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.
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