Gallstone ileus is a rare cause of mechanical bowel obstruction with a high mortality rate of up to 30%. 
The diagnosis is radiological and CT is the gold standard for diagnosis. 
Rigler's triad is pathognomonic for gallstone ileus and may be identified on abdominal x-ray with a gallstone in the terminal ileum, pneumobilia and signs of small bowel obstruction; however not all gallstones are radio-opaque and CT is more sensitive for pneumobilia. 
In this case, findings of a low density gallstone and pneumobilia were only seen on CT.