Pneumobilia due to gallstone ileus

Plain film of the abdomen showing air in the intrahepatic biliary tree (arrows) and dilated loops of small bowel. Pneumobilia implies either a patent cystic duct or a fistula involving the common bile duct. The gallstone cannot be identified on this film. This is a common finding that is due to two factors: most stones are radiolucent, and gas or bony structures can obscure gallstones outside the biliary tree.