After the digital radiographic image is made, it is transferred to a dedicated digital radiography computer for “image processing” (Fig. 22-2). Here, the images can be adjusted as needed by the veterinary technologist. In small practices this may be the only computer available to view, but in most instances the processed image is finalized and then sent to another dedicated computer workstation for diagnostic interpretation by the veterinarian (Fig. 22-3). In large hospitals where multiple diagnostic workstations are necessary, the images will be sent to a main centralized computer (called a server) for storage and distribution to other workstation computers or sent off site via the World Wide Web for review.

Figure 22-2 Making the digital radiograph. A, This German shepherd is prepared for a caudocranial view of the tarsus. The local digital work station and radiology controls are in the background, behind a leaded glass and wall. B, The flat panel detector panel is positioned on the tabletop for caudocranial radiography of the left tarsus. A 10-cm bar marker has been placed lateral to the limb to allow for computer correction of magnification. In this application the detector panel is mobile and can be used for a multitude of positional studies including horizontal beam radiography. The panel can also be placed under the table for conventional radiography, with or without a grid. The radiology technologist is seen in the background in the control area. C, The local digital work station where the radiographic image of the tarsus appears following exposure. On this “Position” screen, the technologist can alter the orientation of the image, magnify the image, and make masking adjustments (black-out the white collimation). Note the tabs to the left of the screen that offer further choices on image manipulation.

Figure 22-3 Diagnostic workstation. This workstation uses a dual-monitor viewing system. The image of the humerus on the left monitor has been magnified for close scrutiny; two images on the right monitor are unmagnified. The workstation software applications allow the veterinarian to perform a multitude of image manipulations to optimize the image for diagnosis.

The term picture archiving and communication system (PACS) is the broad term for computers and components used to capture, transfer, store, and display medical digital information. Digital images can also be printed on high-quality transparent film (laser printers) to be viewed at a conventional illuminated view box, though most practices that make the conversion to digital radiography opt to go “filmless,” one of the major advantages of digital imaging.

LIMITATIONS OF CONVENTIONAL SCREEN-FILM RADIOGRAPHY

Although conventional screen-film radiography has served the medical community well for many years, its limitations make digital radiography an attractive alternative. First, screen-film radiography requires fairly narrow exposure factors to produce a diagnostic quality radiograph. Because x-ray film has a limited linear response (recall the logarithmic toe, linear region, and shoulder of a radiographic film Hunter and Driffield curve), relatively small underexposure or overexposure may yield an unacceptable image. This is why a radiographic technique chart is required and radiographic technique adjustments are necessary for various anatomic areas (thorax, abdomen, skeletal); body part thicknesses (incremental changes in kilovoltage potential (kVp) per centimeter of body part thickness); and different screen-film speeds. In many instances the inherent latitude limitation of conventional screen film radiology means that some areas of the radiograph will be overexposed while other areas will be underexposed. Depending on technical factors chosen, the radiograph can be made with relatively high contrast (e.g., a low kVp bone technique) or wide latitude (more shades of gray, as desired for thoracic radiography), but not both. A compromise is always possible.

Another disadvantage with conventional screen-film radiography is that the radiographic image cannot be adjusted once made. The radiographic film is exposed and then processed and viewed. Any errors in the exposure cannot be remedied; the radiograph must be retaken. This leads to increase in radiation exposure to the technician and patient, increases the cost of the examination, and requires additional technician and veterinarian time. Because the digital image can be manipulated after it is made, overexposure and underexposure problems are essentially a thing of the past with digital radiography.

Traditional radiography requires handling of film for viewing, archiving (storage), and distribution to referring veterinary practices. Reviewing a radiographic examination from a remote location requires that the study either be copied and sent via courier or digitally scanned before electronic transmission. Film storage requires an area large enough to access and sort films, often a separate area from the patient’s medical records.

ADVANTAGES OF DIGITAL RADIOGRAPHY

The Image

Unlike traditional screen-film radiography, kVp has little or no effect on the contrast or latitude of the digital image (this can be endlessly adjusted with software at the digital radiography computer terminal and diagnostic workstation). This flexibility is possible because of the linear relationship of the image receptors used for digital radiography. Simplistically, the number of electrons “trapped” by the digital image receptor during an x-ray exposure is linearly related to the intensity of the x-ray beam. Digital images have more latitude (more shades of gray) than film images and can display high-contrast body parts while simultaneously displaying soft tissues. This high gray-scale (high latitude) resolution is desirable because it allows observation of minor differences in radiation attenuation that may not be visible with film. When compared with conventional screen-film technique charts, digital technique charts do not vary greatly for the body part radiographed or patient thickness.

The higher-contrast resolution (or exposure latitude) of digital radiography has several tremendous advantages over conventional screen-film radiography. The need for retakes resulting from overexposure and underexposure is reduced, and for the most part eliminated. Images that are too light or dark that would be discarded on radiographic film can be adjusted with the digital image management software (Fig. 22-4). Marginal radiographic images, which may have previously been deemed acceptable, are a thing of the past.

Figure 22-4 Lateral thoracic radiographic image of a cat illustrates how overexposure (A) and underexposure (B) can be image processed to produce a perfectly exposed image (C). With conventional film-screen radiography, the improperly exposed studies would require retaking the radiographs.

Computer manipulation of the digital image is a phenomenal advantage that digital radiography has over conventional screen-film radiography. Images can be altered for contrast or latitude (Fig. 22-5) and can be zoomed (magnified) (see Fig. 22-3) to scrutinize the image as if using a magnifying glass to view a radiographic film. Digital viewing software packages offer a variety of ways to view digital images including subtraction tools that make it possible to view bone-only or soft-tissue-only images from a single exposure. Digital radiography makes it possible to see both soft tissue and bony detail in a single image. Because of the high exposure latitude, an unprocessed digital image usually does not look the same as a film/screen radiograph. Although digital images can be manipulated to mimic the appearance of conventional radiographs (Fig. 22-6), usually the image is adjusted to take advantage of simultaneous high contrast and high latitude. Curiously, the appearance of digital images is resisted by some veterinarians who are accustomed to viewing high-contrast radiographs. Getting accustomed to viewing digital images is one small initial disadvantage of digital radiography, but quickly overcome.