X-Ray Projection Imaging
X-ray projection imagingthe familiar ''x-ray"has been and continues to be the most widely used format for diagnostic imaging in spite of important and exciting advances made, for example, in the areas of computed tomography and magnetic resonance imaging.
X-ray projection imaging dates back to R6ntgen's discovery of the x-ray in 1895. In subsequent decades techniques were developed involving the use of contrast materials to enhance the images of blood vessels, gastrointestinal structures, and other anatomy. In 1930 the technique of film subtraction angiography was developed and, in spite of the inconvenient delay required to complete the film processing, enjoyed widespread use for many decades.1
Before the invention of the image intensifier in 1950, x-ray diagnosis was accomplished using either static film images or fluoroscopy, which involved direct viewing of fluorescent screens after the viewer adapted to the dark. The invention of the image intensifier was a major advance not only because it improved fluoroscopy but also because its introduction of an electronic image signal facilitated the development of real-time digital imaging techniques such as digital subtraction angiography (DSA), the digitized analog of film subtraction angiography.
At about the same time that DSA was introduced, digital projection im-
1 Film subtraction angiography is a technique that uses positive and negative film images taken before and after the injection of contrast agents to produce subtraction images showing only the vascular structures affected by the contrast injection, without superposition of other structures (e.g., bones).
ages were obtained with computed tomography (CT) systems and dedicated line scan systems. Line scan images,2which are used routinely to provide localization images prior to CT, benefit from the low scatter associated with the linear collimation employed.
The emergence of digital (as opposed to film) techniques for x-ray projection imaging, combined with the fact that several other diagnostic modalities (e.g., nuclear medicine, CT, and more recently ultrasound and magnetic resonance imaging) provide digital data, has led to increased efforts focused on the possibility of completely digital radiology departments. The movement toward this has been slowed by the complexity of the task and also by the lack of a universally acceptable digital detector for x-ray projection images. While image-intensifier television systems have been adequate for digital fluoroscopy applications, a substitute for film has not yet been found that provides comparable diagnostic accuracy over a wide range of applications.
The development of photo-stimulable phosphor plates in the mid 1980s provided a detector with several attractive properties, including a digital format and greatly increased dynamic range. This modality compares well with film in all but a few applications and appears superior for bedside radiography because of its improved dynamic range. However, it does not have sufficient resolution for mammography and has the disadvantage, shared by film, that cassettes must be handled and processed.
The availability of rapidly obtained digital images has led to investigations of dual energy techniques for chest radiography and cardiac imaging.3Although most of these investigations have been carried out using conventional x-ray sources, there are ongoing investigations of alternative sources for producing narrow band radiation for applications of this type.
While it is recognized that film is unique in its ability to serve as detector, display mechanism, and archival storage medium, the availability of digital techniques offers the possibility of separately optimizing these functions. Significant current research activity revolves around the development and optimization of detectors, sources, and related imaging apparatus and procedures for several imaging applications. The remainder of this chapter summarizes these challenges within the context of some of the most significant areas of clinical applications of projection imaging.
2Produced by sequentially scanning a linear (one line of data at a time) detector over the area of interest.
3Dual-energy techniques involve acquiring two images from the same area using different x-ray spectra (by changing the tube voltage) in order to exploit differences in the attenuation coefficient of different body tissues as a function of the x-ray energy.
In mammography the development of digital techniques is based primarily on the desire to provide a lower-dose examination for improved detection and perhaps characterization of lesions. Because of the desire to image microcalcifications, system resolution must be on the order of 50 microns. Additionally, in order to achieve the required resolution of low-contrast differences inherent in mammography, the detector must be capable of 12-bit contrast resolution.
Although digital detectors already exist for imaging a part of the breast, primarily for stereotaxic localization,4efforts are being made to develop detectors capable of imaging the entire breast. These developments can be divided into systems that perform some form of scan to acquire the images versus area detectors that each simultaneously acquire an image of the entire breast.
2.2.1 Scanning Methods
In some ways the technical requirements for a scanning system using digital detectors are lower than the requirements for area systems because the number of discrete detection elements can be significantly reduced. However, the mechanism to produce the scanning motion can add significant complexity to the system.
In addition to having lower scatter, scanned systems offer the potential for image equalization through spatially varying the exposure. This possibility, previously explored for chest radiography, has recently been investigated in a film system. The film density is equalized throughout the mammogram using a raster scanned x-ray beam with feedback from an opposing detector. This approach offers the advantage that higher-contrast film, which can better display the small structures of the breast, can be used because the large intensity variations through the breast have been reduced. In the case of digital detectors, which enable manipulations of the contrast after image acquisition, scanning equalization still offers the potential advantage of a spatially uniform signal-to-noise ratio and more efficient dose utilization.
4Stereotaxic localization is the three-dimensional localization of lesions for biopsies or surgery using imaging techniques, usually requiring the fixation of the organ under investigation within a mechanical frame during imaging and surgery.
Just as was done in chest radiography, the mammographic equalization system presumably could be implemented in a line-scanned configuration to reduce tube loading and imaging time.
There are several slot or multislot scanning systems reported or under development.5Detector schemes for such systems include phosphor fiber optically coupled to charge coupled devices (CCDs), systems utilizing direct detection of x-rays by glass or plastic scintillation fibers coupled to CCDs, or silicon photodiode arrays.
2.2.2 Area Detectors
Photostimulable phosphor plate detectors have met with some success in chest radiography but have had relatively little acceptance in mammography, presumably owing largely to the limited spatial resolution. In addition, they have reduced detector quantum efficiencies (DQEs)6relative to those of film/screen systems. There is ongoing activity to improve both aspects of the plate performance, and additional phosphor materials are being investigated. In addition, the expiration in the next year or two of some of the patents on this technology should increase competitiveness and accelerate developments in this area. Although much has been borrowed from conventional screen technology, it is likely that significant improvements are still possible.
Several small-field-of-view (5 cm x 5 cm) commercial digital mammography systems are available and are used primarily for stereotactic needle biopsy of the breast. These systems all employ detection of x-rays by fluorescent screens coupled to CCD detectors by a lens or fiber-optic line. With future improvements in CCD technology, which are also being driven by other industries, it is likely that either single or multiple CCD detectors capable of imaging the entire breast will be possible.
Amorphous silicon sheets with multiple discrete detectors are under development. The silicon photodetector is usually coupled to a scintillation screen to convert the x-ray energy to visible light that is then detected by the silicon array. Such arrays have insufficient spatial resolution for mammo-
5Slot scanning imaging systems are similar to line scan imaging and use linear detector systems to sequentially scan the imaging field.
6The quantum efficiency is a measure of detector sensitivity related to its ability to interact with a certain fraction of the incoming x-ray or y-quanta. The quantum efficiency is related to the linear attenuation coefficient. Often geometric efficiencies (active versus inactive area of the detector) are included.
graphy at this time, but continued development may make better resolution possible.
Selenium plates were used previously as an x-ray detector in xeromammography.7Interest in selenium as a detector has been rekindled with the development of some new readout strategies. One of these involves the use of integral discrete detectors on a glass substrate. Another uses a luminescent liquid toner that is scanned by a laser, producing luminescence that is recorded by a charged selenium photodetector plate. One advantage of selenium detectors is that they have inherently high resolution. Whereas previous readout schemes had limited resolution due to the discrete readout process, in this case the luminescent process would be limited by the laser spot size, which can be made very small. Another advantage of selenium is that it can be made quite thick in order to detect a large fraction of the incident radiation without sacrificing resolution. Therefore, the DQE of selenium detectors can be quite good.
Another area of interest is in the development of phosphors that utilize some mechanism to contain the lateral spread of visible light as it exits the detector. Two separate, fairly similar efforts address this objective. The first uses doped glass fibers to produce fluorescent light that stays in the fiber, as in an optical fiber. The other effort involves the use of plastic fibers activated by organic fluorescent dyes to a certain depth at the end of the fiber. In both cases the fiber axis is oriented parallel to the direction of the x-rays. In this way the effective phosphor thickness can be large without sacrificing resolution.
Limitations on intrinsic detector resolution may be relaxed if magnification techniques can be used, although this approach is usually limited by the loss of resolution due to focal spot blurring. The use of x-ray capillary optics is being investigated as a means for achieving magnifications of up to 2.0 without additional focal spot blurring. The capillary optics, which channel the radiation detected at the opposing side of the patient to conventional phosphor plates, prevent focal spot blurring beyond that which exists at the exit plane. Another possible way to achieve large magnifications is through the use of high-output, small-focal-spot x-ray sources, exploration of which has already begun.
7Xeromammography is a filmless method of breast imaging that uses a xerographic technique for image generation: x-rays produce an electrical charge distribution that is reproduced on paper through the familiar photocopying or laser printing process.
2.3 Chest Radiography
2.3.1 Scanning Methods
For chest radiography several slot or area scanning systems employing image equalization have been previously investigated. A scanning slit system capable of producing dual-energy images has also been reported. The narrow slit utilized provided for excellent images but led to high tube loading. It may be possible to increase the slit width to a more favorable size and still maintain image quality. However, these systems have met with limited success.
2.3.2 Area Detectors
Recently there has been significant improvement in conventional film/screen cassettes. The new cassettes utilize asymmetric front and back screens, different front and rear film emulsions, and anticrossover technology so that light from the front screen is not detected by the back emulsion and vice versa. The advantage is that the resolution, contrast, and speed of the front film/screen combination can differ from that of the back combination, permitting variations in image properties throughout the image. This technology has been applied specifically in chest radiography to yield high resolution and contrast in the lung regions and increased film density, good contrast, and somewhat lower resolution in the mediastinum, behind the heart, and below the diaphragm.
Amorphous silicon and selenium detectors (described in the section above on mammographic detectors) are also being investigated for chest radiography. These detectors may have a greater likelihood of success in the chest application, since the demands for resolution are lower for chest radiography than for mammography.
A digital chest radiography system utilizing a selenium detector has recently been developed. The detector is a drum of amorphous selenium. After exposing the selenium, an array of sensitive electrometer probes reads the charge pattern as the drum is rotated at high speed. The dynamic range of the detector is very good and the DQE is reported to be better than that for either phosphor plates or conventional film/screen combinations.
Photostimulable phosphor plate systems are continuing to gain acceptance for bedside chest radiography but are not widely used for dedicated chest radiography, which has much higher requirements for image quality.
However, continued improvements in phosphor plate technology may eventually increase its broader acceptance.
2.4 Digital Fluoroscopy
Digital fluoroscopy includes image-intensifier television camera systems such as those used for DSA, non subtractive cardiac imaging, and gastrointestinal fluoroscopy. Developments in this area include the use of higher-resolution video cameras, potentially higher resolution detectors, image acquisition strategies to permit imaging of moving tables or multiple angles during an angiographic injection, and lower-lag CCD camera systems for cardiac applications.
Compensation for bright spots in the detected x-ray field by means of a selectable configuration of a basic series of spatially variable filter wheels has been investigated and holds promise for solving one of the most important problems in the imaging of low-contrast objects.
It has been known for some time that the use of x-ray contrast materials with k-edges higher than those of iodine and barium would provide greatly improved imaging performance because of the favorable trade-off between contrast loss and increased tissue transmission. Recent work has confirmed the potential advantages of contrast materials based on gadolinium. There is also continued development of iodinated contrast materials. The development of agents with lower osmolality appears to be more significant in terms of patient tolerance than for any advantages in x-ray attenuation. However, these agents may produce fewer misregistration artifacts in subtraction imaging. Agents designed to remain for longer times in the vascular space may be of benefit for quantitative applications where leakage into interstitial space can complicate interpretation of contrast time curves.
Dual-energy cardiac DSA systems using conventional x-ray sources in conjunction with energy switching and rotating filter wheels are currently under investigation and have been shown to be effective in removing tissue-related artifacts that impair various quantitation tasks involving injected iodine.
Specialized sources such as synchrotron radiation, characteristic radiation, channeling radiation, parametric radiation, and laser sources are under consideration as possible sources for dual-energy DSA. The application most often considered is intravenous coronary angiography, although synchrotron radiation has also been applied to achieve high-resolution three-dimensional
microscopy of trabecular bone. Due to the large x-ray doses required with these sources, screening of asymptomatic patients is not justifiable, and the economics of the more expensive of these alternatives would be difficult to rationalize. Additionally, the x-ray approach to coronary angiography, which has progressed slowly in the last decade, should be weighed against recent advances in magnetic resonance imaging of coronary arteries (see Figure 1.3).
2.5 Portal Imaging
Electronic portal imaging devices for verification of radiotherapy treatment fields are rapidly being developed.8These systems offer the advantage, in some cases, of almost real-time operation so that field alignment errors can be corrected quickly.
Several technologies are under investigation. The most common systems use metal plates combined with phosphor materials that are viewed by a video camera. The early systems used a mirror to feed images to the video camera, whereas current systems employ fiber-optic coupling. Amorphous silicon arrays are being evaluated to replace the camera in a direct digital system.
Scanning liquid ionization chambers are also being developed. These devices are similar to conventional ionization chambers except that in place of air they use a liquid. This substitution offers increased detection efficiency in proportion to the ratio of liquid and air densities. Another system under development uses a scanned linear array of high-voltage rectifier diodes as a direct solid-state detector. Other linear scanning array detectors are being developed that use CdTe or zinc tungstate crystals coupled to a photodiode.
2.6 Research Opportunities
· Development of electronic planar array detectors with adequate resolution, size, reliability, and quantum efficiency.
· Development of digital display systems of sufficient resolution and dynamic range.
· Development of means to detect and use the information in scattered radiation, including mathematical correction schemes.
8 These are filmless real-time imaging systems with large apertures that verify the treat-
ment field by employing a geometry similar to the geometry of the therapy equipment.
· Re-evaluation by physicists of the prospects for multiple monochromatic sources, novel x-ray generation techniques, and holographic methods, ambitious research directions that have been resistant to progress.
2.7 Suggested Reading
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3. Boyer, A.L., Antonuk, L., Fenster, A., Van Herk, M., Meertens, H., Munro, P., Reinstein, L.E., and Wong, J., A review of electronic portal imaging devices (EPIDs), Med. Phys. 19 (1992), 1-16.
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5. Gray, J.E., Stears, J.G., Swenson, S.J., and Bunch, P.C., Evaluation of resolution and sensitometric characteristics of an asymmetric screenfilm imaging system, Radiology 188 (1993), 537-539.
6. Kinney, J.H., Lane, N.E., and Haupt, D.L., In vivo three-dimensional microscopy of trabecular bone, J. Bone Miner. Res. 10 (1995), 264-270.
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9. Morris, T., X-ray contrast media: Where are we now, and where are we going?, Radiology 188 (1993), 11-16.
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