Division of Diagnostic Imaging, Faculty of Veterinary Medicine, Utrecht University
Good quality abdominal radiographs of dogs and cats with abdominal complaints may provide valuable information that is often sufficient to obtain a tentative or even definite positive or negative diagnosis.
The full potential of a radiographic examination is only achieved when the radiographs are analyzed in a systematic, well-organized, step-by-step manner.
As successful interpretation of radiographs depends for the better part upon the quality of the radiographs being examined, interpretation begins with an evaluation of the technical quality of the radiographs, with special attention for correct exposure, darkroom technique, and positioning.
Technical characteristics for abdominal radiography include low KVP (60-80 KVP) and consequently high mAs settings. By using this technique, the inherently poor abdominal contrast differences are enhanced and abdominal detail is increased. With exception of cats and small dogs, a grid or buckey must be used. As a consequence, for good quality abdominal radiographs a powerful, high-milliamperage X-ray machine is required. Darkroom technique must be optimal to secure standardized radiographs of high detail. For this reason, the use of a small tabletop developing machine is advised.
Initial survey radiographs of the abdomen include both a left-lateral recumbent projection (DS) and a ventrodorsal recumbent projection (VD) of the abdominal cavity, including the entire region between diaphragm and pelvis. It should be realized that different parts of the abdomen vary in thickness, and, for that reason, require different exposure settings, e.g., the epigastric area under the ribs needs considerable more exposure than the much thinner mid-abdominal region.
Although the presence of food in the gastrointestinal tract and of urine in the urinary bladder represents the actual state of the animal, it causes crowding of the abdominal viscera in the central abdomen and unwanted obscuring of abdominal detail. Therefore, it is advisable to remove as much as possible of these contents before the survey abdominal radiographs are made. The best way to achieve this is by asking the owner to go out for a walk with the dog, prior to the examination. If necessary, a mild laxative can be administered rectally. With cats, preparation is not performed as easily. Here, a fasting period of 24 hours, in combination with a rectal laxative, may provide good results.
With non-cooperative, aggressive or severely traumatized animals, sedation or general anesthesia may be necessary to ensure exact positioning during the radiographic examination.
It is important that each normally visualized structure is identified and determined to be normal or abnormal. For this reason, the principles of radiographic imaging and normal radiographic anatomy should be understood. Visualization of an abdominal structure depends upon the identification of visceral profiles that is determined by the differences in radiographic density of abdominal structures that are lying in close contact with one another. Only, when a visceral structure with a certain radiographic density lies in close contact with another structure with a different radiographic density can its boundary be seen, and from this the appearance of the rest of the structure may be inferred.
The abdominal image is composed out of four radiographic densities, gas - fat - fluid/soft tissue - bone. However, crowding of the fluid-dense abdominal organs in the central abdomen results in a radiographic image that is inherently poor in contrast. The amount of gas and fat in the abdominal cavity and its viscera strongly influences the interpretation of this radiographic image.
Gas (most of it is swallowed air) is normally present within the gastrointestinal tract and provides a ready means for examination of the stomach and bowel loops. Displacement of bowel loops many times presents a meaningful clue for detection of abnormalities of neighboring viscera.
Fat surrounding the abdominal viscera in adult animals provides sufficient contrast to outline the borders of most of the larger abdominal organs. Absence of fat as seen with puppies and kittens or with emaciated animals makes abdominal radiographs unreadable.
Fluid in larger amounts as seen with peritoneal ascites overshadows the contrasting effect of the abdominal fat and, as a result, severely compromises abdominal detail and organ visualization.
Bony structures are surrounding the abdominal cavity, and sometimes are present in the gastrointestinal tract as part of the ingesta.
For interpretation of abdominal radiographs a protocol must be established that will ensure that no structures are overlooked. A complete protocol for interpretation of abdominal radiographs could read as follows:
1. Interpretation begins with an evaluation of the overall abdomen, for size, shape and density. The best way to do this is by viewing the films on the view box, at first from a distance, and then, in second instance, from nearby.
2. The next step is a careful examination of the abdominal borders: musculature of abdominal walls and sublumbar region, spine, pelvic area, diaphragm, and parietal peritoneum.
3. Next, the intra-abdominal density is judged. At this point, different density patterns can be recognized.
4. This is followed by identification of the different abdominal viscera, with evaluation of position, size, shape, density and architecture.
5. At last, spot areas of interest are scrutinized.
Abdominal density patterns
Radiographic density in the abdominal cavity can be normal, increased or decreased
The normal density pattern is present in the mature animal, with good anatomical detail due to the presence of intra-abdominal or retroperitoneal fat.
The increased density pattern can be generalized or localized.
A generalized increased density is seen when intra-abdominal fat is either absent or overshadowed by a large amount of free peritoneal fluid (peritoneal ascites). The result is a homogenous ground-glass appearance, with subsequent loss of abdominal detail and structure recognition. This sign is not specific for either hemorrhage, exudate or transudate, urine, bile, or cellular infiltration. Abdominal paracentesis is required for a more specific diagnosis.
A localized increased density can be ill-defined or well-circumscribed.
A localized, ill-defined density is normally seen when the digestive tract is filled or when a distended bladder causes crowding of the abdominal viscera. Next to this, small amounts of intra-peritoneal fluid as seen with regional peritonitis or localized hemorrhage, adhesive changes, and focal neoplastic infiltration may also produce a localized loss of abdominal detail.
A localized, well-circumscribed density is found in the presence of an abdominal mass, foreign body, or with focal calcification.
Pathologic changes in the abdomen may become radiographically evident by a change in size or shape of a normal structure, the presence of an unusual area of increased density in the (retro)peritoneal cavity, or by displacement of a normal structure from its expected location. Displacement of abdominal viscera follows the path of least resistance, away from the expanding mass lesion. As a consequence, knowledge of the normal position of an organ and its relationship to surrounding structures makes it possible to predict the radiographic appearance of a mass lesion of a certain organ, or makes it possible to determine the most likely structure from which a mass has arisen. Especially, the stomach, large bowel and urinary bladder are structures that will be easily displaced by mass lesions from neighboring visceral structures.
Mass lesions must be described according to their size, shape, position, margination, density and architecture (pattern within the density: homogenous, granular, calcified), and, if possible, to their function.
First, origin and etiology of the mass should be understood. On the lateral radiograph, the mass must be associated with an anatomical area of the abdomen. Next, the mass must be identified on the ventrodorsal film in an attempt to reduce the number of possible organs for the origin of the mass.
Physiologic enlargement of the spleen, stomach, intestines, urinary bladder and uterus may mimic pathology.
Pathologic enlargement of visceral structures can be referred to neoplastic disease (primary or metastatic), hematoma, cystic enlargement, inflammatory or granulomatous changes, congestion due to vascular compromise, or dilatation as seen with ileus, pyloric obstruction, gastric or mesenteric volvulus, or abnormal enlargement of the uterus.
Margination of the mass lesion and possible extension to other organs must be evaluated. A distinct margin of the mass probably means that the lesion is confined to the original organ. An ill-defined, indistinct border of the mass relates either to direct extension of the disease process into surrounding structures or to peripheral tissue reaction as seen with localized peritonitis. Localized fluid collection in the immediate vicinity of the mass produces the same effect, i.e., splenic hemangiosarcoma.
At last, the possibility of more than one mass lesion must always be considered.
And, thoracic radiography for control of metastatic lung disease should always precede surgical intervention of an abdominal mass lesion.
The decreased density pattern can be diffuse resulting from obesity with excessive intra-abdominal fat accumulation or from gaseous distension of the gastrointestinal tract due to meteorismus or paralytic ileus. Severe pneumoperitoneum can be seen following abdominal surgery (days to weeks), and as a result of traumatic rupture/perforation of the gastrointestinal tract.
Focal decreased density originates from segmental gastrointestinal gas collections, free peritoneal gas accumulation due to abdominal wall penetration or acute gastrointestinal rupture or perforation, or from emphysematous gas collections within diseased visceral structures due to gas-producing bacteria.
In understanding and applying the basic principles of abdominal radiography, the following statement is true: "Radiographic findings, both positive and negative, assist in narrowing clinic diagnostic considerations, frequently localize the site of involvement, and at time are typical for a specific disease process"(P.F. Suter).