Ultrasound is the least invasive radiologic modality for imaging the liver and biliary tract. Unlike computed tomographic (CT) scanning and magnetic resonance imaging (MRI), the technique is portable, quick, and can be used to guide interventional procedures. Ultrasound uses no ionizing radiation to create the image, and is therefore the technique of choice in pregnant women, in patients with contrast allergies, or in those in whom MRI is contraindicated .
This topic will discuss the role of ultrasound in the evaluation of the liver and the intra- and extrahepatic bile ducts. The use of ultrasound for the evaluation of the gallbladder is discussed briefly here and in more detail separately. (See "Pathogenesis, clinical features, and diagnosis of acute cholecystitis", section on 'Ultrasonography' and "Uncomplicated gallstone disease in adults", section on 'Transabdominal ultrasound' and "Gallbladder cancer: Epidemiology, risk factors, clinical features, and diagnosis", section on 'Ultrasound' and "Gallbladder polyps and cholesterolosis", section on 'Ultrasonography'.)
Ultrasound relies upon the transmission of targeted sound waves of varying selected frequencies through tissues, with subsequent computerized conversion of the signals from the reflected waves into anatomical images on a screen. The degree of reflection of sound waves depends upon the interface between tissues with different acoustic properties. The degree of echogenicity depends upon the ability of the tissue being evaluated to reflect or absorb the ultrasound waves. A fatty liver will attenuate the ultrasound beam somewhat, limiting full evaluation of the liver parenchyma. Similarly, waves are not transmitted through air; liver lying below interposed loops of bowel therefore will be poorly visualized .
Normal measurements on ultrasound — Measurements of components of the hepatobiliary tree depend upon the skill of the ultrasonographer obtaining the measurements, and there is variability in terms of what is considered "normal." However, some general estimates are available regarding the expected sizes of structures in the hepatobiliary tree:
- Gallbladder: The gallbladder wall should be less than or equal to 2 mm (in a distended or fasting gallbladder). Collapsed gallbladders, seen when the subject has eaten, typically appear thickened. The maximum dimension of the gallbladder is 5 X 10 cm.
- Common hepatic duct: Common hepatic duct (inner wall to inner wall) is usually measured at the level of hepatic artery. In the normal fasting state it should be <7 mm in patients <60 years, and <10 mm in patients older than 60.
- Common bile duct (CBD): The normal CBD diameter increases with age and in patients who have had a cholecystectomy. In patients in their 40s, the normal mean diameter is 4 mm. The normal mean diameter then increases by 1 mm every decade [2-5], and authors have proposed that in elderly patients the normal upper limit of normal be set at 8.5 mm . The CBD is commonly up to 10 mm in patients who have undergone a cholecystectomy.
- Liver: The span of the liver in the right midclavicular line should be less than 16 cm .