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
CR was the first digital radiography system used in medical imaging. These systems have many features analogous to traditional
film-screen systems (Figure 1) because a cassette is used to house a photo-stimulable phosphor sheet.2 The latent image is stored in the phosphor sheet similar to how undeveloped film stores the latent image in film-screen
technology (Figure 2). CR radiographs are obtained in a traditional manner: all the views of the study are taken, and then the cassettes are moved
en masse to a digital processor. Each cassette is then individually fed into the processor where the storage phosphor sheet
is removed and scanned by a laser. The laser imparts energy onto the storage phosphor sheet, liberating light from the latent
image stored therein. The light emitted from the phosphor sheet is digitized to produce the digital radiograph. The processor
requires 60 to 90 seconds to produce an image from each cassette, and the cassettes are re-used numerous times. During this
processing time, the next cassette cannot be fed in, though many processors accept multiple cassettes (two to four) at one
time. CR may slow workflow if the number of radiographic projections exceeds the number of cassette receptacles on the processor.3 This problem may arise in multiple-view studies such as orthopedic or pre-purchase examinations.
Figure 1. A schematic of traditional film in a double-screen cassette system. X-ray photons pass through the cassette to interact
with screens housed on both sides of the film. When an X-ray photon interacts with the screen, light is emitted instantaneously,
which then exposes the film. (Illustration by John H. Doval)
Thus, the CR workflow is similar to that of traditional radiographic film.4,5 Because of the delay in image processing, it will take several minutes to identify errors in radiographic technique, specifically
positioning. In small-animal practice, it may simply entail repositioning of the animal, while mobile equine practitioners
will not recognize the need for retakes until the cassettes are returned to the processor, which is most commonly housed in
a stationary location such as the clinic. More recently, portable CR processors have become commercially available to help
obviate this inconvenience.
Figure 2. A schematic of a typical CR cassette. X-ray photons pass through the cassette to interact with the storage phosphor.
The X-ray energy is stored in the phosphor sheet. When the cassette is inserted into the processor, a laser passes over the
storage phosphor, releasing the stored energy as light. The computer processes this light energy to yield a digital image.
(Illustration by John H. Doval)
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.