THE MERCK VETERINARY MANUAL
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Radiography

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Radiography comprises the majority of diagnostic medical images generated in veterinary practice, but other imaging modalities such as ultrasonography, CT, MRI, and nuclear imaging are also very important and commonly available in specialty practices and academic centers. Imaging provides a large amount of information by noninvasive means. It does not alter the disease process or cause unacceptable discomfort to the animal. Although radiography itself is painless, sedation is often desirable to reduce anxiety and stress associated with the procedure, to promote acquisition of good diagnostic studies with minimal repeats, and to control pain associated with manipulation of animals with painful disorders such as fractures and arthritis.

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 two of the three exposure factors that can be set on most x-ray machines. Kilovoltage potential (kVp) is the highest potential voltage achieved at any given 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 decreased contrast (long-scale contrast) on the final image. 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 with limitations. 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, for a number of reasons relating to the production and absorption of x-rays, 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 are 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 image is related to the total number of photons reaching it. Therefore, increasing mA increases image contrast. Changes in mA settings are relatively linear; increased contrast is desirable when tissue densities are similar (eg, soft-tissue components of the musculoskeletal system). However, increasing mA generally results in more heat loading on x-ray tubes, thus limiting exposure times and reducing tube life.

The third major parameter in making a radiographic exposure is exposure time. Increasing the exposure time increases the number of photons produced and hence the darkness of the image. For exposures in the general diagnostic range, this is a linear function; as is the case with increasing mA, increasing exposure time generally results in greater heat loading of the x-ray tube than increasing kVp, once again potentially shortening tube life.

All three of the above parameters are interdependent, with exposure time and mA so much so that the term milliampere-seconds (mAs) is usually used to indicate the product of these two factors. Increasing the mA and decreasing the exposure time by a proportionate amount results in a radiograph 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 subject and so darkens the image. This effect can be used within limits to correct an underexposure. The converse is likewise true.

When correcting a previously unsatisfactory image, underexposure or overexposure should be corrected for 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 to make 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. However, some generalizations can be made. 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 species, 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 (detector) 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 image exposures between animals. However, appropriate kV settings are needed, and consistent animal positioning is critical. Identical positioning between animals is required to achieve identical images. Placing the heart or lungs over the AEC sensor results in radically different appearing radiographs. AEC is probably most effective when large numbers of images are being done of the same anatomic area by the same personnel.

X-ray machines 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, improving image contrast and detail. Scatter radiation is also the major source of radiation exposure to operators, so proper collimation is important to reduce this risk.

When a radiograph is made, some of the x-rays are scattered. When the object being radiographed is ≥10 cm thick (15 cm for digital systems), 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 if both have the same number of lead lines per centimeter.

Digital recording is rapidly increasing in usage, but radiographic images have traditionally, and still are, commonly stored on specially optimized film. However, even the best silver halide film is relatively insensitive to x-rays. For that reason, the x-ray film is usually placed between specially designed phosphorescent screens—panels composed of microscopic phosphorescent crystals embedded in a plastic matrix that directs propagation of the phosphorescent light toward the film. These screens are much more sensitive to x-rays than is film. When the x-ray strikes a crystal, it causes the crystal to phosphoresce, 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 subject (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 decreases 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, whereas 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.

Use of radiographic film is rapidly being phased out except for special purposes, with digital image capture likely leading to a cessation of film production for most purposes. As advances in digital detector systems continue, it is likely that images produced on digital detector plates will be indistinguishable in detail and quality from those acquired on film. It is already becoming difficult to find film cassettes and screens for medical radiography sold by primary vendors.

As digital image capture replaces radiographic film, darkrooms will no longer be required. However, because radiographic film is still used by many veterinary practices, a brief description of darkroom procedures is provided.

Once the film is exposed, it must be processed in a darkroom to make the latent image recorded on the film visible and fixed so that the image remains unchanged once the film is brought into the light. Care should be taken to be sure 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 the film will not be exposed. Films vary in their spectral sensitivity; therefore, when replacing a safelight filter, the spectral requirements of the filter must be specified.

Automatic processing systems improve processing quality and consistency and reduce the processing time compared with traditional hand film developing. Relatively few films processed per week can 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 inactivates it and requires replacement of the developer. Improper handling of chemicals results in many artifacts on films as well as potential health hazards to the operators.

Image recording systems do not require the use of film, screens, or processing chemicals. They have several advantages over conventional film radiography: 1) radiographs cannot be lost if adequate data safeguards are used; 2) there is no need for film storage and its attendant space and environmental requirements; 3) the process allows for manipulation and enhancement after the image has been recorded; 4) images can be transmitted electronically to a remote location for immediate interpretation; 5) the images are generally available more quickly, usually within 30 sec; and 6) there is no need for a darkroom.

These systems can be divided into two categories: computed radiography (CR), in which a semiconductor plate contained in a cassette is exposed in the usual fashion and then read electronically inside a special reader that detects the magnitude of electrostatic charge on each of the semiconductor elements within the plate; and direct digital radiography (DR), in which a cesium iodide scintillator array absorbs the x-rays, producing a light pulse detected by a large array (millions) of amorphous silicone photodiode/transistor elements. In both systems, the electrical output from each of the detector elements is proportional to the number of x-rays that strike the detector element and is mathematically quantifiable, hence the term “digital images.” In both systems, the data produced are processed by a computer, which generates the image on a monitor according to a previously determined processing algorithm that is specific to the region being radiographed. Processing algorithms are critical to the development of diagnostic images. In many displaced systems, the algorithm can be altered to provide enhancement of various features of the image. The digital images are then stored electronically and made available to any computer with access to the image archive.

The difference between the two 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 because of wear created during the reading process. There is also the issue of whether 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 practice situations in which radiographic images are produced in multiple places. CR systems are also still considerably less expensive than all but the simplest DR systems and generally still have higher resolution capabilities, which may be important for imaging smaller anatomic parts.

DR systems are very complex electronically and subject to the same insults as any complex electronic system. They are particularly sensitive to shock and electronic interference. 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 advantage over CR is image display speed. DR systems have been developed that do not require a cable to communicate between the detector and the computer processing the data into an image. The cable has been replaced by wireless communication on specified electrical magnetic frequencies that are unlikely to be interfered with by other electromagnetic devices such as cell phones and electronic equipment. Although they are still much more expensive than systems incorporating a cable connection between the detector and computer, such systems are particularly suited for use in equine ambulatory practices. Images can also be sent to the storage system in a wireless manner in many areas.

The flexibility and reliability of digital radiography systems, whether wireless or wired, have improved to the point that they are almost universally used in human radiology departments as well as in many veterinary hospitals. As DR systems grow in capability, reliability, ease of use, and resolution, and decrease in cost, it is expected they will eventually replace both CR and traditional film systems. Although currently DR still cannot match the spatial resolution of either standard speed film or CR systems, newer systems are narrowing the gap. This low spatial resolution is offset to a large degree by improved contrast resolution. Because of their inherent high contrast, direct digital systems are also becoming the choice imaging device for very large animals.

The advent of digital imaging has led to the development of special image storage systems and formats. The data stored on computers must be protected from loss and corruption. Loss of data can be guarded against by storing identical sets of data on different computers in different geographic locations and/or by copying the data files to optical storage media that are then kept in a safe location. Protection of the data from corruption is a more thorny issue. Because images stored in a digital format are easily manipulated by various computer programs, it is possible that they could be altered (accidentally or deliberately) to reflect a different situation than the actual one. For this reason, many electronic image formats are not recognized as legal documents and are not acceptable in a court of law. Because of this potential for alteration or abuse, a special medical image format has been developed and agreed on by the American College of Roentgenology, the American College of Veterinary Radiology, and others as the standard format for medical image generation and storage. This is the Digital Imaging and Communication in Medicine (DICOM) III format. The key feature of this format is the presence of a hidden header in the image file that records all manipulations of the image or the header each time the image is saved. The header also contains a large amount of information about the patient and production factors of the image, which must be specified before creation of the image. This makes accidental or malicious manipulation of the image much easier to trace. Another and even more important benefit of the DICOM III format is that it makes images easily transferable to other sites for referral interpretation or patient referral. No digital imaging system should be purchased that does not conform to the DICOM III standard.

A common misconception associated with digital radiography is that it results in decreased radiation requirements to provide a diagnostic image. Although there has been some progress in this direction, most current digital radiography systems require essentially the same amount of radiation to produce a diagnostic image as with film. The computer system will attempt to reconstruct the image with the proper level of contrast and edge enhancement; however, data poor images will often lack detail despite this enhancement.

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

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.

Paradoxically, the development of direct digital radiography systems that allow images to be viewed within 30 sec of production has led to an increase in the number of radiographic images typically produced. Because the images do not have to be processed in a darkroom or through secondary systems such as in computed radiography, individuals making the images often attempt to improve positioning of the animal multiple times. Particularly in instances when the animal is being manually restrained, there is a proportional increase in radiation exposure to both the animal and the holders. This can be avoided by taking a few extra seconds to properly position the animal for the first image.

Proper positioning is also important to maximize the diagnostic content of the x-ray examination. In many cases, improper positioning or radiographic examination can result in a misdiagnosis or inability to appreciate major lesions. Perhaps the best example of this is in thoracic radiography. Both right and left lateral recumbent radiographs are recommended in dogs and cats. This is done because positioning of the animal on its side results in rapid relocation of fluids to and atelectasis of the downside lung. The result is compression and increased radiographic opacity of the dependent lung. Soft-tissue nodules, sometimes of considerable size, can be obscured by this phenomenon. Similarly, lesions affecting the pylorus may be more evident on a left lateral radiographic examination of the abdomen than on a right lateral. Another example of positioning affecting interpretation is frequently encountered when evaluating the coxofemoral joints for hip dysplasia in dogs. If the legs are excessively abducted, the femoral necks will appear thickened, mimicking the production of osteophytes and potentially leading to a misdiagnosis. Radiography of the spine without the aid of anesthesia in an acutely paraparetic or paraplegic animal may result in inability to properly position the animal for optimal visibility of vertebral structures because of the pain such positioning produces.

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 the hands and body of personnel from scattered radiation. 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 reduce exposure from the primary beam by only a factor of ~10. Thyroid shields are considered mandatory, and eye shields in the form of lead-impregnated plastic “glasses” are also recommended, especially when radiographing large animals, because the exposures used are sometimes quite high and the orientation of the beam is more likely to be horizontal. Upper limb, cervical spine, and skull studies in horses are particularly likely to result in substantial exposure to anyone holding the film/detector or the horse.

Proper collimation of the x-ray beam is an important and integral part of radiation protection. If the x-ray beam extends beyond the animal, then that radiation contributes to increased scatter radiation and personnel exposure. Any image in which the entire field of the detector or film is exposed is probably under-collimated, unless the animal extends to the limits of the detector. In addition, with digital radiography systems, excessive amounts of exposure outside the subject can result in false interpretation of the data by the reconstruction algorithm and substantially degrade image quality. If this occurs, the exposure must be repeated with proper collimation to achieve an acceptable image. In most instances, the x-ray beam should be collimated to ~1 cm outside the subject limits to provide optimal image quality and radiation protection for personnel.

Pregnant women and any personnel <18 yr old should refrain from direct involvement in taking radiographic images whenever possible. If a pregnant woman is directly involved in taking radiographs, she should wear an apron that completely encircles her abdomen.

Although federal and state authorities have set maximal limits for both extremities and whole-body radiation exposure for occupationally exposed personnel, the principal of "as low as reasonably achievable" (ALARA) should always be adhered to. The currently set limits allow occupational whole-body exposure to be roughly the same as that which occurs from environmental sources. However, in many veterinary teaching hospitals with large radiographic case loads, the occupational exposure is held to <10% of the permitted values by use of proper protective equipment and radiographic techniques. There is no reason for veterinarians or technicians in private practice to ever receive exposures approximating the allowed limits unless they are heavily involved in specialized interventional radiography.

Individuals involved in taking 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. Several companies provide this service for a relatively nominal fee.

Radiographic images are complex two-dimensional representations of three-dimensional subjects that are generated in a format unfamiliar to the average individual. Substantial experience and attention to detail is required to become proficient in interpretation. The start 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 decreases the amount of information obtained from the radiograph.

Although interpretation is aided by experience, conscious use of a systematic approach to evaluation of the image will improve the reading skill of even very experienced individuals and ensure that lesions in areas not of primary interest or near the edge of the image are not missed. However, many studies have shown that experience is the best teacher with regard to evaluation of radiographs. So, although anyone will become more adept at image interpretation with time, those individuals who interpret large numbers of images will be the most proficient. Even proficient individuals can miss lesions that are unfamiliar to them or so-called "lesions of omission." A lesion of omission is one in which a structure or organ generally depicted on the image is missing. A good example of this is the absence of one kidney or the spleen on an abdominal radiograph. Therefore, particular attention to systematic evaluation of the image is very important. It is perhaps best to begin interpretation of the image in an area that is not of primary concern. For instance, when evaluating the thorax of an animal with a heart murmur, the vertebral column and skeleton should be evaluated first because if a substantial lesion is identified for the heart, it is possible that the skeletal structures may not be examined.

It is essentially impossible to evaluate radiographs without a preexisting bias as a result of knowledge of the history, physical examination findings, and previously performed laboratory results. This bias can easily promote under-evaluation of the image by focusing on only the area of interest associated with the bias.

Interpretation of radiographic images depends on a pleural knowledge of anatomy and understanding of disease pathology. Anatomic changes, such as in size, shape, location/position, opacity, and margin sharpness represent the basis of radiographic interpretation. In addition, the degree of change, whether generalized throughout an organ or associated with other abnormalities, must be evaluated. The presence of lesions that do not affect the entirety of an organ, such as focal enlargement of the liver or focal opacification of the lung field, are strongly suggestive of localized disease such as tumors or bacterial infections. Conversely, lesions causing generalized change throughout an organ such as the liver or kidneys are most suggestive of a systemic disease such as viral infections or toxicities. Combinations of lesions in different locations or organs also help narrow down the potential diagnosis. Careful attention to the basic principles of interpretation and use of a careful systematic approach will often provide answers not readily apparent on initial examination.

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. However, many diseases can cause similar radiographic lesions, and radiographs must be interpreted in light of the entire gestalt of lesions present and not based on any single lesion if multiple abnormalities are present. In many cases, it is appropriate and advisable to seek the opinion of a radiologist for interpretation of radiographic images, particularly as the number of radiographic studies available and potential diagnoses proliferate.

Radiographic exposure of film alone lacks sufficient contrast to evaluate many structures; therefore, contrast procedures are used to increase the native contrast of organs and lesions, to separate them from surrounding tissues. Contrast media are radiopaque compounds that have extremely low toxicity, although there are well-recognized hemodynamic alterations seen after administration of IV contrast agents. These consist of primarily a reflex hypotension followed by a rebound mild hypertension. In extreme cases, the hypotension can lead to vascular collapse and even anaphylaxis. This effect is thought to be primarily associated with the hypertonic nature of ionic contrast agents and is markedly less evident when nonionic agents are used. For this reason, nonionic agents have almost completely replaced the ionic agents as IV contrast material. IV and intra-arterial contrast agents are generally iodine based and increase the opacity of the blood, making 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 are also iodine based and allow evaluation of the spinal cord and meninges. Many of these contrast procedures have been largely supplanted by modern imaging procedures, but many of them are still the best way to image the structures they are designed to evaluate and should not be forgotten if modern imaging procedures fall short. 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 extensive experience and training in the interpretation of radiographic images.

Last full review/revision December 2014 by Jimmy C. Lattimer, DVM, MS, DACVR, DACVRO

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