An elderly man presented with abdominal pain
Free intraperitoneal gas was seen over the liver and extending across the upper abdomen. A small amount of free fluid was seen in the hepatorenal space and inferior to the liver which contained small locules of gas.
Free gas must be distinguished from gas within the bowel. Free gas lies immediately beneath the peritoneal line, which is smooth, whilst gas within bowel will be deeper due to the bowel wall, which has mucosal folds. In addition free gas is mobile throughout the abdomen, so in doubtful cases the patient may be repositioned (similar to positioning for a lateral decubitus xray). Bowel interposition should be considered if gas is seen over the liver (also known as Chiliaditti's anomoly). The presence of gas locules within the free fluid (ie. outside bowel) confirms the diagnosis of intraperitoneal free gas.
Laparotomy and subsequent pathology revealed a sigmoid colon with diverticular disease and stricture which had obstructed and perforated.
For another example of pneumoperitoneum from a different cause, click here