Airport Passenger Screening Using Backscatter X-Ray Machines Compliance with Standards (2015) / Chapter Skim
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6 Review of X-Ray Backscatter Advanced Imaging Technology Studies
6 Review of X-Ray Backscatter Advanced Imaging Technology Studies
Pages 56-85
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From page 56... ...
Paulter, Assessment of the Rapiscan Secure 1000 Single Pose (ATR version) for Conformance with National Radiological Safety Standards, NIST report for the TSA, interagency agreement HSHQDC-11-X-00585, April 19, 2012; latest version available is J.L.
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From page 57... ...
5 F. Cerra, Assessment of the Rapiscan Secure 1000 Body Scanner for Conformance with Radiological Safety Standards, Food and Drug Administration, Arlington, Va., July 21, 2006.
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From page 58... ...
Thus, the effective filtration may increase slightly with tube use. The photon spectrum is essentially independent of the anode current, but at a fixed anode voltage, the fluence rate is directly proportional to the anode current.
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From page 59... ...
The measure ment of kerma was generally done with an ion chamber calibrated for a specific photon spectrum. If the photon spectrum produced by the AIT system does not exactly match the spectrum used for the calibration, small errors in the measure ment of kerma may occur.
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From page 60... ...
The ion chamber was orientated so that the incident beam was parallel to the long axis of the cylindrical detector. To shield the ion chamber from extraneous radiation, a piece of lead was placed on top 8 Calibrationto a specified photon spectrum corrects the measured kerma for attenuation in the detector wall, but that attenuation continues to be included in a depth per dose or HVL measurement.
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From page 61... ...
SOURCE: (a) Department of Homeland Security, Radiation Safety Engineering Assessment Report for the Rapiscan Secure 1000 in Single Pose Configuration, August 2010, http://www.dhs.gov/advanced-imaging-technology-documents; (b)
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From page 62... ...
The Rapiscan was oper ated in the full scanning mode with vertical translation of the X-ray assem bly, horizontal collimator, and rotating chopper wheel. Lead shielding was wrapped around the ionization chamber to shield from ambient radiation and inscatter, but the beam was not collimated before the attenuation foils (Figure 6.1b)
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From page 63... ...
Measurements were made using the full vertical and horizontal raster 13 Glover et al., Assessment of the Rapiscan Secure 1000 Single Pose (ATR version) for Conformance with National Radiological Safety Standards, 2012.
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From page 64... ...
Measurements of air kerma were also taken at the ANSI-specified Reference Location representing the point of maximum exposure, but no closer than 30 cm to the surface where the beam exits, at a height of 185 cm above the floor, midway between the entry and exit portals and 30 cm from where the beam exits the front panel. The ionization chamber was calibrated using the M50 beam quality at the NIST X-ray calibration facility.
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From page 65... ...
The result was 12.6 ± 0.8 nSv per screening at the position that resulted in the maximum kerma, near the top of the scan range. A more realistic reference effective dose, typical of potentially occupied spaces, was 9.23 nSv per scan (18.5 nSv per screen)
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From page 66... ...
Procedures and Interlocks The NIST report did not indicate that potential failure mechanisms or verified safety interlocks were considered during the AIT system evaluation. The AAPM Report The aim of the AAPM report15 was to utilize the expertise of the medical physics community to make independent, detailed measurements of the Rapiscan Secure 1000 single-pose system output.
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From page 67... ...
The ion chamber was orientated so that the incident beam was perpendicular to the long axis of the cylindrical detector. The ionization chamber was calibrated using the M50 beam,17 which has a HVL of 1.04 mm of aluminum.
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From page 68... ...
adult, and a CDC 90th percentile adult using the PCXMC Monte Carlo code. The input data for beam quality was 50 kV, 20 degree anode angle, and 1 mm of aluminum total filtration to give the measured 0.93 mm of aluminum HVL.
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From page 69... ...
The use of these passive dosim eters avoids the possible complications of high dose rate in the beam, which has been mentioned as a concern when the effective dose, E, is determined based on measurements of kerma, K, using a large ion chamber. The most sensitive passive dosimeters available for this type of measurement are aluminum oxide crystals, with the absorbed dose determined by optically stimulated luminescence (OSL)
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From page 70... ...
The reference effective dose was calculated using the ANSI N43.17 formula and ion chamber data, resulting in EREF equal to 33 nSv.
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From page 71... ...
Procedures and Interlocks The USAPHC report includes potential failure mechanisms that could result in overexposure to the person being screened or the bystanders or screeners, in cluding overvoltage of the X-ray tube and the vertical scan stopping during the screening and increasing the exposure dose to one area of the passenger. They did not verify design or interlocks that would prevent these scenarios but indicate that the operator cannot adjust the power to the X-ray tube or the scan speed, so these failure modes cannot be caused by operators.
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From page 72... ...
Radiation scattered from a phantom was measured with the 1800 cc ion chamber (calibrated for the M50 spectrum) at two locations, on the mid-plane of the phantom and on the plane of the AIT system back plate (100 cm above the floor and 30 cm outside the plane of the AIT system side surface in both cases)
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From page 73... ...
Detector Scan Spectrum Detector Scan Leakage Detector USAPHC Single 1800 cc Full Unknown 1800 cc Full OSL FDA Dual 1800 cc Full Solid state Full 1800 cc GM survey JHU/APL Single 1800 cc Full Unknown 1800 cc Full 1800 cc GM survey AAPM Single 1800 cc Full M50 6 cc thimble None 1800 cc 1800 cc full NIST Single 1800 cc Full M50 1800 cc Line 1800 cc GM survey NOTE: GM, Geiger-Muller tube; HVL, half-value layer; N/A, not applicable; OSL, optically stimulated luminescence. TABLE 6.3 Summary of Rapiscan Secure 1000 Measurement Results in the Reviewed Reports KStd Kmax HVL1 (nGy)
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From page 74... ...
b Per anterior scan. SUMMARY OF THE INTERLOCK TOPIC FROM REVIEWED STUDIES Three of the reviewed studies considered the potential for accidental or delib erate malfunction of the Rapiscan Secure 1000 that could result in X-ray overex posure to the person being scanned, bystanders, or the operators.
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From page 75... ...
In order to make these calculations, a great deal of information is required about the scanning AIT system, including the beam spot size, the photon spectrum, the beam scanning rate, the location and efficiency of the scattered photon detectors, and much more. However, most of this informa tion was not available to the authors because it was considered proprietary by the 20 K
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From page 76... ...
Meets ANSI requirement N43.17.2002 6.2.2 "Subject exposure during a malfunction" AAPM X-ray tube can be operated 13 Power supply cannot No evaluation with higher voltage resulting in provide voltage higher higher X-ray photon flux and than tube specification higher X-ray photon energies Vertical scan stops and X-ray 13 If beam stops, the No evaluation exposure is focused on one image would be affected area of the body for enough and the operator would time to overexpose the notice "immediately" passenger
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From page 77... ...
R eview of X - R a y B a ck s c a t t er A dv a nced I m a g in g T ec h no l o g y S t udie s 77 TABLE 6.5 Continued Report Potential Malfunction Page Conclusion How Verified USAPHC X-ray tube can be operated 2 Operator cannot adjust No evaluation with higher voltage or current X-ray tube voltage or resulting in higher X-ray photon current flux or higher X-ray photon energies Vertical scan speed is lower 2 Operator cannot adjust No evaluation than set value and increases scan speed X-ray exposure, resulting in overexposure to the passenger TABLE 6.6 Summary of Rapiscan Secure 1000 Malfunction Investigations from the Committee's Investigation of the Unit at NIST Report Potential Malfunction Conclusion How Verified Committee X-ray source stays on X-ray source turns off Verified by EMO activation on after emergency-off (EMO) when EMO button is engineering AIT system button is pressed pushed; vertical scan bar completes full travel to top or bottom X-ray source activated X-ray source only activated Verified by measurement on by "on" switch without by "scan" button engineering AIT system pressing "scan" button X-ray source can be Key can be removed only Verified on engineering AIT activated with power key with power switch in "off" system removed position X-ray source can be Panel doors are Verified on engineering AIT activated for a scan with interlocked, and AIT system panel doors removed system will not scan if interlock is tripped X-ray beam fails to turn No X-rays are emitted after Verified by measurement on off at the end of a scan a scan is complete engineering AIT system Vertical scan can be Cannot initiate scan if Verified on engineering AIT locked into a single vertical scan motor is system position turned off; can activate X-ray source with vertical scan locked only in engineering mode, which requires a separate password
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From page 78... ...
Although they express the opinion that the difference between their calcula tion and measured results is likely due to recombination in the ion chambers used to measure air kerma, considering the tenuous nature of their estimates of pixel size and number of scattered photons it is perhaps more likely that the difference is due to differences between their estimated input values and those actually used in AIT systems. The authors also assert that if there is a failure of the scanning AIT system, the beam would have to be turned off within ~15 milliseconds to avoid exceeding an effective dose of 250,000 nSv.
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From page 79... ...
Most radiation exposures, including those delivered by inspection systems, do not deliver the same energy per mass to all parts of the body. However, the exposures received by the survivors of the nuclear bombs used in Japan, the source of most of the data used to set radiation protection limits, were nearly uniform.
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From page 80... ...
If one assumes that the photon spectrum produces an average linear energy of 5 keV/µm and that cells are 10-µm diameter spheres, the average dose to a cell hit by the radiation is 0.01 Gy, and at a kerma of 100 nGy, more than the kerma per screening produced by most AIT systems, the chance of a cell being hit is 10–5 -- that is, about 1 in 100,000 of the cells in the skin are hit. 26 International Commission on Radiation Units and Measurements, Report 85: Fundamental Quantities and Units for Ionizing Radiation, Journal of the ICRU 11(1)
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From page 81... ...
They used the estimated photon spectrum to calculate energy deposited in the 1800 cc ion chamber used by JHU/APL in order to determine the kerma to fluence conversion, but they did not do additional calculations to test the accuracy of the photon spectrum. This process has been contested because the ion chamber was simulated as a water cylinder instead of air, which results in an overestimate by a factor of about 3 in the source fluence.28 They found the effective dose for a person at 30 cm from the beam exit surface to range from 50 nSv for an adult male to 70 nSv for the 6-year-old male child.
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From page 82... ...
DDE is defined as the dose equivalent at a tis sue depth of 1 cm and is not an assessment of the dose equivalent to organs and tissues distributed throughout the entire body; therefore, it should not be used as a surrogate for effective dose received by airline passengers from bilateral scans during a backscatter screen. FINDINGS AND RECOMMENDATIONS ON EXPOSURE AND DOSE Individual Being Screened Key Finding: Previous radiation dose studies employed different methodolo gies and instrumentation to estimate the dose delivered by Rapiscan Secure 1000 X-ray backscatter AIT systems.
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From page 83... ...
Recommendation: During kerma measurements, one should use a parallel plate ion chamber with precisely known effective center and a relatively small diameter. Half-Value Layer Measurements Finding: In most cases, the HVL was measured using a large cylindrical ion chamber with relatively thick walls (3 mm air equivalent plastic)
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From page 84... ...
However, the average energy of the photon spectrum scattered from a person being scanned will be less than the average energy of the incident spectrum. As a result, an ion chamber calibrated to detect radiation from the incident spectrum may not accurately detect radiation scattered from a person being screened.
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From page 85... ...
Use of detectors that are appropriate for other applications but not ideal for measuring dose in a X-ray backscatter AIT system may result in inaccurate measurements. As discussed in Chapter 7, for both AIT systems the committee studied, the radiation measured outside the inspection area by the National Research Council subcontractor was found to be so low as to be statistically indistinguishable from zero (7.3 ±8.0 nGy)
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