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Imaging Studies of the Liver
The diagnosis of hepatobiliary disease frequently requires the use of imaging studies to characterize the presence of vascular and parenchymal changes which may facilitate diagnosis and management. Traditional imaging studies include abdominal ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI). More recent imaging techniques which incorporate liver elastography have emerged as important clinical tools to facilitate the diagnosis of liver cirrhosis, which remains an essential step in management. Several of the most commonly used techniques include vibration-controlled transient elastography (VCTE), shear wave elastography, acoustic radiation force impulse elastography (ARFI), and magnetic resonance elastography (MRE). Techniques such as VCTE are now routinely used in clinical practice, whereas others primarily used in research contexts such as MRE are now emerging as potentially important clinical tools for chronic liver diseases such as nonalcoholic fatty liver disease (NAFLD).
Abdominal Ultrasonography
Abdominal US is often the first imaging modality used to evaluate the liver. US is easily performed, does not require intravenous (IV) access, and can provide a large amount of clinically relevant information in evaluating the liver and biliary tract. The use of Doppler technology also enables examination of the hepatic and portal venous system and the hepatic arterial flow. These latter developments have been particularly useful in the evaluation of patients after orthotopic liver transplantation and in those who have undergone placement of a transjugular intrahepatic portosystemic shunt (TIPS).
US is particularly helpful in evaluating suspected cystic lesions and in excluding the presence of dilated bile ducts. Liver cysts are often found during US and are usually asymptomatic. Based on US features, these can usually be classified as simple cysts. Simple cysts are anechoic and show posterior enhancement, which means there is an echogenic region behind the cystic lesion. In questionable cases, cyst aspiration can be performed to obtain cells for cytologic examination or to institute drainage if infection is a concern.
US is the most sensitive method for determining the presence of dilated bile ducts. Both intrahepatic and extrahepatic ductal dilatation can be identified, particularly in slender patients without fatty liver; thus both the presence and the level of bile duct obstruction can be identified. In patients with acute biliary obstruction (e.g., acute choledocholithiasis), however, US may not reveal dilated ducts. A number of focal liver lesions may be identified, including bacterial, fungal, and parasitic abscesses and benign and malignant lesions (e.g., hepatic adenomas, hemangiomas, hepatocellular carcinoma).
However, CT and MRI are superior to US because of the ability to administer contrast and to obtain images in various phases—arterial, portal, and hepatic venous—which can further increase the specificity of the diagnosis.
Ultrasonography is also helpful to evaluate the hepatic parenchyma. In patients with fatty liver infiltration associated with obesity, hyperlipidemia, or type 2 diabetes, diffusely increased echogenicity may be observed in the liver. Increased echogenicity of the liver is also observed in patients with cirrhosis. In addition, US may reveal features of portal hypertension, such as splenomegaly, perisplenic varices, portosystemic collaterals, and reversal of flow (hepatofugal) in the portal vein. In some patients with cirrhosis, particularly resulting from alcohol, the left lobe may be enlarged.
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