Proper measurement and positioning are important to producing diagnostic-quality radiographs.
Safety should be the primary objective when working with radiation.
Reading radiographs requires the ability to recognize and analyze different radiographic opacities.
Radiographs are extremely useful in identifying emergency and general conditions in veterinary medicine. However, radiographs may be misinterpreted if not taken properly, so proper measurement and patient positioning are important. Radiation safety is also a concern when taking radiographs. Veterinary technicians who are knowledgeable about radiology and can take diagnostic-quality radiographs are valuable members of the health care team.
Please Note that while the veterinarian needs to know the technical details of taking a radiograph their responsibility is in interpretation of the films relative to the case.
Discovery of X-Rays
X-rays were discovered by accident. In November 1895, Wilhelm Conrad Röntgen, a German physicist, discovered x-rays while experimenting with a Crookes tube, an evacuated glass tube containing two electrodes through which an electrical current could pass (similar to a fluorescent light bulb).1,2 When the tube was shielded with heavy, black cardboard, a green fluorescent light could be seen on a screen a few feet away. Röntgen realized that the tube was radiating an “invisible light”1 through the cardboard, and through further experiments, he found that this radiation passed through most substances, including human tissues, and produced images of solid objects on film. Röntgen named the new type of radiation “x-rays” because it was unknown to the scientific world (“X” is used to designate an unknown in mathematics1) and because light is considered to travel in rays.
Safety should be the primary objective during radiography. Ionizing radiation (e.g., x-rays) is hazardous3 and can cause cellular damage if safety precautions are not followed. Sources of ionizing radiation in the radiography room are the primary beam, scatter radiation, and the x-ray tube head. Exposure to scatter radiation — x-rays that are diverted from the path of the primary beam through interaction with the patient — is a common radiologic hazard. Each veterinary hospital should establish a radiology safety program to protect its technical staff and patients from scatter radiation. This safety program needs to meet regulations set by the state department of health.
When radiation comes in contact with living cells, it can4,5:
Pass through cells with no effect
Produce cell damage that is repairable
Produce cell damage that is irreparable
Kill cells within the body
All living cells can be damaged by ionizing radiation, but rapidly dividing cells are the most sensitive.4,5 In human adults, tissues that are readily sensitive to radiation include the bones, lymphatic tissues, dermis, hematopoietic (blood cell-forming) tissues, and epithelial tissues.3-5 The eye and thyroid are also sensitive.
Persons under 18 years of age and pregnant women should not be involved in radiographic procedures. Younger individuals are still growing and are easily affected by radiation.3 They should only be involved with radiation if it is prescribed by a medical doctor, and they should not receive more than 0.005 sievert (Sv) per year, which is 10% of the maximum permissible dose for an adult.3
Pregnant women should not take radiographs because the developing fetus is sensitive to the effects of ionizing radiation. The degree of sensitivity depends on the stage of pregnancy and the dose received. The fetus is most sensitive to ionizing radiation during the first trimester. Exposure to ionizing radiation during the pre-implantation period (0 to 9 days) is most likely to be lethal to the embryo.4,5 During the organogenesis period (10 days to 6 weeks), radiation exposure can cause the fetus to suffer from skeletal and dental malformations and overall growth retardation. The least sensitive time for the fetus is the fetal period (6 weeks to term)4,5; however, growth and mental retardation can still occur during this period. Although not recommended, a pregnant woman who chooses to continue working around radiation should wear an additional dosimeter at waist level underneath her lead gown to monitor the dose of radiation the fetus might be receiving.3
Two types of biologic damage can occur from radiation — somatic and genetic.3″5 Somatic damage describes damage to the body that is manifested within the lifetime of the recipient.3,5 Although radiation can produce immediate changes within cells, the body’s ability to repair itself may delay the appearance of changes for a long time after exposure.3 In some cases, cellular damage may never be visible. Somatic damage is more extensive when the body is exposed to a single massive dose of radiation than when smaller, cumulatively equivalent, repeated doses are received,3 but it can result from several smaller doses administered over many years.5 Cancer, cataracts, aplastic anemia, and sterility are examples of somatic damage.3 Genetic damage from radiation occurs in the form of injury to the DNA in the genes of reproductive cells3″5 and is not detectable until future generations are produced.3,5 No amount of radiation is non-damaging.
Even under the best conditions, some exposure to scatter radiation will occur. The National Committee on Radiation Protection and Measurement (NCRP) has set the maximum annual permissible dose for occupational exposure to radiation at 0.05 Sv.3 Every facility involved with radiation should follow the NCRP guidelines to help protect staff from being exposed to excess radiation. Some states prohibit staff who are occupationally exposed to radiation (e.g., radiation therapy staff) from restraining patients for diagnostic radiography; therefore, staff not routinely involved with radiation need to assist.3 It is the responsibility of the person taking radiographs to limit his or her exposure to ionizing radiation in order to avoid exceeding the maximum permissible dose. Radiographs should not always be taken by the same technical personnel, especially if the facility takes multiple sets of radiographs daily.
Safety Monitoring Devices
Safety monitoring devices record the amount of radiation received during radiologic procedures. Monitoring devices must be worn during every procedure that involves radiation, and exposed devices must be sent to a federally approved laboratory for processing.3 Monitoring devices are usually worn for 1 month before they are sent to the laboratory, and replacements should be issued immediately so that there is no time when the radiography staff are not monitored.3 The amount of radiation each device receives is recorded under the wearer’s social security number, allowing a cumulative radiation dose to be recorded for each person. An individual total lifetime dose can then be calculated.3,5There are four main types of monitoring devices: film badges, thermoluminescent dosimeters (TLDs), optically stimulated luminescence badges, and ion chambers.3,5Film badges are the most common devices used to monitor scatter radiation.3,5 These badges consist of a lightproof plastic holder that contains a film sensitive to radiation. Wrist, clip-on, and ring badges are available. These badges can be worn on the belt, hand, or collar, depending on the anatomic area considered at most risk (e.g., gonads, hands, thyroid). Radiation passes through the plastic holder and exposes the film. The processing laboratory then develops the film and measures the level of exposure.3,5TLDs contain lithium fluoride or calcium fluoride crystals, which store energy produced by radiation exposure. These dosimeters are placed into small containers and worn by technical staff. When evaluated at the laboratory, the crystals undergo heat processing during which light is emitted from the crystals in an amount proportional to the amount of radiation the dosimeter has received. TLDs have advantages over film badges in that they can be worn for a longer time period, are reusable, and store information for years.3,5Optically stimulated luminescence badges are more advanced than film badges or TLDs. A thin layer of aluminum oxide is sandwiched between three element filters in a tight, lightproof package. The badges are analyzed by using a laser to stimulate the aluminum oxide layer. The laser causes illumination in proportion to the amount of radiation the individual has received. These badges are extremely sensitive to radiation, and even the lowest dose of radiation is recorded.5
The ion chamber is the most complex of the four monitoring devices.5 This device comes in the form of a pen and can be easily clipped onto a pocket. The chamber is charged before use, and with each exposure to radiation, it discharges.3,5 At the end of the monitoring period, the amount of ion discharge is read. The amount of discharge is proportional to the amount of radiation received.3,5Minimizing Exposure
Three ways to minimize exposure to radiation are commonly used — wearing lead shields, increasing distance, and reducing exposure time5 — but the best way to minimize exposure is to avoid retakes when possible.
Lead shields are required for all staff members involved in radiographic procedures. Lead gowns, thyroid shields, and gloves are available, as is leaded-glass eyewear. Lead shields that are worn during radiography should contain a layer of lead at least 0.5 mm thick.3″5 Lead shields are expensive and should be handled with care. Gowns should be laid flat or draped over a special vertical rack without any folds or wrinkles to prevent cracks in the lead.5 Gloves can be stored on a special vertical rack, or metal soup cans with both ends cut out can be placed inside the gloves to prevent cracks and provide air circulation to the liner, reducing the moisture build-up that can accumulate in lead gloves when not in use.3,5Gloves and gowns should be manually checked for cracks or holes frequently. Gloves should also be radiographed every 6 months; gowns, every 12 months.4,5 A common technique used for radiographing lead shields is 80 kVp and 5 mAs.4,5 This technique can be adjusted as needed to obtain proper density on the film. The radiograph of the protective apparel should remain clear after processing. Cracks in the lead will appear as increases in density or blackness on the radiograph.3
According to the laws of physics, the intensity of x-ray radiation decreases as the distance from the source increases; for example, if the distance from the source is doubled, the intensity is reduced to one-fourth its original strength.3,5 (This is known as the inverse square law.) In the radiography room, this means that staff exposure to x-rays may be decreased by simply standing farther away from the primary beam. A distance of at least 6 feet is considered to be safe from scatter radiation. Any personnel who remain in the radiology room to restrain the patient should always lean back and look away from the beam to protect their eyes during exposure. Gloves should always be worn properly, not simply draped over the hand, because scatter radiation can come from any direction, including under the table top.4,5Time
Reducing the time of film exposure is another factor in reducing radiation exposure to staff and patients. Exposure time can be reduced by using the fastest film-screen combinations available.4,5 Reducing the number of retakes and collimating down to the area of interest will also help reduce staff exposure to radiation.
A technique chart is a valuable resource for the technical staff taking radiographs. By providing a consistent method of choosing the proper exposure to create a diagnostic radiograph, a chart helps reduce the need for retakes, thereby reducing exposure to both the radiographer and the patient.2,6,7 Each x-ray machine should have its own technique chart because all machine models are different.2,6 A technique that works for one machine may not be the best technique for another machine even when the setting and anatomic area being radiographed are the same.
Developing a technique chart requires radiographing an “average” patient. The ideal subject is a 40-lb (18-kg) dog that is neither underweight nor overweight. The dog should be anesthetized so that the process runs smoothly. Several factors influence the development of the technique chart: screen speed, screen age, film speed, source-image distance, amount of beam filtration, temperature and time of film processing, and type of grid.6 Once the chart is developed, the correct technique setting (kVp and mAs) for use in individual patients is chosen based on tissue thickness and anatomic area of interest.
Proper patient positioning is important because radiographs of inaccurately positioned patients can be misinterpreted. The American Committee of Veterinary Radiologists and Anatomists has standardized the terms and abbreviations used in reference to positioning.2,8,9 These terms define the position and direction of the primary beam. In a two-letter abbreviation, the first letter designates where the beam enters and the second designates where the beam exits.2,8,9 For example, the abbreviation DV (dorsoventral) indicates that the primary beam enters through the dorsal side of the animal and exits through the ventral side. The terms left (L) and right (R) refer to the side of the animal facing the table; therefore, the designation “R lateral abdomen” indicates that the primary beam enters through the left side of the abdomen and exits through the right side of the abdomen.
Reading radiographs is based on recognition and analysis of structures with different relative radiographic opacities.10Each radiograph should be interpreted by a veterinarian, and a written report of the interpretation should be recorded as part of the patient’s medical record. It is recommended that a radiologist also read radiographs, if indicated, to provide an expert’s interpretation. Radiologists can also suggest additional imaging that could help in the diagnosis and treatment of a patient.
Radiographic opacity describes the ability of x-rays to penetrate tissue and other substances. Substances that allow fewer x-rays to pass through appear whiter on radiographic film; those that allow more x-rays to pass through appear darker. Radiographic opacity runs the spectrum from radiopaque (white) to radiolucent (black).10 The difference in the radiographic opacity of different substances and structures in the body makes their differentiation possible. The most common substances found in the body — gas, fat, soft tissue (water/muscle), bone, and metal — have distinctly recognizable radiographic opacities.4,10,11Gas is the most radiolucent substance in the body4,10 and can easily be recognized as the darkest areas on radiographs. Because gas is very radiolucent, it provides good contrast to visualize the more radiopaque structures or organs within the body.10 For example, soft tissues in the chest (e.g., heart, aorta) are easily visualized because contrast is provided by the air-filled alveoli and airways of the lungs.10 Some radiographic procedures introduce air into structures or organs (e.g., urinary bladder) to enhance radiographic contrast to help better visualize those structures.
Fat is more radiolucent (appears darker) than bone and soft tissue but more radiopaque (appears lighter) than gas. Fat provides good radiographic contrast to differentiate and visualize the edges of many organs and structures.10 For example, the omentum between the stomach and the spleen allows differentiation of the gastric wall from the spleen. If a young or emaciated animal is being radiographed, the lack of fat can prevent the visualization of many organs and structures.
Water and muscle appear as shades of gray on radiographs. This spectrum of radiopacities is considered normal for soft tissue. Soft tissues seen on radiographs can be either solid or fluid-filled organs or muscle (e.g., heart, liver, spleen, kidneys, urinary bladder). Variations in volume, thickness, and degree of soft tissue composition create variations in opacity that help identify different organs.10
Bone, which is primarily composed of calcium and phosphorus, absorbs more x-rays than muscle and appears whiter (more radiopaque) on a finished radiograph.4,10 Some variation in radiographic opacity within and between bones is normal because of differences in composition (e.g., the ratio of compact bone to spongy bone and of the cortex to the medullary canal). When bone is radiographed, it is assessed as more or less radiopaque than normal.10 When the patient is not being radiographed for bone pathology, the clarity and radiopacity of bone on the radiograph are two good indicators of the overall quality of the radiographic technique. Clear, white bones usually mean that the organs are as differentiated as possible.
Metal appears the whitest (most radiopaque) on a finished radiograph because it absorbs most x-rays.10,11 Metal may be seen on radiographs if the animal has swallowed a metal object (e.g., fishing hook, coin, sewing needle, key) or has sustained a gunshot wound with a retained bullet. Metal objects should be relatively easy to identify on radiographs because they will be the most radiopaque, but their position may be confirmed with surgery or endoscopy if they warrant removal.
Because radiographs are two dimensional, a minimum of two diagnostic radiographs should be taken at right angles to one another. Using different angles will help the interpreting veterinarian “see” the structures in three dimensions. Mistakes are often made when the veterinarian tries to save the client money by basing a diagnosis on only one view.
Standard projections for thoracic radiographs are right and left lateral and DV or VD. It is suggested that either the right or left lateral view be used consistently on every patient so that all staff members become familiar with the projections in terms of radiographic interpretation. When taking right and left lateral chest radiographs, the patient is placed in lateral recumbency with the front limbs pulled forward to avoid superimposing the triceps muscle on the cranial part of the lung.8,12 The hindlimbs should be pulled forward to maintain proper symmetry of the thorax.12 The cranial landmark for collimating the radiograph field is the manubrium, and the caudal landmark is the first lumbar vertebra.8,12 When taking a VD chest radiograph, the patient is placed on its back with its front limbs extended cranially.8,12 The landmarks are the same as those for lateral projections, but the radiographer needs to make sure that the sternum is superimposed or aligned over the spine.8,12 Positioning for DV chest radiographs is the same as for VD radiographs except that the patient is on its stomach.
The lungs, which are located inside the ribcage, are the largest organs in the thorax. Dogs and cats have six lung lobes, four in the right lung and two in the left lung. The cranial aspect of each lung is called the apex; the caudal aspect of each lung adjacent to the diaphragm is called the base.13 The borders of the lungs are referred to as cranial, dorsal, caudal, and ventral.13
When focusing on the lungs, the views recommended are right and/or left lateral and DV or VD. Lung radiographs should be taken at peak (full) inspiration. The patient’s breathing should be watched closely before taking the radiograph to ensure that exposure takes place during inspiration.12 VD projections produce better images of the lungs because they allow the patient to be positioned symmetrically and enable better inspiratory effort.8,12,13 VD images also allow better visualization of the accessory lung lobes and caudal mediastinum.12 Both right and left views are recommended when looking for lung diseases (e.g., pneumonia, metastatic tumors).13
In dogs, the cardiac silhouette can vary in size and shape depending on the size of the patient’s thorax (e.g., deep and narrow, shallow and wide). The shape of the dog’s heart is generally oval, but it appears more vertical and narrow on lateral views and more rounded and smaller on DV views. The heart is normally between 2.5 to 3.5 intercostal spaces in width (depending on the breed of dog) on lateral views and two-thirds the width of the chest cavity on DV views.14 In cats, the cardiac silhouette is more slender in diameter and has a more pointed apex, but it is still generally oval. A normal feline heart is about two intercostal spaces in width and occupies about 70% of the distance from the sternum to the thoracic spine.14 Young animals appear to have larger hearts than mature animals.
When focusing on the heart, a right lateral view should be taken along with a DV or VD projection. Right lateral is the recommended lateral view because a normal heart has a more consistent position on this projection.14 The choice of VD or DV view depends on doctor preference. A DV view seems to be preferred because it provides greater consistency of the heart’s position (closer to the sternum) and makes the heart easier to visualize and assess.14 The position of the patient for DV radiographs is also less compromising for patients in respiratory distress. If pleural effusion is suspected, a VD image is recommended because the lungs and heart can be obscured on DV view.14 Heart base tumors, canine dilated cardiomyopathy, congenital heart diseases, and congestive heart failure are among the diseases affecting the heart that radiographs can help diagnose.
Standard projections for abdominal radiographs are right and/or left lateral and VD views. Other views can be taken if needed (e.g., DV, right or left oblique, standing lateral with horizontal beam). The right lateral is the most commonly used projection in radiology because it allows longitudinal separation of the kidneys.12 For a right lateral radiograph, the patient is placed in right lateral recumbency with the hindlimbs extended caudally. Extending the hindlimbs caudally eliminates superimposition of the femoral muscles over the caudal portion of the abdomen. A foam pad can be placed between the femurs to help eliminate rotation of the pelvis; another can be placed under the sternum to elevate it to the level of the thoracic spine.12
For VD radiographs, the patient is placed in dorsal recumbency with its hindlimbs in normal flexion. Sandbags or a V-trough can help keep the patient in proper VD position. When collimating the radiographic field, the cranial landmark is the diaphragm and the caudal landmark is the femoral head.8,12 The exposure should be taken during the expiratory phase of respiration so that the diaphragm is cranially displaced and not compressing the abdominal contents.8,12 If the animal is large, it may be difficult to image the entire abdomen on one film. In these cases, two radiographs are needed — one of the cranial abdomen and one of the caudal abdomen. Abdominal radiographs can be used to help diagnose many conditions, such as enlargement of the liver, spleen, kidney, or stomach; splenic abscess or rupture; gastric dilatation”volvulus; intestinal obstruction; linear foreign body; perineal hernia; ruptured bladder; and cystic calculi.
The liver consists of six lobes in the dog and cat: left medial, left lateral, right medial, right lateral, caudate, and quadrate. Normal liver appearance on radiographs varies with the patient’s body conformation, age, overall body condition, stage of respiration, and posture. The liver usually has sharp margins. The cranial surface of the liver touches the diaphragm and appears larger on right lateral than on left lateral images. The liver can appear smaller during expiration than during inspiration because of the different positions of the normal diaphragm.15
The gallbladder is a pear-shaped, fluid-filled structure that consists of a body, a rounded end or fundus, and a neck that tapers into the cystic duct. It is located between the quadrate lobe of the liver medially and the right medial lobe of the liver laterally and is part of the biliary system. The gallbladder cannot be seen separately from the liver; therefore, it cannot be seen on radiographs unless there is a problem with it (e.g., if severely enlarged, the gallbladder might be seen as a mass protruding from the liver).15
The spleen is a flat, elongated, soft tissue organ located in the cranial left abdomen, caudal to the stomach. It consists of a dorsal extremity (the head), a midsection (the body), and a ventral extremity (the tail). Normally, the spleen has sharp, smooth margins. Its location depends on the position of the patient when the radiograph is taken and the condition of adjacent abdominal viscera. For example, if the stomach is empty, the spleen might be seen in the left abdomen, but if the stomach is distended with fluid, gas, or ingesta, the spleen might be seen more caudally in the central abdomen.15
The pancreas is a small, dense, soft tissue organ located adjacent to the cranial border of the transverse colon and consists of a left and a right limb. The left limb is located adjacent to the caudal border of the gastric body, and the right limb is medial to the descending duodenum. The normal pancreas is not visible on radiographs.15
Located within the rib cage, the stomach comprises the cardia, fundus, body, pyloric antrum, and pyloric canal. The radiographic appearance of the stomach depends on the volume and type of gastric contents (e.g., fluid, gas, ingesta) and the position of the animal when the radiograph was taken. Gravity usually moves fluid stomach contents to the dependent area (down side) of the stomach; gas rises to the nondependent area (up side).15
The small intestine includes the duodenum, jejunum, and ileum. Distribution of the small intestine within the abdomen varies depending on the patient’s conformation, its nutritional status, and the size, shape, and position of adjacent abdominal viscera (e.g., empty versus distended urinary bladder). In obese animals, fat within the abdomen can displace the small intestine into the central abdomen between the stomach and urinary bladder. In thin animals, the small intestine can extend from the liver to the pelvic cavity.15
The large intestine comprises the cecum, colon, rectum, and anal canal. In dogs, the cecum is the shape of a corkscrew or “C.” In cats, the cecum is a small, cone-shaped structure. The colon can be recognized by the shape, size, and location of the ascending, transverse, and descending sections. The rectum is located between the descending colon and the anal canal. The normal large intestine can be identified by the presence of gas, fluid, and fecal material within the lumen. The amount of matter in the colon affects its position, shape, and size. As a rule of thumb, the normal diameter of the colon is less than the length of the seventh lumbar vertebra and no larger than three times the diameter of the small intestine.15
The kidneys are located in the dorsal retroperitoneal space of the abdomen in normal dogs and cats. In dogs, the right kidney is located near T13 to L1; in cats, it is located near L1 to L4. The position of the left kidney is more variable but is near L2 to L4 in dogs and near L2 to L5 in cats. Both kidneys should have smooth margins and be similar in shape and size. A normal canine kidney is bean shaped; a normal feline kidney is more rounded. The kidneys can be easily seen in cats and obese dogs. They are difficult to visualize in young cats and dogs, emaciated animals, or animals with retroperitoneal disease.16
The urinary bladder is a rounded or teardrop-shaped organ located within the caudal abdomen cranial to the pubis and ventral to the rectum and descending colon. The urinary bladder can vary in size and, when full, can be displaced cranially in the small intestine.16
Radiographs are a valuable diagnostic tool in veterinary medicine, and radiography technology continues to advance. Digital radiography now makes taking radiographs even easier by eliminating the traditional steps of using a film cassette and developing the film in the darkroom. Instead, the image appears on a computer screen within seconds of exposure, allowing the waiting veterinarian to begin interpretation sooner and reducing delay in patient care. However, until the use of radiation is eliminated altogether, proper safety precautions, radiographic technique, and patient positioning will continue to be important.
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7. HURD C: DEVELOPING A SMALL ANIMAL TECHNIQUE CHART, IN HAN C, HURD C (EDS): PRACTICAL DIAGNOSTIC IMAGING FOR VETERINARY TECHNICIANS, ED 3. ST. LOUIS, MOSBY, 2005, PP 44-48.
8. HAN C: SMALL ANIMAL RADIOLOGY, IN HAN C, HURD C (EDS); PRACTICAL DIAGNOSTIC IMAGING FOR VETERINARY TECHNICIANS, ED 3. ST. LOUIS, MOSBY, 2005, PP 57-123.
9. LAVIN L: GENERAL PRINCIPLES OF POSITIONING, IN LAVIN L (ED): RADIOLOGY IN VETERINARY TECHNOLOGY, ED 3. PHILADELPHIA, WB SAUNDERS, 2003, PP 147-153.
10. BIERY D, OWENS J: PRINCIPLES OF RADIOGRAPHIC INTERPRETATION, IN BIERY D, OWEN J (EDS): RADIOGRAPHIC INTERPRETATION FOR THE SMALL ANIMAL CLINICIAN, ED 2. BALTIMORE, WILLIAMS & WILKINS, 1999, PP 9-13.
11. HAN C: ACHIEVING RADIOGRAPHIC QUALITY, IN HAN C, HURD C (EDS): PRACTICAL DIAGNOSTIC IMAGING FOR THE VETERINARY TECHNICIAN, ED 3. ST. LOUIS, MOSBY, 2005, PP 10-21.
12. LAVIN L: SMALL ANIMAL SOFT TISSUE, IN LAVIN L (ED): RADIOLOGY IN VETERINARY TECHNOLOGY, ED 3. PHILADELPHIA, WB SAUNDERS, 2003, PP 225-234.
13. BIERY D, OWENS J: THORAX (NONCARDIAC), IN BIERY D, OWEN J (EDS): RADIOGRAPHIC INTERPRETATION FOR THE SMALL ANIMAL CLINICIAN, ED 2. BALTIMORE, WILLIAMS & WILKINS, 1999, PP 147-184.
14. BIERY D, OWENS J: HEART, IN BIERY D, OWEN J (EDS): RADIOGRAPHIC INTERPRETATION FOR THE SMALL ANIMAL CLINICIAN, ED 2. BALTIMORE, WILLIAMS & WILKINS, 1999, PP 185-216.15. BIERY D, OWENS J: GASTROINTESTINAL SYSTEM, IN BIERY D, OWEN J (EDS): RADIOGRAPHIC INTERPRETATION FOR THE SMALL ANIMAL CLINICIAN, ED 2. BALTIMORE, WILLIAMS & WILKINS, 1999, PP 223-260.
16. BIERY D, OWENS J: URINARY SYSTEM AND ADRENAL GLANDS, IN BIERY D, OWEN J (EDS): RADIOGRAPHIC INTERPRETATION FOR THE SMALL ANIMAL CLINICIAN, ED 2. BALTIMORE, WILLIAMS & WILKINS, 1999, PP 261-277.