Radiography - The Merck Veterinary Manual

Radiography

Equipment

Darkroom

Filmless Radiography

Animal Restraint

Radiation Safety

Interpretation of Images

Contrast Procedures

Radiography is one of the most commonly used diagnostic tools in veterinary practice. It provides a large amount of information to the veterinarian by noninvasive and economical means. It does not alter the disease process or cause unacceptable discomfort to the animal. Although radiography is painless, sedation is often desirable in order to reduce anxiety and stress associated with the procedure.

Equipment:

Radiographs are made using a specialized type of vacuum tube that produces x-rays. The tube current, measured in milliamperes (mA), and voltage, measured in kilovolts (kV), determine the strength and number of x-rays produced and are 2 of the 3 exposure factors which can be set on most x-ray machines. Kilovoltage potential (kVp) is the highest potential voltage achieved at a certain kV setting.

Higher kV settings produce more penetrating beams in which a higher percentage of the x-rays produced penetrate the subject being radiographed. There is also a decrease in the percentage difference in absorption between tissue types. This results in a decrease in contrast (long-scale contrast) on the final film. High kVp techniques are most useful for studies of body regions with many different tissue densities (eg, thorax). Higher kVp techniques are appropriate for larger and thicker animals. Increasing kV is not a linear function, and small increases in kVp settings may substantially increase the number of x-rays penetrating the animal. However, this effect is much less dramatic above 85 kVp.

Increasing the mA setting on the machine increases the number of x-rays produced. The energy spectrum of the x-ray beam is essentially unchanged, as is the relative numbers of x-ray photons penetrating tissues of different densities such as bone, soft tissue, and fat. However, the amount of darkening on the film is related to the total number of photons reaching it. Therefore, increasing mA increases film contrast. Changes in mA settings are relatively linear. Increased contrast is desirable where tissue densities are similar (eg, musculoskeletal system).

The third major parameter in the making of a radiographic exposure is exposure time. Increasing the exposure time increases the number of photons produced and hence the darkness of the film. For exposure in the general diagnostic range, this a linear function.

All 3 of the above parameters are interdependent. Exposure time and mA are so much so that the term milliampere-seconds (mAs) is usually used to indicate the product of these 2 factors. Increasing the mA and decreasing the exposure time by a proportionate amount results in a radiograph that is less likely to be degraded by motion. As a rule, it is best to minimize the exposure time but maintain an appropriate mAs and scale of contrast. Increasing kVp increases the number of photons penetrating the patient and also darkens the film. This effect can be used within limits to correct an underexposure. The converse is likewise true.

When correcting a previously unsatisfactory film, underexposure or overexposure should be corrected by adjusting the mAs when examining areas of high contrast (skeleton) or by adjusting the kVp when examining areas of low contrast (thorax). This will maintain the same relative contrast for that anatomic area while adjusting the film darkness.

Establishing a technique chart for making radiographs makes it easy for the operator to arrive at a technique by simply correcting a standardized protocol for the size of the animal being examined and the anatomic area under consideration. It also ensures that radiographs of the same anatomic region will have a consistent appearance from animal to animal. A technique chart must be made for each machine. Some generalizations can be made, however. Exposure factors for the thorax should have mAs values ≤5 unless the animal is very large. Values of 10 for the abdomen and 15-20 for skeletal studies are appropriate. In many modern x-ray machines, the technique chart is built into the machine. The operator need only enter the body part and thickness and the machine automatically sets the technique. This is convenient and reduces mistakes in technique, but the settings may need to be altered to suit the specific equipment, film-screen speed, and the viewer’s preferences (eg, contrast level).

Automatic exposure control (AEC) is a system in which the operator sets the kVp and mA and the machine terminates the exposure at the appropriate time. If used properly, this system results in nearly identical film exposures between animals. However, appropriate kV settings are needed, and animal positioning is critical. Identical positioning between animals is required to achieve identical films. Placing the heart or lungs over the AEC sensor results in radically different appearing radiographs. AEC is probably most effective when large numbers of films are being done of the same anatomic area by the same personnel.

X-ray machines today are equipped with collimators that allow adjustment of the size of the beam to the size of the area being radiographed. This reduces the amount of scatter radiation generated, thus improving image contrast and detail. Scatter radiation is also the major source of radiation exposure to operators, so proper collimation is important in reducing this risk.

When a radiograph is taken, some of the x-rays are scattered. When the object being radiographed is ≥10 cm thick, scattering becomes a problem by causing unwanted exposure of the x-ray film. A grid, which is a thin plate made up of alternating thin strips of lead and plastic, can be placed between the animal and the film to reduce the scattered x-rays from exposing the film. The ability of a grid to remove scattered radiation is measured by the grid ratio. The grid ratio is determined by the height of the lead strips divided by the distance between them. A grid with an 8:1 ratio will eliminate more scattered radiation from exposing the film than will a grid with a 6:1 ratio.

Recording of the radiographic image has traditionally been done on specially optimized film. However, even the best silver halide film is relatively insensitive to x-rays. For that reason, the film is usually placed between specially designed phosphorescent screens—panels composed of microscopic phosphorescent crystals embedded in a plastic matrix that directs the spread of the phosphorescent light toward the film. These screens are much more sensitive to x-rays than film. When the x-ray strikes a crystal, it phosphoresces and the light exposes the film secondarily. This process of recording the x-ray image is much more efficient than using film alone and markedly reduces radiation exposure to the patient (sometimes by a factor of 100 or more) and the operator. It also reduces the amount of scatter radiation recorded on the image. The screens and film are contained in a lightproof cassette, which is transparent to x-rays.

Screens and film must be matched for spectral emission and sensitivity. Films produced by one company are generally not optimally sensitive to screens made by another, and it is inadvisable to mix screen and film brands. Screen and film combinations come in different speeds. The larger the crystals in a screen are, the more likely it is to interact with an x-ray and the greater the amount of light produced. Unfortunately, larger crystals also produce larger areas of light, which tend to decrease the detail of the film. Likewise, film with larger silver halide grains is more sensitive to the light creating the exposure but also reduces the detail or resolution of the final image. Therefore, fine grain films are matched to fine crystal screens, resulting in very detailed images that take more radiation to produce. The converse is true for large grain film and large crystal screens. The speed of these combinations is designated by a rating of 100-1,600, with 100 being relatively slow but with very good detail and 1,600 being very fast but with limited detail. Film-screen combinations with speeds of 200-800 are generally used in veterinary medicine. 200-speed systems are used for small body parts and skeletal imaging, while 800-speed systems are used for large abdomens in small animals and thoracic radiography in large animals. Choice of the proper speed system for a specific use is based not only on the area being radiographed but also on the capabilities of the machine. Small portable x-ray machines can be used for larger body parts with fast film-screen combinations, substantially improving the utility of these machines.

Darkroom:

Once the film is exposed, it must be processed in a darkroom to make the latent image recorded on the film visible and fix it so that the image remains unchanged once the film is brought into the light. Care should be taken to make sure that no exterior light enters the darkroom. Even very small amounts of white light will markedly fog a film and decrease its diagnostic quality. Safelights used to illuminate darkrooms include filters that remove the frequencies of the light to which the film is sensitive, so that the film will not be exposed. Films vary in their spectral sensitivity; when replacing a safelight filter, the spectral requirements of the filter must be specified.

Developing was traditionally done in hand tanks by placing the film on a rack and immersing it in tanks full of the processing chemicals. However, automatic processors are now readily available and economically feasible. Automatic processing systems improve processing quality and consistency and reduce the processing time. Relatively few films processed per week will justify the purchase of an automated processing system. In any case, film processing must be done in strict accordance with the specified time and temperature requirements of the film being used. These requirements have been standardized for many years, and automated systems are designed to meet them.

Whether processing is manual or automated, the chemicals must be handled with care. Contamination of the darkroom with chemicals can ruin film, screens, and clothes. Fumes from the chemicals may be harmful, and some people may be more sensitive than others, especially to the fixer solution. Cross contamination of the developer solution with fixer will inactivate it and require replacement of the developer. Improper handling of chemicals will result in many artifacts on films as well as potential health hazards to the operators.

Filmless Radiography:

Image recording systems developed recently do not require the use of film, screens, or processing chemicals. They have several advantages over conventional radiography: 1) radiographs cannot be lost; 2) there is no need for film storage; 3) the process allows for post-processing manipulation; and 4) images can be transmitted to a remote location for interpretation. These systems fall into 2 categories: 1) computed radiography (CR), in which a semiconductor plate contained in a cassette is exposed in the usual fashion and then read electronically; and 2) direct digital radiography (DR) in which a large array of photoelectric crystals is exposed in the usual fashion and then read in place electronically. In both systems, the radiographs are read and processed electronically by computer, which generates the image on a monitor. The digital images can then be stored electronically and made available to any computer with access to the image archive. The difference between the 2 systems lies in the intermediate step of exposing a plate in CR, which is then placed in a reader. These plates must be replaced periodically due to wear created during the reading process. There is also the issue of whether or not the latent image recorded by the reader is an accurate representation of the true image. The portability of the cassettes is an important benefit in large animal and equine radiography.

Direct digital systems expose the image recording device, which is then directly read by the computer. This is done by embedding millions of photodiodes or similar devices in the surface of the recording plates. Each element (referred to as a pixel) is connected to an electronic recording system that reads the exposure of each pixel and transmits that information to a computer. The computer then reconstructs the image based on the amount of exposure received by each of the photosensitive elements. The image is displayed rapidly and can be viewed before the animal is moved or repositioned. DR systems are very complex electronically and subject to the same insults as any complex computer system. However, when properly cared for, DR systems are durable and reliable. They do not require handling of the image recording plate, which reduces wear and tear on the system. Their main advantages over CR are image display speed and improved spatial and contrast resolution. The flexibility and reliability of these systems is rapidly improving and has reached the point of being introduced to clinical settings. As DR systems grow in capability and acceptance and decrease in cost, it is expected they will eventually replace both CR and traditional film systems in the majority of medical practice.

Animal Restraint:

Animals must be adequately restrained and positioned to obtain quality radiographic images. Many animals may be manually restrained by people dressed in appropriate protective apparel; however, manual restraint should be kept to a minimum. In some states, a manual restraint is not allowed except under explicitly defined circumstances. Sedation or short-acting anesthesia is often necessary. Chemical restraint lessens the need for manual restraint, which leads to fewer poor or unacceptable radiographs and usually shortens the amount of time required to complete the examination. In many instances, animals can be restrained using sandbags, tape, and foam pads. With some practice it is often possible to complete the radiographic examination in essentially the same time that it could have been performed using manual restraint, with the added benefit that the animal is less likely to injure personnel.

Animal motion may also be minimized by decreasing exposure time and maximizing mA to achieve the required mAs for the body region examined. Other technical adjustments, such as increasing the kVp or shortening the film focus distance, may be made in some cases. However, major changes in film focus distance will likely cause serious degradation of the image. In most instances it is preferable to chemically immobilize the animal as long as there is not a medical contraindication.

Radiation Safety:

Radiographic examinations must be performed with proper respect for radiation safety procedures. Diagnostic x-ray machines are potent sources of radiation and can, if improperly used, result in injurious exposure to personnel over time. The exposure factors used in modern x-ray systems are substantially lower than those used in the past but can still result in injury. It is never acceptable to hold animals without the use of lead-impregnated aprons and gloves to decrease exposure to personnel. Leaded gloves should not be used within the primary beam of the x-ray machine. These gloves and aprons reduce exposure from scatter radiation by a factor of ~1,000 but only reduce exposure from the primary beam by a factor of ~10. Thyroid shields and eye shields are also recommended, especially when radiographing large animals, as the techniques used there are sometimes quite high. Upper limb and skull studies in horses are particularly likely to result in substantial exposure to anyone holding the film or the horse.

Pregnant women and any personnel <18 yr of age should refrain from direct involvement in the making of radiographs whenever possible. If a pregnant woman is directly involved in the making of radiographs, she should wear an apron that completely encircles her abdomen. Individuals involved in the making of radiographic images should be monitored for radiation exposure. This is essential to identify and correct conditions that can result in excessive radiation exposure to personnel. Monitoring of exposure also provides evidence of proper adherence to radiation safety standards if questions arise as to whether an employee’s medical condition could be related to radiation exposure.

Interpretation of Images:

Radiographic images are complex 2-dimensional representations of 3-dimensional subjects that are generated in a format unfamiliar to the average individual. Interpretation of radiographic images is difficult for the novice. Substantial experience and attention to detail is required to become proficient. The cornerstone of radiographic interpretation is a properly positioned and exposed study. Studies that are poorly or inconsistently positioned are difficult to interpret, and improper technique further decrease the amount of information obtained from the radiograph.

Although interpretation is aided by experience, conscious use of a systematic approach to evaluation of the film will improve the reading skill of even very experienced individuals and will ensure that lesions in areas not of primary interest or near the edge of the film are not missed. Once all of the lesions on the study are identified, a rational cause for those lesions can be formulated. The maximum amount of information is derived from the radiographic study when interpretation is done in light of the clinical and clinicopathologic information available. In this way, the most likely cause for the animal’s condition can be determined.

Contrast Procedures:

Shortly after radiography developed as a diagnostic specialty, it became evident that radiographic exposure of film alone lacked sufficient contrast to evaluate many structures. Contrast procedures were developed to increase the native contrast of organs, in order to separate them from surrounding tissues. Contrast agents are iodine-based compounds that are radiopaque. IV and intra-arterial contrast agents increase the opacity of the blood and make vascular structures visible. Iodinated contrast agents are cleared primarily by the kidneys, making the collecting system of the urinary tract visible. Orally administered agents, primarily barium sulfate-based compounds, outline the mucosa and lumen of the GI tract. Intrathecal contrast agents allow evaluation of the spinal cord and meninges. These contrast procedures have been largely supplanted by modern imaging procedures, but many of them are still the best way of imaging the structures they are designed to evaluate. Many contrast procedures do not require special equipment and can be performed in the average veterinary practice, but interpretation is best performed by someone with experience.