PneumoperitoneumWhat is the diagnosis?

• Lucency beneath the diaphragm, separating it from the underlying viscera
• On CT, free gas collects anteriorly in front of the liver and stomach
• The abdominal film in the second patient also contains abnormal free gas outlining bowel and peritoneal structures

Free intraperitoneal gas without recent surgery means a perforated hollow viscus until proved otherwise. An erect chest radiograph is the classic first test, but CT is far more sensitive and often shows the cause.

On supine films, look for Rigler's sign (both sides of the bowel wall visible), a visible falciform ligament and the football sign. Beware mimics such as Chilaiditi sign, where interposed colon shows haustral folds.