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L. Gaschen1
1Louisiana State University, Veterinary Clinical Sciences, Baton Rouge, USA
Lorrie Gaschen, PhD, DVM,,, DECVDI
Louisiana State University School of Veterinary Medicine, Baton Rouge, LAHepatobiliary Organs
The feline liver can sometimes be tricky to examine
in obese cats. A common error is to mistake the
falciform fat for the liver, commonly seen in beginning sonographers. Careful attention to the hyperechoic liver capsule helps the sonographer to see the separation between the fat and the liver. Comparatively, the falciform fat should be hyperechoic compared to the liver.
The use of ultrasound and ultrasound-guided tissue sampling has now surpassed the use of survey radiography for the diagnosis of many liver diseases
due to its broad availability and greater sensitivity. Diffuse parenchymal disease generally affects all
lobes and may appear normal, iso- or hyperechoic. Examples include: cholangiohepatitis, diffuse prenodular (early) metastatic carcinoma or sarcoma, round cell neoplasia (lymphoma, mast cell disease and histiocytic sarcoma), patchy or diffuse fatty infiltration, vacuolar hepatopathy, storage diseases (amyloidosis, copper), toxic hepatopathy and early degenerative changes associated with micronodular hyperplasia and fibrosis. The overall accuracy of ultrasound as the sole criterion for discriminating among the categories of diffuse liver disease is <60% in cats. It is generally not possible to make a final diagnosis based on the combination of sonographic findings and biochemical and hematological data with diffuse liver disease. Tissue sampling, preferably for histological examination, is required for a definitive diagnosis in most instances, even if the liver appears sonographically normal.
Vacuolar changes in the liver associated with lipidosis usually cause hepatomegaly in conjunction with
diffuse hyperechogenicity and rounded borders. Inflammatory disease can be associated with diffuse hypoechogenicity. If acute hepatitis or cholangiohepatitis is present, the liver may appear to have high
contrast; a hypoechoic parenchyma with pronounced hyperechogenicity of the portal veins. Chronic inflammation of the liver will usually result in hyperechoic or mixed echogenicities. When fibrosis or cirrhosis is
present, the liver may be smaller and hyperechoic. If nodular hyperplasia develops such as with vacuolar hepatopathy, the liver may appear more heterogeneous and nodular such as in neoplastic disease. Other differentials for this pattern include amyloidosis in cats.
Focal or multifocal changes in the liver parenchyma are easier to identify sonographically than diffuse changes. Hypo-, hyper- and anechoic lesions are easy to identify as they contrast better with the surrounding parenchyma. Therefore, cystic lesions are the easiest to detect, even when very small.
Anechoic cavitary structures in the liver can be due to necrosis, neoplasms or cysts. Cysts structures generally have sharply defined borders, can be round or irregular in shape and may even contain hyperechoic septa within them. Causes include congenital cysts, due to cavitations following trauma, biliary pseudocysts or parasitism. Unfortunately, biliary cystadenomas and cystadenocarcinomas may appear similarly.
Neoplastic disease of the liver may manifest as
diffuse, multifocal or focal disease sonographically. Diffuse disease is usually due to round-cell neoplasia. Lymphoma, histiocytic sarcoma and mast cell tumor are the most common neoplasms that may lead to diffuse changes that remain sonographically undetectable. Carcinomas tend to be diffusely spread throughout the liver and often lead to a mixed pattern.
Malignant nodules have a highly varied appearance and size. They may appear as hypo- or hyperechoic nodules, target lesions or heterogenous ill-defined nodules. Hypoechoic nodules can be due to nodular hyperplasia, metastases, lymphoma, histiocytic sarcoma, primary neoplasia, necrosis, hematomas and abscesses. For this reason, tissue sampling is critical to a definitive diagnosis and the presence of hepatic nodules is not synonymous with malignancy. Hepatic target lesions have a positive predictive value for malignancy of 74% and emphasizes the fact that histological type cannot be predicted by the presence of target lesions.
Hepatic abcessation occurs rarely in small animals and may appear similar to a primary tumor, granuloma or hematoma due to their highly variable sonographic features. Sonographically, they may be round to irregular in shape with either a hypoechoic central region or of mixed echogenicity. Reverberation artifacts may be detected due to gas accumulations within the necrotic tissue. Focal peritonitis may be seen with abcessation and include free peritoneal fluid and focal hyperechoic mesentery.
The feline gall bladder is typically ovoid but can be bi-lobed. The cystic duct is highly tortuous in the cat. The gallbladder wall is approximately 1mm thick in the cat and the bile is anechoic. Gallbladder wall thickening and sludge are indicative of gallbladder disease.
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