The supine abdominal x-ray shows a massive pneumoperitoneum. There is a large radiolucency outlining the abdominal cavity, causing the ‘Football’ sign (i.e. resembling the shape of a football). There is also a positive double wall or ‘Rigler’ sign, wherein the bowel wall is outlined by intraluminal and free peritoneal air.

A CT scan of the abdomen showed a massive pneumoperitoneum due to a perforated gastric ulcer.

Further reading here.