The B-scan ultrasonic appearances of gas in the biliary tree

Gas in the biliary tree has a striking but not unique ultrasonic appearance consisting of acoustic shadows trailing behind well-defined discrete high amplitude echoes deep within the liver parenchyma, and may be mimicked by intrahepatic gall stones and calcified tumours and granulomas. It interferes with assessment of liver structure, and renders ultrasound B-scan unreliable for the detection of liver metastases. Computed tomography remains unaffected by biliary gas.