One of the first steps in switching to digital radiography is deciding what type of system to purchase. Two broad classes of digital radiography equipment are available for veterinarians: cassette-based, or computed, radiography (CR) and direct (capture) radiography (DR).1,2
This article focuses on primary capture devices instead of secondary capture devices such as image digitizers (scanners) and digital cameras. In general, secondary capture devices have important limitations from both a medicolegal and an image quality standpoint.
CR requires purchasing the cassettes, a processor, and equipment for viewing and storing the images. The cassettes have the same physical dimensions as film-based cassettes and do not require any modification to the X-ray generator, tables, or grids. In general, CR is a slightly less expensive digital radiographic modality that will provide many of the advantages of digital imaging to a veterinary practice.
DR, or direct X-ray (DX), generally refers to one of three system types. The unifying characteristics of these systems are that an image is produced nearly instantaneously and that the same X-ray detector is used for every exposure.1,2,6-8 Two systems fall into the flat panel detector category. These systems are a commonly used DR system in veterinary medicine. The other type of DR system is a charge-coupled device (CCD) camera.
Flat panel detectors
The various vendors' panels have different physical dimensions, but all have a slightly smaller active detection area than the overall dimensions of the panel. The size of the radiograph is limited to the detector size, and care should be taken to choose the correct imaging surface size for the types of patients and studies being produced. Generally, the dimensions of this type of detector will be different from that of conventional film-screen cassettes, occasionally requiring alterations in radiographic examination tables or preexisting cassette holders (equine practice). Furthermore, to electronically synchronize the DR system with the existing generator, a minor system modification, which is usually done by the vendor, is necessary. As with CR systems, all DR systems require purchasing a means of image storage and review.
CCD camera radiographic systems generate an X-ray image when X-rays interact with a fluorescent (light-producing) screen that is attached to the underside of the X-ray table. The light produced from this interaction is then focused by a lens and captured by the CCD chip. In essence, the CCD camera photographs the light produced from X-rays interacting with a screen.
CCD chips are relatively small (2.5 x 2.5 cm to 8 x 8 cm). The large demagnification factor needed to reduce the size of the patient's image to the size of the CCD may result in loss of information, which has limited the quality of CCD-based images in the past.2,10 Advances in CCD technology, however, have overcome this limitation, and CCD-based machines are now accepted in human radiographic applications.12-15 The veterinary experience with CCD cameras is more limited than with CR and flat panel DR detectors. CCD radiography is used extensively in human and veterinary dental radiography because of the small field of view.
This article provides an overview of the technologies used for primary capture of radiographic information. Before the information is useful it must undergo image processing. Each vendor has a different system for image processing that may involve several steps that vary in complexity. Image processing contributes in large part to the differences in image appearance when evaluating the finished product. Although there are marked technical differences among the radiographic systems from the different vendors, much of the difference in image quality will depend on the vendors' ability to effectively process the image and not necessarily on the type of equipment used to obtain the image. CR is a relatively inexpensive system of digital radiography that shares a similar workflow with film-based radiography. DR technologies enable veterinarians to generate radiographic images within seconds of acquisition.
Sarah M. Puchalski, DVM, DACVR
Department of Surgical and Radiological Sciences
School of Veterinary Medicine
University of California
Davis, CA 95616
Editors' note: This article updates "Exploring your digital radiography options," which appeared in a supplement to Veterinary Medicine's December 2006 issue.
1. Bruce R. CR versus DR—what are the options? AuntMinnie, 2003. Available at: www.auntminnie.com/index.asp?sec=ser&sub=def&pag=dis&ItemID=58851.
2. Bushberg JT, Seibert JA, Leidholdt Jr, EM, et al. The essential physics of medical imaging. 2nd ed. Philadelphia, Pa: Lippincott, Williams & Wilkins, 2002.
3. Puchalski S, Hornof W. A limited image quality comparison of film-screen, computed radiography and digital radiography, in Proceedings. Am Coll Vet Radiogr Annu Mtg 2004.
4. Andriole KP. Productivity and cost assessment of computed radiography, digital radiography, and screen-film for outpatient chest examinations. J Digit Imaging 2002;15:161-169.
5. Andriole KP, Luth DM, Gould RG. Workflow assessment of digital versus computed radiography and screen-film in the outpatient environment. J Digit Imaging 2002;15(suppl 1): 124-126.
6. Digital x-ray systems. Part 1. An introduction to DX technologies and an evaluation of cassette DX systems. Health Devices 2001;30;273-310.
7. Digital x-ray systems. Part 2. An overview of digital radiography concepts and an evaluation of cassetteless DX systems. Health Devices 2001;30:381-401.
8. Fischbach F, Ricke J, Freund T, et al. Flat panel digital radiography compared with storage phosphor computed radiography: assessment of dose versus image quality in phantom studies. Invest Radiol 2002;37:609-614.
9. Goo JM, Im JG, Kim JH, et al. Digital chest radiography with a selenium-based flat-panel detector versus a storage phosphor system: comparison of soft-copy images. AJR Am J Roentgenol 2000;175:1013-1018.
10. Hornof W. Digital radiography: the available technologies and how to separate hype from reality, in Proceedings. Am Vet Med Assoc Annu Convention 2005.
11. Peer S, Neitzel U, Giacomuzzi SM, et al. Direct digital radiography versus storage phosphor radiography in the detection of wrist fractures. Clin Radiol 2002;57:258-262.
12. Kroft LJ, Geleijns J, Mertens BJ, et al. Digital slot-scan charge-coupled device radiography versus AMBER and Bucky screen-film radiography for detection of simulated nodules and interstitial disease in a chest phantom. Radiology 2004;231:156-163.
13. Kroft LJ, Veldkamp WJ, Mertens BJ, et al. Comparison of eight different digital chest radiography systems: variation in detection of simulated chest disease. AJR Am J Roentgenol 2005;185:339-346.
14. Veldkamp WJ, Kroft LJ, Boot MV, et al. Contrast-detail evaluation and dose assessment of eight digital chest radiography systems in clinical practice. Eur Radiol 2006;16:333-341.
15. Veldkamp WJ, Kroft LJ, Mertens BJ, et al. Digital slot-scan charge-coupled device radiography versus AMBER and Bucky screen-film radiography: comparison of image quality in a phantom study. Radiology 2005;235:857-866.