Gallstone ileus represents a mechanical obstruction of the small intestine by a gallstone, usually lodged in the proximal duodenum (Bouveret's syndrome) or in the terminal ileum. This is a complication of chronic cholecystitis when fistula between the gallbladder and duodenum is formed and a gallstone passes into the small intestine, and lodges in narrow parts of it. Less than 1% of cases of intestinal obstruction are caused by this mechanism.

CT is considered superior to plain abdominal films or US in the diagnosis of gallstone ileus cases, with a sensitivity of up to 93%. There are three radiographic signs pathognomonic of gallstone ileus:

1. partial or complete intestinal obstruction
2. pneumobilia or contrast material in the biliary tree
3. an aberrant gallstone