with an area equivalent to a 3 mm × 3 mm square beam, the approximate dimensions of the properly collimated scanning beam under normal

TABLE 7.18 Comparison of the Absorbed Dose per Scan to Either the Total Skin Volume (Dermis and Epidermis) and the Presumed Radiosensitive Epidermal Stem Cell Layer (50 to 100 µm) in a Cylindrical Stylized Torso Phantom

Absorbed Dose per Screen (nGy)
Stylized Phantom Hybrid Phantom
Target - Total skin 37.3 43.5a
Target - Radiosensitive layer 37.9 44.2b
Ratio (total skin/radiosensitive layer) 1.02

a Sex-averaged dose with adult hybrid phantoms.

b Hybrid phantom total skin dose × 1.02.

NOTE: The simulations showed only 2 percent increase in dose. Corresponding doses to skin in the 50th percentile adult male phantom are shown as well.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 7.19 Reference Geometry AIT Absorbed Doses Under Conditions of a Standard Screening Compared to Those Incurred Under Conditions of Maximum Exposure due to Two Different Modes of Equipment Failure

Tissue Normal Screen Failure Mode 1a Failure Mode 2b Tissue Reaction Threshold
(nGy) (nGy) (nGy) (nGy) (Gy)
Lens 43 29,000 1,100,000 500,000,000 0.5
Skin 44 26,000 870,000 2,000,000,000 2
Breast 23 310 7,400

a Failure Mode 1: Beam fixed vertically but not horizontally (chopper wheel operational).

b Failure Mode 2: Beam fixed vertically and horizontally (chopper wheel not operational).

operation. The beams of both failure modes were oriented toward the targets of interest and were assumed to emit the same number of photons produced under normal operation for a full body scan. Table 7.19 summarizes the approximate doses to the lens, breast, and skin under these failure modes. For the second failure mode, peak skin dose was calculated assuming a 1 × 1 cm2 area on the respective skin region. The maximal skin and lens doses (failure mode 2) of 0.87 mGy (870 µGy or 870,000 nGy) and 1.1 mGy (1,100 µGy or 1,100,000 nGy), respectively, are well below the minimum threshold values for deterministic effects of both skin (2 Gy or 2,000,000,000 nGy)37 and eye lens (0.5 Gy or 500,000,000 nGy).38 The skin determines the threshold limit for the breast due to the sensitivity for necrosis of the skin compared with clinical issues involving breast tissue.39

Summary

The NRC subcontractors and the committee performed a detailed computational assessment of the doses received during a security screening process involving X-ray backscatter AIT systems. This involved a detailed Monte Carlo

_______________

37 S. Balter, J.W. Hopewell, D.L. Miller, L.K. Wagner, and M.J. Zelefsky, Fluoroscopically guided interventional procedures: A review of radiation effects on patients’ skin and hair, Radiology 254:326-341, 2010.

38 F.A. Stewart, A.V. Akleyev, M. Hauer-Jensen, J.H. Hendry, N.J. Kleiman, T.J. MacVittie, B.M. Aleman, A.B. Edgar, K. Mabuchi, C.R. Muirhead, R.E. Shore, and W.H. Wallace, ICRP Publication 118: ICRP Statement on Tissue Reactions and Early and Late Effects of Radiation in Normal Tissues and Organs—Threshold Doses for Tissue Reactions in a Radiation Protection Context, Annals of the ICRP 41:1-32, 2012.

39 F.A. Metler and A.C. Upton, Medical Effects of Ionizing Radiation, Third Edition, Saunders, Elsevier, Philadelphia, Pa., 2008.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

simulation using a representative source term and state-of-the-art hybridized human phantoms. The model used in the computations is referred to as the reference X-ray backscatter AIT system. The model represents a dual scan–single-pose configuration with an anterior unit providing an anterior image and a posterior unit producing a posterior image. The spatial dimensions were obtained from the previous NIST report evaluating a Rapiscan Secure 1000. This also included the size of the collimated X-ray beam, vertical span of the X-ray source, angular pitch of the X-ray source, and horizontal properties of the scanned beam. These results can be compared with previous studies reviewed in Chapter 6 and summarized in Table 6.4.

X-Ray Source

Typically, the energy distribution of the X-ray photons incident on the person scanned are derived from the operating voltage, anode angle, and measurements of the first HVL (HVL1) in aluminum. This information serves as input to a standardized computer model that yielded the desired energy distribution. In this case, the information came from NRC subcontractor measurements made on an AS&E SmartCheck and a Rapiscan Secure 1000 AIT system. These measurements also included a central-axis PDD in tissue-equivalent material. The objective was to use the measured PDD as a validation of the Monte Carlo simulation using the computer-generated energy distribution.

Beam Intensity

The beam intensity serves as a scale factor for estimating the absorbed dose to the passenger as well as a calibration of the Monte Carlo computations. The quantity used for this is the air kerma per scan (in nGy) at a reference location between the anterior and posterior units. The Monte Carlo method transports one incident photon at a time. A simulation of the air kerma measurement yielded a normalization factor in terms of the number of photons required to generate 1 nGy of air kerma.

Results

In this section, a screen is considered to be the combination of an anterior and a posterior scan of 50 kV with an HVL of 1.18 mm Al and air kerma of 68 nGy per scan. It should be noted that the results will scale linearly with air kerma if the other conditions remain the same.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
  • The conventional method for computing the energy distribution of the incidents photons for 50kV X-ray backscatter AIT systems did not adequately predict the data obtained for PDD in tissue.
  • The absorbed doses to individual tissues and organs for the reference adult phantoms located midway between the anterior and the posterior were all less than 50 nGy per screen.
  • In general, the absorbed doses decreased in the adult phantoms as the BMI increased.
  • The sex-averaged effective dose for the adult phantom ranged from 15 nSv at the 5th percentile BMI to 6 nSv for the 95th percentile BMI. (The ANSI EREF for these conditions is 20 nSv.)
  • The sex-averaged effective dose for the adult phantom was significantly greater for the anterior scan compared with the posterior scan. This difference in all cases depends on the distance of the organ from the X-ray source and on the organ’s location within the individual.
  • The absorbed doses to individual tissues and organs for the pediatric phantoms located midway between the anterior and the posterior AIT system units were all less than or equal to 60 nGy per screen.
  • The absorbed doses to tissues and organs remained relatively constant in the pediatric phantoms as the BMI increased.
  • The sex-averaged effective dose for the pediatric phantoms ranged from 25 nSv at the 5th percentile BMI to 22 nSv for the 95th percentile BMI. (The ANSI EREF for these conditions is 20 nSv.)
  • The sex-averaged effective dose for the pediatric phantoms was significantly greater for the anterior scan compared with the posterior scan.
  • The absorbed doses to the fetus of a pregnant female located midway between the anterior and the posterior AIT system units were all less than 10 nGy per screen.
  • The absorbed doses to the active bone marrow of the developing fetus of a pregnant female located midway between the anterior and the posterior AIT system units ranged from 16 nGy at 15 weeks post-conception to 11 nGy at 38 weeks post-conception.
  • The absorbed doses to individual tissues and organs for the reference adult phantoms located midway between the anterior and posterior AIT system units were all greater for a plane-parallel incident-X-ray beam compared with the exposure conditions in an actual AIT system.
  • The sex-averaged effective dose for the adult phantoms increased as the phantom was located closer to the anterior unit.
  • The absorbed doses to the locations of radiosensitive cells, at depths between 50 µm and 100 µm, were not significantly larger than the dose averaged over the complete layer of skin.
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
  • The localized absorbed doses for a stationary beam in both vertical and horizontal directions and normal scan time duration while centered upon the lens of an eye, a female breast, or central skin of the chest were on the order of 1,000,000 nGy for the lens of the eye and the skin and 7,000 nGy for the breast.
  • For a constant air kerma, the sex-averaged effective dose for the adult phantoms ranged from 12 nSv at a tube voltage of 50 kV (HVL = 1.18 mm Al) to 32 nSv at 100 kV (HVL 2.86 mm Al). The ANSI EREF ranged from 20 nSv to 49 nSv under these conditions.

FINDINGS AND RECOMMENDATIONS

Measurements

Key Finding: Using appropriate detectors, the estimated values of the radiation outside the inspection area that might affect a bystander are so low as to be statistically indistinguishable from the background radiation.

System Design

Although the radiation measurements and dose computations were performed for the committee in a detailed manner, the committee was unable to unequivocally determine whether the X-ray backscatter AIT systems studied have adequate operating safety interlocks that will prevent the AIT system from exceeding the ANSI/HPS N43.17-2009 standard under every imaginable situation for the following reasons:

  • The committee was not given an opportunity to independently verify how all of the interlocks would perform in different situations, with the exception of simple functions such as termination of operation if a door was opened. Such testing would need engineering support from the manufacturer and require unique testing tools, and it would require dismantling portions of the AIT systems, potentially causing damage to them.
  • The committee was not given a demonstration of how interlocks are checked at the manufacturer level from either Rapiscan or AS&E.
  • Detailed electrical and mechanical drawings and computer code descriptions and documents describing internal functions at the most fundamental level of the AIT systems are either restricted from public access or were not made available by the manufacturers to either the committee or the sponsor.
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

However, the committee was able to inspect the interior of both the Rapiscan Secure 1000 and the AS&E AIT system. AS&E representatives also described to the committee how many of the interlocks are intended to perform on their second-generation prototype.

With the above limitations noted, having evaluated as many aspects of the AIT systems as mechanically and electrically as possible, and combining that knowledge with the measurements and computations performed, the committee can make the following statements:

Key Finding: It appears that the X-ray backscatter systems adhere to the recommended safety mechanisms described in the ANSI/HPS N43.17-2009 standard.

Key Finding: Given the results obtained by the committee on radiation measurements and calculations for the X-ray AIT systems investigated, normal screening of an individual would need to extend for more than 60 seconds for an individual to be exposed to radiation that exceeds the ANSI/HPS N43.172009 limit. In comparison, a typical screen takes about 6 seconds.

Key Recommendation: Future X-ray advanced imaging technology (AIT) systems should have some independent mechanism to ensure that the AIT system does not screen any person for longer than the time needed to acquire the appropriate image while keeping radiation exposure compliant with the safety principle of as low as (is) reasonably achievable.

Key Recommendation: Any future testing procedures should at a minimum continue to follow the indicators, controls, and safety interlocks requirements of the ANSI/HPS N43.17-2009 standard, or similar testing procedures, and include daily verification of safety parameters by a test piece.

Dose Computations

The committee’s approach in examining the dose to the individual being screened differs from that of previous investigations in two ways:

  • It made use of sensitive detectors with tissue-equivalent phantoms to verify beam intensity, X-ray quality, and penetration; and
  • It performed computations using estimates of beam intensity, scanning geometry, and digitized human phantoms that have realistic dimensions and morphology.
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

Based on these improved conditions, the committee states that:

Key Finding: Under routine operations, the computed effective doses using realistic computational X-ray sources and scanning geometries, coupled with the digitized hybrid phantoms, are similar to the ANSI reference effective dose and an order of magnitude below the limit of 250 nSv/screen, as set forth in the applicable ANSI standard.

Key Finding: For either the Rapiscan Secure 1000 or the AS&E SmartCheck systems, as determined by the committee for adults and children:

  • The effective doses are about the same as those calculated following the simplified formula for the reference exposure dose identified by the ANSI/HPS N43.17-2009 standard;
  • The effective doses are lower than those in previous reports using plane-parallel X-ray beams with stylized geometrical (low-fidelity) human phantoms; and
  • Sensitivity analysis showed that under a range of different conditions, including passenger position in the AIT system and increases in the energy (i.e., by increasing the tube high voltage) of the X-ray beam, the computed effective dose would not increase by more than a factor of 3 and, even so, it would remain well below the limit specified in the ANSI/HPS N43.17-2009 standard.

Section 6.1.1.1 of ANSI/HPS N43.17-2009 specifies that the exposure limitations is “based on a computational adult model and is not always indicative of the actual effective does, especially for small children.” Section 6.1.1.1 also refers to a radiation dose for a full-body scan. The NRC subcontractor’s calculations enabled the committee to estimate variations in absorbed dose not captured in the ANSI standard. This included details of absorbed dose distributions in a wide variety of body types, including children and the developing fetus, as well as specific organs in each of those body types. Under standard operating conditions, the committee found that:

Key Finding:

  • No person, regardless of age and weight modeled, would exceed the effective dose limit per screen (i.e., 250 nSv/screen), as defined by the ANSI/HPS N43.17-2009 standard;
  • The absorbed dose per screen to the developing fetus at any of the three stages post-conception is less than 0.0003 percent of the recommended limit for radiation protection of the fetus during the entire gestation period;
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
  • The absorbed dose to the epithelial layer of radiosensitive cells in the skin is not significantly elevated (~1.6 percent) compared to the average dose to the skin; and
  • The dose received by the lens of the eye, skin, or female breast during a stationary beam of X rays for the duration of the scan were at least 2 orders of magnitude below thresholds where tissue injury might occur.

It might be worth mentioning that Mowery et al.40 indicate that during the worst-case scenario, when all interlocks are defeated by malicious code, the AIT system can be instructed to deliver the whole radiation dose from a scan to a single random point on the body. If this worst-case scenario occurs and the random delivery of the dose actually happens to be in the worst place possible, the eye, the computations presented above indicate that this dose is at least 2 orders of magnitude below the threshold where tissue injury might occur. This fact points out how important the previous recommendation is that there is some independent mechanism to ensure that the AIT does not screen any person for longer than the time needed to acquire the appropriate image.

Key Finding: The agreement between the estimated dose results from the NRC subcontractor and the results from previous studies confirms that the calculations performed in previous studies were adequate to establish compliance with effective dose limits recommended in ANSI/HPS N43.17-2009.

_______________

40 K. Mowery, E. Wustrow, T. Wypych, C. Singleton, C. Comfort, E. Rescorla, S. Checkoway, J.A. Halderman, and H. Shacham, Security analysis of a full-body scanner, Proceedings of the 23rd USENIX Security Symposium, 2014, https://www.usenix.org/conference/usenixsecurity14/technical-sessions/presentation/mowery.

Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 86
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 87
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 88
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 89
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 90
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 91
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 92
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 93
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 94
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 95
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 96
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 97
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 98
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 99
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 100
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 101
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 102
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 103
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 104
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 105
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 106
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 107
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 108
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 109
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 110
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 111
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 112
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 113
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 114
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 115
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 116
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 117
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 118
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 119
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 120
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 121
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 122
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 123
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 124
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 125
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 126
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 127
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 128
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 129
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 130
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 131
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 132
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 133
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 134
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 135
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 136
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 137
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 138
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 139
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 140
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 141
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 142
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 143
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 144
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 145
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 146
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 147
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 148
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 149
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 150
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 151
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 152
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 153
Suggested Citation:"7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 154
Next: Appendixes »
Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards Get This Book
×
Buy Paperback | $69.00 Buy Ebook | $54.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Passenger screening at commercial airports in the United States has gone through significant changes since the events of September 11, 2001. In response to increased concern over terrorist attacks on aircrafts, the Transportation Security Administration (TSA) has deployed security systems of advanced imaging technology (AIT) to screen passengers at airports. To date (December 2014), TSA has deployed AITs in U.S. airports of two different technologies that use different types of radiation to detect threats: millimeter wave and X-ray backscatter AIT systems. X-ray backscatter AITs were deployed in U.S. airports in 2008 and subsequently removed from all airports by June 2013 due to privacy concerns. TSA is looking to deploy a second-generation X-ray backscatter AIT equipped with privacy software to eliminate production of an image of the person being screened in order to alleviate these concerns.

This report reviews previous studies as well as current processes used by the Department of Homeland Security and equipment manufacturers to estimate radiation exposures resulting from backscatter X-ray advanced imaging technology system use in screening air travelers. Airport Passenger Screening Using Backscatter X-Ray Machines examines whether exposures comply with applicable health and safety standards for public and occupational exposures to ionizing radiation and whether system design, operating procedures, and maintenance procedures are appropriate to prevent over exposures of travelers and operators to ionizing radiation. This study aims to address concerns about exposure to radiation from X-ray backscatter AITs raised by Congress, individuals within the scientific community, and others.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!