CHAPTER 7 The Abdomen

Standard radiographic views of the abdomen include a lateral view and a ventrodorsal (VD) view; dorsoventral (DV) views of the abdomen are rarely made and are not described herein. In general, abdominal radiographs should be made without fasting or enema administration. This way the image represents the natural condition of the patient and avoids the alteration of normal bowel content patterns that would result from an enema. In addition, the gastric contents are sometimes relevant in terms of making the correct diagnosis. All figures in this chapter are of patients for which no special preparation was undertaken prior to radiography. The instances in which preparation for abdominal radiography is needed is beyond the scope of this work.

The distribution of gas in the gastrointestinal tract often helps lead to a correct interpretation. Obtaining both right and left lateral radiographs in addition to the VD view has merit and is recommended for all abdominal examinations. This is especially relevant when the gastrointestinal tract is the primary organ of interest, because the distribution of gas in opposing lateral views will be different and can lead to clarification of findings. Left versus right recumbency also influences the radiographic appearance of some other organs, notably the spleen and kidneys. The effect of left versus right lateral recumbency on the radiographic appearance of the abdomen will be illustrated for the stomach, spleen, and kidneys.

Pulling the pelvic limbs caudally during radiography can complicate interpretation. In the VD view, pulling the pelvic limbs caudally leads to skin folds, which create linear opacities that can interfere with assessment of the caudal aspect of the abdomen. It is preferable to flex the pelvic limbs for the VD radiograph so that the caudal aspect of the abdominal wall is not taut but is relaxed with greater lateral expansion and less crowding (Figure 7-1). In lateral view, femurs should be approximately perpendicular to the lumbar spine and neither pushed cranially nor pulled caudally.

Figure 7-1 Ventrodorsal radiographs from two dogs. In A, the pelvic limbs are flexed, allowing relaxation of the caudal abdominal muscles and greater expansion of the caudal aspect of the abdomen. In B, the pelvic limbs are pulled caudally, creating skin folds (solid white arrows) that can interfere with interpretation, and the caudal aspect of the abdominal cavity is narrower and more crowded. Also in B, the edge of the positioning trough has created a linear opacity (solid black arrows) that can also interfere with interpretation. The edge of positioning devices should not be included in the primary x-ray beam.

Radiographic visualization of abdominal organs requires that there be adequate normal surrounding fat to provide contrast. This is true for the intraperitoneal and retroperitoneal spaces. Fat provides contrast to abdominal organs because it is of lower physical density and lower effective atomic number (Figure 7-2). Fat mainly provides enhanced visualization of the periphery of organs; the character of the interior of abdominal organs relates to their constituency. Most abdominal organs are of soft tissue opacity, but the lumen of the gastrointestinal tract commonly contains material that is either more radiopaque than soft tissue, such as bone fragments, or less radiopaque than soft tissue, such as gas. The homogeneity and radiographic opacity of the intraperitoneal fat and retroperitoneal fat should be similar. Discrepancies between the opacity of fat in these two compartments can indicate the presence of retroperitoneal or intraperitoneal disease.¹

Figure 7-2 Lateral radiograph of a 6-year-old Labrador retriever (A), and close-up view of the cranioventral (B) aspect of the abdomen. Various organs are visible. Fat provides contrast for visualization of the outer margin of organs (hollow white arrowheads in B).

A relative lack of fat in the abdomen, as seen in emaciated animals, reduces the conspicuity of the margins of abdominal organs (Figure 7-3). Serosal margin conspicuity is also reduced in very young patients, likely due to a relatively reduced volume of abdominal fat (Figure 7-4). It has been theorized that peritoneal fat in young patients may be more hydrated than fat in adults, thus increasing its opacity and decreasing its ability to provide radiographic contrast.²

Figure 7-3 Lateral radiographs of a 16-year-old domestic cat (A) and a 2-year-old Basenji (B). The conspicuity of the margin of abdominal organs in each patient is poor due to the lack of intraperitoneal and retroperitioneal fat to provide contrast. With this appearance, radiographic assessment of the abdomen is compromised.

Figure 7-4 Lateral radiograph of an 11-week-old golden retriever. Conspicuity of serosal margin detail is diminished due to the relative lack of abdominal fat; this is a common finding in young animals. Margin visualization is, however, better than in patients that are undernourished and emaciated (compare with Figure 7-3).

Cats, particularly those in a state of positive caloric balance, can accumulate massive amounts of intraperitoneal and retroperitoneal fat. The largest collections of abdominal fat in cats are in the retroperitoneal space and in the cranioventral aspect of the abdomen. Fat accumulation in the cranioventral aspect of the abdomen, some of which is in the falciform ligament, can create a large mass effect in this region (Figure 7-5). The falciform ligament is formed by the two layers of the ventral aspect of the mesogastrum, thus fat in the falciform ligament is actually extraperitoneal.³ Falciform fat is commonly misinterpreted as peritoneal effusion by inexperiencd interpreters; basic principles of radiographic opacity dictate this cannot be true or there would be border effacement of the liver margin and loss of serosal margin detail.

Figure 7-5 Right lateral radiograph of an 8-year-old domestic cat. There is a large collection of fat in the cranioventral aspect of the abdomen (solid black arrowheads) that is causing dorsal displacement of the liver and caudal displacement of the small bowel. This fat, which is in the falciform ligament, is often misinterpreted as peritoneal fluid. This cannot be fluid, however, due to its lower radiographic opacity than adjacent soft tissue organs. Fluid would cause border effacement of liver and bowel. This cat also has abundant fat in the retroperitoneal space surrounding the kidneys and in the inguinal region.

LIVER

The liver is located in the cranial aspect of the abdomen, between the diaphragm and stomach. Most of the visible liver mass is to the right of midline (Figure 7-6). The liver should be of homogeneous soft tissue opacity.

Figure 7-6 Left lateral (A) and ventrodorsal (B) radiographs of a 5-year-old German shepherd. The liver is the homogeneous soft tissue opacity between the diaphragm and the stomach. In B, note that most of the visible liver mass is to the right of midline. A1 and B1, corresponding labeled radiographs. In A1, the solid lines represent the accepted normal range of the gastric axis (the gastric axis is an imaginary line connecting the fundus and pylorus). The dotted line represents the approximate location of the gastric axis in this dog. The visible portions of the liver are shaded gray in A1 and B1; this shading does not encompass the entire liver volume because there are portions of the liver that are superimposed on stomach that cannot be seen distinctly. Antrum, gastric antrum; fundus, gastric fundus; LI, liver; SPL, spleen.

There is individual variation in the size of the normal liver in the dog and the cat, making the distinction between a mildly enlarged or mildly small liver from a normal liver highly subjective and relatively inaccurate. The position of the stomach is one parameter that is commonly used as an indicator of liver size. It is generally accepted that if the liver is normal, a line connecting the gastric fundus with the gastric pylorus in a lateral radiograph, termed the gastric axis, will range between being perpendicular to the spine or angled such that the gastric axis is parallel with the intercostal spaces (see Figure 7-6, A1). If the gastric axis extends outside of this range, cranially or caudally, then the possibility that the liver is decreased or increased in size, respectively, should be considered. It is important to note that the accepted normal range of the gastric axis in lateral radiographs is only a guideline, and slight deviation in the position of the gastric axis outside of the accepted normal range can be encountered when there is no pathologic liver abnormality.

Another commonly used criterion for liver size in lateral radiographs is extension of the liver beyond the costal arch (the costal arch is formed by the border of the last few costal cartilages). However, the costal arch is not a precise demarcation between a normal liver and an enlarged liver, and it cannot be used as a definitive landmark to assess liver size. It is important to recognize that a normal liver can extend beyond the costal arch (Figure 7-7). Unfortunately, the cutoff point between a normal size liver and an enlarged liver based on extension beyond the costal arch is vague.

Figure 7-7 Lateral (A) and ventrodorsal (B) radiographs of a 4-year-old miniature schnauzer and corresponding labeled radiograph (A1). In A1, the costal arch, defined by the caudal limit of the costal cartilages, has been marked with a dotted line. The liver in this patient clearly extends beyond the costal arch but is normal. There were no clinical or laboratory signs of liver dysfunction, and sonographic examination of the liver was normal. Extension of the liver beyond the costal arch cannot be taken as indisputable evidence of liver enlargement. LI, Liver; SP, spleen.

Another situation in which a normal liver might extend beyond the costal arch is a distended stomach placing pressure on the liver and causing its ventral aspect to slide caudally beyond the costal arch (Figure 7-8).

Figure 7-8 Lateral radiograph of a 9-year-old Rottweiler that ingested an excessive amount of dog food. The stomach is markedly distended with food. The liver extends beyond the costal arch in the cranioventral aspect of the abdomen, which likely is due to displacement caused by pressure from the enlarged stomach and not liver enlargement.

A change in shape to the margin of the portion of the liver that extends caudal to the costal arch from a gradual taper (see Figures 7-6, A, and 7-7, A) to a more rounded configuration is another finding often associated with liver enlargement.

In a deep-chested dog (such as an Afghan Hound or collie), the gastric axis is normally more perpendicular. In contrast, the gastric axis in a shallow-chested dog (such as a Dutch pug or a bulldog) typically is more caudally angled.

The gastric axis is not used as a measure of liver size in the VD view. Rather, the position of the pyloric part of the stomach is an alternate landmark. With liver enlargement, the pylorus is displaced caudally and medially in the VD view. In the lateral view, liver enlargement causes dorsal and caudal displacement of the gastric pylorus.

The criteria for evaluating liver size in cats are the same as in dogs. However, it is important to recognize that the gastric pylorus in cats is normally located more medially than in dogs, residing nearly on the midline. Thus, the position of the gastric pylorus is rarely used to assess liver size in VD abdominal radiographs in cats (Figure 7-9).

Figure 7-9 Ventrodorsal radiographs of a 12-year-old domestic cat without (A) and with (B) barium in the stomach, and of a 7-year-old miniature schnauzer with barium in the stomach (C). In the cat (A), note the midline location of the gastric pylorus (P). In A, the patient was slightly rotated when the radiograph was made, giving the pylorus a mildly exaggerated midline position. The pylorus is easier to identify when there is barium in the stomach (B). The undulations in the pyloric antrum seen in A and B are due to normal peristalsis. In the dog (C), note the more rightward position of the gastric pylorus (P) versus the position in the cat.

The gallbladder is situated between the quadrate lobe medially and the right medial lobe laterally, in the cranioventral aspect of the liver. In most patients, the normal gallbladder is not seen radiographically. Occasionally, especially in cats, a round opacity is superimposed on the ventral aspect of the liver or extends slightly beyond the margin of the ventral aspect of the liver. This round opacity can be is created by a normal, but modestly distended, gallbladder (Figure 7-10). This can be seen when there is no associated evidence of liver or gallbladder disease. However, visualization of this opacity is not always a normal finding and, if clinically indicated, the liver should be assessed using other methods.

Figure 7-10 Lateral radiograph of a 1½-year-old Persian (A) and a 4-year-old domestic cat (B). In each radiograph there is a round opacity extending beyond the ventral margin of the liver that is consistent with enlargement of the gallbladder (solid black arrows). This can be found without evidence of biliary disease and can be a normal variant in some subjects.

SPLEEN

The spleen, a lymphatic organ, is located in the left hemiabdomen. The spleen should be of homogeneous soft tissue opacity. The proximal aspect of the spleen, commonly referred to as the head of the spleen, is located in close approximation to the gastric fundus. It is held loosely in this position by the gastrosplenic ligament and the short gastric arteries and veins that communicate with the splenic vessels.⁴ These short gastric arteries and veins can be less well-developed in the cat.⁵ The distal extremity of the spleen, commonly called the tail of the spleen, is more mobile and variable in position than the proximal extremity.

The size of a normal spleen is highly variable, especially in the dog. In the dog, the spleen is usually of adequate size to be identified radiographically. It is common for the spleen to be seen in both the lateral or VD projections of the canine abdomen. In general, the spleen is smaller in the cat than in the dog. In the cat, the spleen usually can be identified in the VD but not the lateral radiograph. Occasionally, no portion of the normal feline spleen can be seen radiographically. If the spleen is seen in lateral and VD abdominal radiographs of a cat, the possibility of enlargement should be considered.

As noted, the size of the normal spleen varies widely, according to its functional status and whether chemical restraint has been used for radiography. Determining the clinical significance of a spleen that appears radiographically enlarged is not usually possible without other assessments, such as ultrasonography and cytologic sampling.

When the spleen is visualized radiographically, it is important to realize that the entire spleen is not seen in a single radiographic view. When the x-ray beam is oriented along or parallel to (called end-on projection) a length of the spleen, it typically has a triangular shape. If the body of the spleen is struck perpendicularly by the primary x-ray beam (called side-on projection), there usually is insufficient absorption of the x-rays to see that portion of the spleen (Figure 7-11). Sometimes, portions of the spleen proximal and distal to a region struck end-on by the x-ray beam also absorb sufficient x-rays to be conspicuous. The extent of the spleen that can be seen depends on the thickness of the spleen, its orientation to the primary x-ray beam, and the contrast resolution of the imaging system.

Figure 7-11 Diagram of a spleen being struck by a primary x-ray beam oriented either side-on (perpendicular) or end-on (parallel) to the long axis of the spleen. When the long axis of the spleen is struck side-on, there is inadequate absorption of x-rays for the spleen to be seen. When the long axis is struck end-on, the number of x-rays absorbed is increased, creating opacity sufficient to be seen. What is seen radiographically when the spleen is struck end-on is only a fraction of the total spleen volume.

In lateral abdominal radiographs of the dog, the distal extremity of the spleen is the portion seen most commonly (Figure 7-12, A-B). In lateral recumbency, the distal extremity lying adjacent to the ventral abdominal wall is oriented end-on to the primary x-ray beam. Occasionally, as mentioned previously, the portions of the spleen extending peripherally from the distal extremity can also be seen (see Figure 7-12, A1 and B1; white arrows).

Figure 7-12 Left lateral (A) and right lateral (B) radiographs and corresponding labeled left lateral (A1) and right lateral (B1) radiographs of a normal 9-year-old Dachshund. The distal extremity of the spleen has its typical triangular shape in the ventral aspect of the abdomen. The spleen is relatively large in this dog, likely due to the chemical restraint used to facilitate radiographic positioning. Conclusions regarding splenic disease cannot be made from a spleen with this appearance. As noted in the text, the triangular structure is only a fraction of the total splenic volume. In both the left and right lateral views, a portion of the spleen extending from the triangular region can be seen (solid white arrows in A1 and B1). Identifying this opacity as spleen is facilitated by it being contiguous with the familiar triangular splenic opacity; in isolation this opacity would likely not be diagnosed as spleen. In A, the two focal opacities ventral to the spleen, and in (B) the focal opacity superimposed on the spleen, are summation shadows due to superimposed nipples. LK, Left kidney; RK, right kidney; SP, spleen; SP?, opacity possibly due to proximal extremity of spleen. For legend see facing page.

With a moderately sized normal spleen, the proximal portion might be oriented in a way that creates an ill-defined mass effect in lateral radiographs in the region between the gastric fundus and the kidneys, without being visualized as a distinct triangular structure (see Figure 7-12, A1 and B1). Other imaging modalities would be needed to interpret this finding correctly, because a mass effect between the gastric fundus and kidneys in lateral radiographs could represent an abnormal mass.

In VD abdominal radiographs of the dog, it is the proximal extremity of the spleen that typically is seen (Figure 7-13). Again, this is due to the portion of the proximal extremity lying adjacent to the left abdominal wall being oriented end-on to the primary x-ray beam. Similar to lateral views, portions of the spleen extending peripherally from the proximal extremity can sometimes be seen (see Figure 7-13).

Figure 7-13 Ventrodorsal radiograph of a normal 9-year-old Dachshund (A), and corresponding labeled radiographs (A1, A2). The triangle lateral and caudal to the gastric fundus in (A) is the typical appearance of the proximal extremity of the spleen as seen in ventrodorsal views. In this dog, portions of the spleen extending from the region struck end-on by the primary x-ray beam can be seen (solid white arrows in A1, dotted lines in A2). These peripheral regions of the spleen cannot be seen in every dog. Their identification as spleen is facilitated by the fact that they are contiguous with the familiar triangular splenic opacity. SP, Spleen.

In the dog, due to its mobility, the distal extremity of the spleen can be located in the cranioventral aspect of the abdomen and can create an oblong opacity that is often misdiagnosed as an enlarged liver lobe (Figure 7-14). The key to correctly distinguishing between spleen and liver in this circumstance relates to the location of the cranial aspect of the suspicious opacity. If the opacity is the distal extremity of the spleen, its cranial margin will not extend cranial to the gastric pylorus and there will be a plane of fat opacity between it and the liver (see Figure 7-14), except in emaciated subjects. If the suspicious opacity is an enlarged liver lobe, the opacity can be traced cranial to the gastric pylorus and there will not be a plane of fat opacity between it and the liver.