A List of Radiographic Changes and Differential Diagnoses
The Liver and Biliary Tract
Ultrasound examinations of the liver and biliary tract is beneficial when the following signs are noted clinically or radiographically: icterus, microhepatica, hepatomegaly, or liver mass. For metastasis, check for multifocal neoplasia such as mast cell tumors and lymphosarcoma, and increased liver enzymes.
The liver is a relatively homogeneous organ. It is isoechoic to the falciform fat and the cortex of the right kidney. The liver is hypoechoic to the spleen. You should see two vascular systems: hepatic veins and portal veins. The hepatic artery is too small to be recognized. The portal veins are located in the portal triad and have a thick wall; hence, they appear to have echogenic walls. The hepatic veins do not usually show distinct walls except for large branches near the caudal vena cava. It is important to evaluate all liver lobes. You may need to approach the liver through the intercostal spaces. A ventrocranial approach may not be adequate, especially in case of microhepatica. The intrahepatic biliary tract is not normally visible. In normal cats, the cystic duct and common bile duct can be visualized. It may normally be up to 4 mm in cats. It is rare to see the common bile duct in normal dogs due to gastrointestinal gas and contents.
Ultrasound Abnormalities of the Liver
Ultrasound abnormalities can be categorized into two groups: diffuse vs. focal or multifocal.
Diffuse Abnormalities: Diffuse changes are either hyperechoic or hypoechoic. Hyperechoic liver can be recognized based on reduced visualization of portal vein “walls” than usual. When the liver shows diffusely increased echogenecity, portal veins are difficult to see. In addition, the liver is hyperechoic to the falciform fat and the cortex of the right kidney. On some occasions you may be able to see the liver isoechoic to the spleen. Differential diagnoses for diffuse hyperechoic liver are steroid hepatopathy (Cushing's or iatrogenic), cirrhosis, and lymphosarcoma in dogs and lipidosis and lymphosarcoma in cats.
Diffuse hypoechoic liver is recognized when you see many small portal veins (“portal radicals”). Again, you may compare the hepatic echogenecity to the kidney, spleen, and falciform fat. Differentials include congestion (usually enlarged liver with truncated and prominent hepatic veins), lymphosarcoma, and hepatitis. Early cirrhosis may also appear diffusely hypoechoic with an apparently reduced size.
Recognition of diffuse liver changes is difficult. When it is seen, one should think that the condition is in an advanced stage. Liver biopsy or aspirates may be beneficial to further differentiate lesions.
Focal or Multifocal Abnormalities: Focal or multifocal lesions can be hyperechoic, hypoechoic, or isoechoic to surrounding hepatic tissues. Sometimes, focal lesions may appear as “bull's eye” or “target” lesions that have layers of hyperechoic and hypoechoic appearances. Unfortunately, differentials for these lesions range from benign regenerative nodules, cysts and abscess to malignant neoplasia. Hence, ultrasound diagnosis is not definitive. When these lesions are noted, you need to decide if they are clinically significant. Further laboratory tests and tissue sampling (biopsy or fine needle aspirate) are usually needed.
Portosystemic shunt is highly suspected in young animals when small liver is noted on abdominal radiographs and bile acid level is increased. Ultrasound examinations are helpful to differentiate an intrahepatic shunt (usually carries worst prognosis) and an extrahepatic shunt (surgically treatable especially now with the use of a ring vascular constrictor). An intrahepatic shunt is usually a patent ductus venosus. It is located adjacent to the diaphragm. Since the liver tissues surround the shunt vessel, the detection rate is relatively high. An extrahepatic shunt can be visualized, but the detection rate is variable dependent on the degree of patient preparation for better acoustic windows in the porta hepatis. A transverse scan plane should be used. First, find portal veins and caudal vena cava. Any large vessels seen in the transverse plane are highly suggestive of a shunt vessel. A single extrahepatic shunt is usually detected at the level of right renal cranial pole. If gas is excessive in the gastrointestinal tract, the study should be repeated after four to five hours of cage rest. The false negative rate ranges widely. Thus, a normal appearance should not be considered as normal. Further studies should include an operative jejunal vein portogram, percutaneous splenic portogram, and transcolonic scintigraphy.
Presence of sludge in the gallbladder is a common finding in sick animals and should be considered as clinically insignificant. When a hyperechoic focal structure with acoustic shadowing is seen in the lumen, a gallstone should be considered. Sometimes, you may see mixed echogenic luminal contents with apparent thickening of the wall. This may be due to cholecystitis. A thickened appearance of the gallbladder may be caused by the presence of free peritoneal fluids. Thus, the diagnosis of cholecystitis needs to be made cautiously.
In a recent report, ultrasonographic appearances of a gallbladder mucocele are described. These animals usually show chronic icterus and anorexia. Ultrasonographically, a kiwi fruit-like appearance of the gallbladder lumen is noted. Alternately, mixed echogenic, mosaic-like appearances may be seen. The key point is that these contents do not show any movement in a real-time study. The condition often requires early surgical intervention, and thus, a careful evaluation of the gallbladder is warranted in a patient with icterus.
Biliary duct dilation may be noted in chronic obstruction. My experience is usually in cats with chronic pancreatitis or animals with bile duct carcinoma. The intrahepatic bile ducts are dilated and tortuous. If possible, a Doppler study differentiates abnormal bile ducts from hepatic blood vessels. Bile duct stones may also be seen with a strong acoustic shadowing in the liver parenchyma.
Indications for ultrasonography of the spleen include radiographic detection of splenomegaly, splenic mass lesions (ruling out hemangiosarcoma and other neoplasms), peritoneal effusion, and potential neoplastic metastasis of mast cell tumor and lymphosarcoma.
The spleen is always visible in dogs. Since the size of the spleen is small and thin in cats, the feline spleen needs to be scrutinized. It is important to clip the hair of left lateral abdominal wall enough to see the entire spleen. The spleen has uniform echogenecity and is hyperechoic to the liver and kidneys. Normal size for dogs or cats has not been established. The spleen has multiple vessels that enter near the hilus. This anatomical feature should be used to differentiate an enlarged liver from the spleen.
Diffuse lesions are detected, when splenic echogenecity is compared with that of the liver and kidney. Differentials for diffuse hypoechoic spleen include congestion, lymphosarcoma, and splenic torsion. Usually, fine needle aspiration is performed to find cytological abnormalities. Focal mass lesions are often seen. In older dogs, neoplasia, especially hemangiosarcoma, should be considered. Unfortunately, fine needle aspirate may not yield cytological diagnosis for this particular tumor. Splenectomy should be considered as excisional biopsy, if the lesion is considered significant. Hematoma, hemangioma, and hemangiosarcoma cannot be differentiated on ultrasound. Presence of peritoneal effusion is not a good indicator for malignancy either. The liver is the common metastatic site for hemangiosarcoma.
Splenic lymphosarcoma can be diffuse or focal/multifocal and hyperechoic or hypoechoic. A target lesion may be seen. This means that there is no pathognomonic sign for neoplasia. Other splenic neoplasms include leiomyoma and mast cell tumor.
Abscesses can be hypoechoic or anechoic with a slightly irregular margin. Since it can be anechoic, it may look like a cyst. Gas can be seen as showing reverberation artifacts. Splenic infarct can be diffuse: non-homogeneous and hypoechoic. However, it may be focal as hypoechoic and appear lacy.
Since a mass lesion in the spleen can be benign or malignant, tissue diagnosis becomes imperative. Thoracic radiographs should be made to check for metastases. Surgical excisional biopsy may be more appropriate than biopsy of the spleen.
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