Principles of Radiographic Interpretation of the Abdomen

Donald E. Thrall

As in the thorax, acquisition of abdominal radiographs is one of the more common radiographic examinations performed in small animal practice. Abdominal radiography is rarely performed in the adult horse, except for the assessment of the cranioventral and/or ventral aspects of the abdomen for sand collections or enterolith detection. The entire abdomen can be radiographed successfully in foals and miniature horses, but the conspicuity of abdominal organs is less than in dogs or cats because of the relatively reduced amount of peritoneal fat and the larger volume of the equine abdomen occupied by the gastrointestinal tract.

Abdominal sonography is also used commonly to assess the canine and feline abdomen, and it has found many applications in these species. However, in dogs and cats, sonography should not be viewed as a replacement for abdominal radiography. Sonography provides for real-time assessment of organ texture and blood vessels that cannot be obtained with radiography, but sonography does not provide for a global assessment of the abdomen, and the quality of the information obtained from an abdominal ultrasound examination is highly dependent on the abilities of the operator. With regard to equine abdominal radiography, the relatively poor quality of abdominal radiographs in adult horses has led to abdominal sonography assuming a more primary role for abdominal imaging in many equine patients.

Computed tomography (CT) imaging is also useful for evaluation of the canine and feline abdomen, and it would be expected that most, if not all, abnormalities detected using radiography and sonography would also be detected using CT. Overall, however, the use of CT use for canine and feline abdominal disease is low compared with radiography and sonography. Magnetic resonance imaging (MRI) is rarely used for abdominal imaging in dogs and cats, except for assessment of portosystemic shunts. Neither CT nor MRI is used for abdominal imaging in the horse.

Nomenclature

As with the thorax, the naming of lateral abdominal radiographs made with the patient recumbent, as commonly done in dogs and cats, violates the point-of-entrance to point-of-exit nomenclature system described in Chapter 5. If that system were followed, a lateral view made with the dog or cat in left recumbency would be termed a right-left view because the x-ray beam strikes the right aspect of the abdomen and exits the left aspect of the abdomen. But, for the abdomen, the terminology for lateral projections has been abbreviated to describe the side of the patient lying on the x-ray table. Thus, for example, an abdominal radiograph acquired with a dog or cat lying on the left side would be termed a left lateral.

In the horse, where lateral abdominal radiographs are usually made with the patient standing and a horizontally directed x-ray beam, the point-to-entrance to point-of-exit system should be used. Thus, lateral radiographs should be described as either left-right or right-left views.

In all species, ventrodorsal (VD) and dorsoventral (DV) radiographs of the abdomen are named according to the point-to-entrance to point-of-exit system.

Preparation

In the dog and cat, unless one is (1) looking specifically for small abdominal masses, small mineralizations such as urethral calculi, or abnormalities of the peritoneum; or (2) preparing the patient for an abdominal contrast study, such as a barium upper-gastrointestinal examination or an excretory urogram, no special preparation such as fasting or enema administration is needed before abdominal radiography. If a bowel obstruction is being considered, enema administration is contraindicated because it will alter the native pattern of bowel gas and fluid. The native appearance of bowel gas and fluid is critical to the radiographic diagnosis of bowel obstruction and should not be altered by enema administration.

There are no specific circumstances in the horse where abdominal fasting or enema administration is recommended before abdominal radiography.

Positioning—Dog and Cat

A lateral and a VD radiograph will be adequate for assessing the abdomen completely in many canine and feline patients. However, the abdomen contains a valuable inherent contrast medium—gas. Thus, by routinely acquiring left and right lateral radiographs in addition to the VD view, redistribution of bowel gas between the two lateral views can provide valuable additional information that can make the difference between obtaining the diagnosis or not (Fig. 35-1). Dorsoventral radiographs of the abdomen are rarely made and should not form the basis of the orthogonal view to complement the lateral projections. A DV view may have to suffice when a patient cannot be positioned in dorsal recumbency for a VD view, but the abdomen will be more crowded, organ conspicuity will be reduced, and portions of the pelvic limbs are often superimposed on the abdomen (see Fig. 35-9 for an example).

Fig. 35-1 Right **(A)** and left **(B)** lateral radiographs of the cranial aspect of the abdomen of a cat with acute vomiting. In the right lateral view **(A),** the stomach contains gas and there is heterogeneous material in the duodenum *(white arrow),* but there is no diagnosis. In the left lateral view **(B),** gas from the stomach fills the proximal aspect of the duodenum outlining a foreign object *(black arrow),* supporting a diagnosis of bowel obstruction.

As in the thorax, the effort required to complete an abdominal radiographic examination is obviously related to the number of projections. There is no reason to limit the number of projections acquired routinely, especially with a direct digital system where the image is available instantly and there are no expendable supplies. Not acquiring both right and left lateral views routinely in addition to the VD view will result in some diagnoses being missed. Admittedly, the increased number of exposures per patient has the potential to increase the occupational radiation dose to personnel, but as long as proper protection principles are followed, this potential increase will not be realized.

It is important that the entire abdominal cavity be included in the image. The field of view should extend from just cranial to the diaphragm to a few centimeters caudal to the coxofemoral joints. For lateral views, the pelvic limbs should not be pulled caudally but should be kept perpendicular to the spine. This relaxes the caudoventral aspect of the abdominal wall and reduces crowding. Likewise, in the VD view, the pelvic limbs should be flexed into a so-called frog-leg position rather than being pulled caudally. If the pelvic limbs are pulled caudally for the VD view, crowding will be increased, and skin folds will be created in the lateral thigh and/or caudal abdomen region. These skin folds create noticeable superimposed opacities that will interfere with assessment of the caudal aspect of the abdomen (Fig. 35-2).

It is usually possible to include the entire abdominal cavity in one 14-inch × 17-inch image, even in the largest dogs. For the occasional huge dog where this is not possible, then each projection (left lateral, right lateral, VD) should be divided into cranial and caudal portions. This effectively doubles the requisite number of images to evaluate the entire abdominal cavity.

Right and left lateral and VD views of the abdomen will suffice for almost all canine and feline patients. If the status of the urethra in male dogs is of concern, as for assessment of urethral calculi, the right and left lateral views should be supplemented with a third lateral view made with the pelvic limbs pulled cranially. This provides an unobstructed view of the ischial arch and os penis, allowing for assessment of urethral calculi without superimposition of the pelvic limbs (Fig. 35-3).

A compression radiograph can be used to clarify a suspicious finding that cannot be confirmed because of superimposition.¹ The theory is to apply mild compression to displace superimposed organs from the field of view, leaving an unobstructed view of the region of interest. This technique can be applied to assess structures such as the uterine body (lateral view) or the caudal aspect of the retroperitoneal space (VD view). Specialized inflatable compression paddles are available, but a wooden spatula or wooden spoon can also be used. If using a film-screen system, the kVp should be decreased by approximately 15% because the compressed part will be reduced in thickness. Of course, care should be taken not to place compression on organs that are obviously diseased, and the amount of pressure placed should always be gentle (Fig. 35-4). The increased use of ultrasound for abdominal assessment has reduced the need for compression radiographs, but they remain a convenient method to increase the accuracy of radiographic interpretation in some patients where ultrasonography is not possible.