School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA
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. Cyst 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 emphasize the fact that histological type cannot be predicted by the presence of target lesions.
Hepatic abscessation 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 abscessation and includes free peritoneal fluid and focal hyperechoic mesentery.
The feline gall bladder is typically ovoid but can be bilobed. The cystic duct is highly tortuous in the cat. The gallbladder wall is approximately 1 mm thick in the cat and the bile is anechoic. Gallbladder wall thickening and sludge are indicative of gallbladder disease.
Cholecystocentesis is helpful in these instances for performing bacteriology and cytology. Extrahepatic bile duct obstruction often results in dilation of the common bile duct, between the porta hepatis and the major duodenal papilla. The papilla is fairly easy to identify in cats as it is round and echogenic, located at the wall of the cranial duodenum, a short distance from the pylorus. The common bile duct is easily visible in most cats as a thin anechoic tube ventral to the portal vein, another large and easy to identify structure in the cat. Only about half of cats with obstructive biliary disease have a dilated gallbladder. The bile duct should be traced and observed for wall thickening, intraluminal echogenic or hyperechoic shadowing material and for papillary masses. Inflammatory disease of the papilla can be as obstructive as a cholelith.
The spleen is small in healthy cats and is very laterally located in the left cranial abdomen. Sometimes it is necessary to scan intercostally to find the spleen. Usually if one cannot find the spleen, it is small and dorsal and the sonographer needs to strive to find it intercostally.
Splenomegaly in the cat can be caused by extramedullary hematopoiesis, hyperplasia, chronic inflammation, immune-mediated disease, neoplasia, and infectious organisms. Lymphoma is by far the most common neoplasm affecting the spleen. Mast cell tumor and histiocytic sarcoma are also possible and often cause splenomegaly. Histoplasmosis is a systemic fungal infection that affects many organs in the cat, including the spleen. Sonographically, it appears enlarged and diffusely hypoechoic and in some instances can be mottled.
Gastritis is very difficult to diagnose sonographically and there are few specific signs. Cats rarely get ulcers unless induced by an overdose or chronic use of anti-inflammatory agents. Ulcers can lead to a focal wall thickening that, if not masked by gas and ingesta, can be identified sonographically. Linear foreign bodies anchored at the tongue may also cause bunching up of the stomach in addition to the jejunum. Hair balls are identified as a heterogenous structure with gas reverberations and shadowing.
The gastric wall in the cat is usually about 3 mm thick. Thickening with loss of layering is mainly due to neoplasia. Gastric neoplasia can only be identified with sonography approximately 50% of the time compared with endoscopy. Gastric lymphoma often leads to a transmural, hypoechoic wall thickening, often diffuse or within a large section of the stomach.
The duodenum is very midline and to find it sonographically, one must focus on the porta hepatis, identify the stomach and trace it rightward and the pylorus and duodenum will be easily visible. Continuing, trace the duodenum along its short length and notice the small papilla at its cranial end for entrance of the common bile duct and pancreatic duct.
Randomly distributed, 2–3 mm in thickness with distinct five wall layers. The most common abnormality of the small intestine is muscularis thickening seen in both inflammatory bowel disease and lymphoma. Linear foreign bodies have a specific appearance where the bowel is plicated together and often a thin hyperechoic band is evident pulling them together.
The Ileocecocolic Junction (ICCJ)
The feline cecum is a small bulbous organ having a subtle curvature (concave side toward the ileum), located in the right abdomen ventral to the descending duodenum. A small constriction demarcates the transition with the colon and the ileum enters the ascending colon obliquely from the left, just distal to the cecocolic transition. The position of the ileum relative to the cecum is fixed by the presence of the ligamentum ileocecalis, in which the ileocecal lymph nodes are located. These lymph nodes are usually paired and can be found along the concavity of the cecum. The ascending mesocolon contains one to five colonic lymph nodes.
Normal kidneys are 30–45 mm in length. Fat deposited in the renal tubules in cats leads to a more hyperechoic cortex in some animals. Bilateral pelvic dilation is often due to obstruction in male cats. Uni- or bilateral pelvic or ureteral dilation in cats often occurs due to inflammation secondary to ureterolithiasis. If ureterolithiasis is suspected and ureteral dilation is noted in ultrasound, abdominal radiographs following an enema should be performed to screen for uroliths, which may be challenging to find sonographically but easy to see radiographically. Any toxin that cat ingests can also affect the kidneys. Ethylene glycol and Easter lily are two and the retroperitoneal space may develop fluid secondary to acute renal injury. Ethylene glycol-affected kidneys may become so hyperechoic that shadowing will result. Polycystic kidney disease in cats results in several cysts in both kidneys to complete absence of recognizable renal tissue. The cysts are thin walled, with a near and far wall hyperechoic border and anechoic content.
Congenital defects of the kidneys include hypo- and dysplasia as well as aplasia. When young cats have small and irregularly shaped kidneys, dysplasia is more likely than chronic renal disease. Chronic renal injury in cats is due to chronic nephritis, glomerulonephritis, amyloidosis and nephrocalcinosis. In all cases the kidneys are smaller in length, hyperechoic and often irregularly shaped. Loss of corticomedullary distinction is also apparent. Occasional small round anechoic cortical cysts are identified as well. Chronic renal injury can also lead to uremic gastropathy, which sonographically appears as hyperechoic mucosal borders. Cryptococcus and feline infectious peritonitis can both lead to chronic renal injury and the sonographic findings are variable.
Renal neoplasia in the cat is often due to lymphoma which can appear as nodules or diffuse cortical echogenicity and enlargement. Specific to cats is a hypoechoic halo around the kidney that can be present with lymphoma, but also with renal infection, such as with FIP.
Suspended echoes that do not cause acoustic shadowing, reverberation, or twinkle artifact distal to the echo may be due to urine lipid. Clumping of these echoes may be present. Hyperbilirubinemic animals can have similar findings as can those with hematuria. Cystic calculi are hyperechoic and dependent and shadowing, not suspended. Cats may have accumulations of fine crystals that collect and shadow but, when agitated, break apart and look like a snow globe. Chronic cystitis can affect the cranioventral wall which becomes focally thickened with an irregular mucosa. Cats will develop a hyperechoic and sometimes shadowing border facing the lumen due to mucosal necrosis gas bubble entrapment, much like an ulcer. Pedunculated and broad-based masses can develop and can be benign polyps as well as malignant neoplasms.
Chronic pancreatitis is poorly described in cats. The pancreas may be of normal size or enlarged with a heterogenous appearance. Hyperechoic foci with acoustic shadowing may represent mineralizations. Multiple hypoechoic round foci of a few millimeters in diameter may also be recognized. These may represent nodular hyperplasia or dilated pancreatic ducts.
Cavities of the pancreas in cats are typically either due to abscesses or pseudocysts and appear as anechoic or hypoechoic cavities, possibly with a thickened wall. A number of investigators have attempted to assess the sensitivity and specificity of ultrasound compared to other imaging modalities for diagnosing pancreatitis in cats, however, with greatly varying results. Ultrasound will most likely remain one of the most important diagnostic tools in both dogs and cats, as it allows not only assessment of the pancreas, but also that of other organs that may be involved in the inflammatory process.
Identifying important landmarks is critical to localizing the pancreatic limbs as the pancreatic parenchyma can be difficult to differentiate initially from the surrounding mesentery. It may have indistinct margins and be isoechoic with the mesentery in the normal situation. The left lobe of the pancreas and body are easier to see than the right. Pancreatic ducts are easy to see in most cats. Also, the major duodenal papilla is the common entrance of the pancreatic and common bile ducts in cats. The main landmark for identifying the left lobe and body in the cat is the portal vein. The pancreatic body lies directly ventral to the portal vein caudal to the stomach. The left lobe is caudal to the stomach and cranial to the transverse colon on the left side of the portal vein. It may continue caudally for a small distance to the level of the splenic hilus. The pancreatic duct is more commonly identified in cats and is seen as a small anechoic tubular structure in the body and left pancreatic lobe. The right lobe of the pancreas is small in the cat and is more difficult to identify. It is adjacent to the duodenum and follows it caudally. A small pancreatic duct can also be identified in it. The major duodenal papilla appears as a small nodule attached to the duodenal wall close to the cranial flexure.
The following are parameters that should be assessed when examining the feline pancreas:
- The left and right limbs as well as the body should be examined and measured for thickness in the sagittal plane. Normal: Body: 0.5–0.9 cm thick, left lobe: 0.4–10 cm thick, right lobe: 0.3–0.6 cm. In acute disease, the pancreas may become enlarged as in dogs. However, this finding is much more inconsistent in cats. In chronic disease, the pancreas may be of normal size or smaller. Unremarkable changes do not rule out pancreatitis in cats. Pancreatic size does not increase with increasing age.
- Echogenicity: Normal: iso- to hypoechoic with the mesentery. Abnormal: The pancreas usually becomes hypoechoic in acute disease. In chronic disease, the pancreas may have a normal, hypo- or hyperechoic appearance. Pancreatic echogenicity does not change with increasing age.
- Abnormal echotexture: Often the pancreas appears heterogeneous. It can become nodular with irregular borders. Nodular hyperplasia has the appearance of small hypoechoic distinct nodules throughout the parenchyma. It is commonly seen in older cats. Nodules may be up to 1 cm in diameter and the pancreas may be enlarged.
- Pancreatic duct size: Normal: 0.5–2.5 mm diameter. Too little is known about the size of the duct in disease. However, there is a slight increase in size of the duct in older cats.
Normal feline adrenal glands are 1 cm in length, 3.7–4.9 mm pole height, have an ovoid shape and are hypoechoic and can have mineralizations in older animals. Cats with hyperthyroidism may have 20% larger glands, which can be explained by the stimulation of the hypothalamic-pituitary-adrenocortical axis by hyperthyroidism. Treated hyperthyroid cats had a gland length of 1.1 cm and up to a 4.9 mm pole height. Untreated hyperthyroid cats can have a gland length of 1.5 cm and pole height of up to 4.9 mm. Adrenal tumors most always lead to much greater size changes, rather in the centimeter than mm range. Adrenal tumors in cats with hyperaldosteronism are typically 2–4 cm in size. Cats with acromegaly can have bilateral enlargement also.