One of the first steps in switching to digital radiography is deciding which type of system to purchase. Two broad classes
of digital radiography equipment are available for veterinarians: computed radiography (CR), which is cassette-based, 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
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. (Illustrations by John H. Doval.)
CR was the first digital radiography system used in medical imaging. CR systems have many features analogous to those of traditional
film-screen systems (Figure 1) because a cassette is used to house a photo-stimulable phosphor sheet.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 (Figure 2). 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. During this processing time, the next cassette
cannot be fed into the processor, though many processors accept multiple cassettes (two to four) at one time. CR may even
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.
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.
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 techniques, specifically
positioning. For small-animal practice, this may simply entail repositioning the animal. But mobile practitioners won't 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.
CR requires purchasing the cassettes, a processor, and equipment for viewing and storing the images (see the article "Getting the most out of digital image viewing" and the article "How to store digital images and comply with
medical recordkeeping standards" ). 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 in a veterinary practice.