Charles E. Willis, PhD

During the 20-odd years since its introduction into the United States, computed radiography (CR) has become established as the preeminent technology for acquiring ordinary radiographic projections in digital form. CR is extremely flexible and promises to replace virtually all conventional screen-film radiography, including mammography, scoliosis, and Panorex studies. Unlike competing digital technologies, CR is especially suited to bedside radiographic examinations, which are traditionally the worst images in the hospital because of the urgent conditions under which the examinations are performed. CR’s wide latitude and automatic density adjustment dramatically improve the consistency of bedside radiographs. As is the case with all digital modalities, the appearance of the CR image can be modified as desired after acquisition. The CR image can be distributed virtually anywhere electronically, can be viewed simultaneously by multiple care providers, and can be reprinted, as necessary. CR imaging plates are reusable image media (RIM), which can be erased and imaged thousands of times, eliminating consumption of film and chemicals.

Figure 1a. QC reports from spatial tests by manufacturer A.
Figure 1b. QC reports from contrast tests by manufacturer A.

The success of CR led to a misconception that quality control (QC) processes were no longer necessary. Early adopters of CR claimed that retake rates decreased to zero. If there is anything new in the realm of CR quality assurance (QA), it is the general recognition that QC processes for CR are no less important than they are for conventional screen-film radiography.

Experienced practitioners of CR, having witnessed the consequences of uncontrolled CR imaging, now understand the need for routine QC. Just like conventional radiography, most bad images are the result of mispositioning. 1 In order to maximize the contrast for the anatomy of interest, CR needs to locate the boundaries of the radiation field in the image. Inappropriate collimation and mispositioning can confuse CR and make an image that is uninterpretable. 2,3 There are a variety of errors that can result in bad CR images, and this means that departments must institute QC processes to detect and correct these errors before images are released to physicians for interpretation. 4

The need for routine inspection and intervention means that a complete CR system should include a QA/QC workstation. This application has been integrated into the CR acquisition station in some models. Common features include the ability to modify examination and demographic information, to annotate the image, and to apply a shadow mask. If the intent is for the technologist to modify the appearance of the image, then it is only reasonable to provide the technologist with the same viewing capabilities as the physician. That is, the display must have a similar luminance range and have a DICOM Part 14-compliant grayscale display function. The display must provide the technologist with the ability to view the entire image dataset, without the image being minimized to make room for excessive control icons. Otherwise, technologists are going to be releasing images that look good to them, but look terrible to the physician. At this writing, only one of the four major CR manufacturers allows appropriate displays on their QA/QC workstation.

Figure 3a. QC summary results by manufacturer C.
Figure 3b. MTF test by manufacturer C.

Most practitioners appreciate that it requires more radiation exposure to produce a CR image that is comparable in terms of noise quality to a conventional 400 speed class screen-film system. 5 Practitioners and vendor applications personnel are beginning to understand that the amount of digital image processing that you can apply to a CR image is directly related to the exposure technique that you use to create the image.

Practitioners now also appreciate the potential for overexposure in CR, and the need for vigilance and targeting of the values of derived exposure indicators. 6,7 Manufacturers and imaging scientists are working together to standardize these exposure indicators. The American Association of Physicists in Medicine (AAPM) recently formed a task group on the subject. The “International Electrotechnical Commission (IEC) Standards Committee 62B Working Group 27: Image quality and dose for x-ray imaging equipment” is also considering this topic.

CR users have learned that imaging plates do not last forever. There are mechanical, chemical, and environmental processes that degrade the performance of CR plates over time. The lifetime of plates is highly variable depending on usage, care, and cleaning. This means that users must be on the lookout for plate defects, and smart users maintain an annual budget for replacing some fraction of their imaging plates and cassettes.

BUILT-IN SOFTWARE

CR manufacturers now recognize the importance of QC processes and are building software into their QA/QC workstations to accommodate processes such as reject/repeat analysis and exposure indicator tracking. Manufacturers have made great progress in developing test objects and automated analysis software. Four manufacturers showcased their QA/QC products this year at the AAPM 46th Annual Meeting and Summer School in Pittsburgh.

Manufacturer A first introduced QC test objects and automated evaluation software in 1997. Their system consists of two test objects: a spatial test object that provides information on the frequency response and geometric performance of the system (eg, square-wave response, scan linearity, dimensional accuracy), and a contrast test object that shows how the CR system is registering signal intensity (eg, SNR, sensitivity, dose linearity, dynamic range). Images of both test objects are read and evaluated automatically by the software, which displays or prints a report (including historical results, if desired) on all relevant aspects of system performance, including acceptance limits, and whether the system passed each test (Figure 1a and 1b). With the assistance of a service engineer, the results of the tests are also available in the form of ASCII text files. The product includes a carrying case that holds the test objects and user guide.

Figure 4a. Exported QC data from manufacturer B’s QC workstation database.
Figure 4b. Exported QC data from manufacturer B’s QC workstation database.
Figure 2. Test phantom by manufacturer B.

Manufacturer B has a single test object (Figure 2) that incorporates features to test laser jitter, contrast, sharpness, measurement accuracy, shading and sensitivity, and linearity. The tests follow those outlined in AAPM Task Group #10 on CR Acceptance Testing and QC. The resulting images are interpreted manually, by inspecting either Group #10 on CR Acceptance Testing and QC. The product has three primary components: a manual that provides specific testing instructions, the QA phantom, and the QA Worksheet, onto which the user enters and tracks all data. The QA Worksheet will run on any Windows-based PC that has Excel software.

CONCLUSION

Practitioners of CR are cognizant of the fact that a QA program is more than a collection of tests and measurements. 8 QA is an ongoing process that encompasses all activities that affect the quality and efficiency of the imaging operation. This includes installation, configuration, calibration, maintenance, and operation of the CR system. An effective QA program requires active participation by radiologists, radiology administrators, technologists, clinical engineers, informatics personnel, medical physicists, as well as vendor applications and service personnel.

References:

  1. Honea R, Elissa Blado M, Ma Y. Is reject analysis necessary after converting to computed radiography? J Digit Imaging. 2002;15(suppl 1):41-52.
  2. Willis CE, Thompson SK, Shepard SJ. Artifacts and misadventures in digital radiography. Applied Radiology. 2004; 33(1):11-20.
  3. Willis CE. Computed radiographic imaging and artifacts. In: Siegel EL, Kolodner RM, eds. Filmless Radiology. New York: Springer-Verlag; 1999:137-154.
  4. Willis CE. Quality assurance: an overview of quality assurance and quality control in the digital imaging department. In: Quality Assurance, Meeting the Challenge in the Digital Medical Enterprise. Great Falls, Va: Society for Computer Applications in Radiology; 2002:1-8.
  5. Huda W, Slone RM, Belden CJ, Williams JL, Cumming WA, Palmer CK. Mottle on computed radiographs of the chest in pediatric patients. Radiology. 1996;199:249-252.
  6. Freedman M, Pe E, Mun SK, Lo SCB, Nelson M. The potential for unnecessary patient exposure from the use of storage phosphor imaging systems. Proc SPIE. 1993;1897:472-479.
  7. Gur D, Fuhman CR, Feist JH, Slifko R, Peace B. Natural migration to a higher dose in CR imaging. Proceedings of the Eighth European Congress of Radiology. 1993;154:12-17.
  8. Willis CE. Computed radiography, QA/QC. In: Practical Digital Imaging and PACS. Medical Physics Monograph No. 28. Madison, Wis: Medical Physics; 1999:157-175.