Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

256 Small Animal Radiolo g y and Ultrasono graphy
slightly caudal to the thirteenth rib and chondral cartilage, whereas in deep-chested breeds
(e.g., Collies) the edge of the liver may not even extend to the thirteenth rib. In older dogs, the
liver may be more ventrally and caudally located than in younger ones. Regardless of its position,
the caudal ventral border of the liver should come to a relatively sharp point. A useful
parameter for liver size is the fundic–pyloric axis. The axis is defined by a line drawn on the lateral
view through the center of the gastric fundus to the center of the pylorus. This axis should
be parallel to the last two to three intercostal spaces. The caudal margin of the caudate lobe of
the liver may be identified cranial to the right kidney in obese dogs. It can be recognized
because of its triangular shape and position adjacent to the cranial pole of the right kidney. The
appearance of the liver in cats is similar to that in dogs; however, in obese cats a large amount
of fat may accumulate in the area of the falciform ligament, ventral and caudal to the liver, and
may markedly displace the liver dorsally (Fig. 3-7). This radiographic appearance tends to suggest
falsely that the liver is abnormally small. On the ventrodorsal view, the liver frequently is
hard to delineate because many structures are superimposed over it. It is bordered cranially by
the diaphragm. The caudal border is located at the level of the cranial wall of the nondistended
stomach. The liver should extend to both lateral body walls, but in obese animals there may be
fat between the body wall and the liver. The gallbladder, although located slightly to the right
of the midline and in the cranial ventral area of the liver, is not seen routinely on survey
abdominal radiographs because there are rarely significant amounts of fat between it and the
surrounding hepatic lobes. In the cat, the ventral margin of the gallbladder may be seen on the
lateral radiograph as an oval structure extending ventrally beyond the ventral liver margin.
The liver is not always accessible to ultrasonographic scanning, because a portion of it
may be obscured by the overlying stomach. In dogs with deep thoracic conformation, the
liver may be cranial to the last rib and relatively inaccessible to subcostal scanning. Scanning
through the intercostal spaces may be hindered by the overlying lung. The entire liver should
be examined systematically starting on one side and sweeping across to the other. This may
require using an approach in which you start beneath the costal arch on one side and
progress across the abdomen to the other side. The liver can be recognized during an ultrasonographic
examination because of its position cranial to the stomach and caudal to the
diaphragm. When the liver architecture is abnormal, identification of the bright echogenic
line of the diaphragm is a reliable indicator of the liver’s position. The liver parenchyma is
less echogenic (blacker on a black background) than the spleen and ranges from similar to
more echogenic (whiter) than the renal cortex. The architecture of the liver is composed of
a uniform texture, which is interrupted by short, highly echogenic paired parallel lines surrounding
an anechoic lumen that represent the portal veins and anechoic linear structures
that represent the hepatic veins (Fig. 3-8). The resulting pattern is heteroechoic with the uniformly
echogenic hepatic cells interrupted by the portal and hepatic veins. The hepatic and
Fig. 3-7 A 4-year-old female domestic
short-haired cat with a 2-day
history of vomiting. The lateral view
revealed marked accumulation of fat
in the region of the falciform ligament
(black f), which separated the
normal liver from the ventral body
wall. The cat responded to symptomatic
therapy. Diagnosis: Obesity.