Most gallstones (80-85%) are not visible on plain abdominal radiographs (cf. renal calculi which are normally visible). They are present on this image but would be easy to miss, and this is a classic 'corner of the film' abnormality. As it turned out, it seemed that the gallbladder calculi were not the cause of the pain, although no other clear cause was demonstrated on the CT (not provided in full) which was performed only 3 hours after the AXR.