The detection of air in the peritoneal cavity is an important radiological sign as most patients have perforation of the bowel. In this era of sophisticated imaging techniques, it is easy to underestimate the diagnostic value of relatively simple investigations such as radiographs of the abdomen and chest. A chest radiograph taken with the patient in an upright position is still an excellent way to demonstrate air under the right hemidiaphragm. Even small volumes of air (1–2 ml) can be detected, particularly if the patient is placed in the left lateral decubitus position for 15 to 20 minutes before moving to an upright position. Sensitivity may also be increased if films are taken in midinspiration or midexpiration. The obvious principle is that air rises to the highest point in the peritoneal cavity.

When upright films are difficult or impossible, the radiologist needs to recognize the presence of intraperitoneal air on films taken in the supine position. One helpful sign was first reported by Dr Rigler in 1941 and is often called Rigler's sign. The sign is illustrated in Figure 1. The radiograph was taken from a 34-year-old man with Crohn's disease who presented with an ileal perforation. There is air on both sides of the bowel wall and the wall itself is seen as a thin, white stripe (arrows). There is also an abnormal lucency in the right upper quadrant of the abdomen (arrowhead) that may represent air on the undersurface of the liver. The supine radiograph in Figure 2 was from a 42-year-old man who also had Crohn's disease and an ileal perforation. Air was present bilaterally in the subphrenic spaces and he also had air on both sides of the falciform ligament (arrows). The latter is sometimes called the falciform ligament sign. Other radiological signs of intraperitoneal air on supine images include visualization of the undersurface of the diaphragm, air in the posterior hepatorenal space (Morison's pouch) and triangular lucencies when air becomes trapped between loops of small bowel. The ‘football sign’ is largely applicable to infants and occurs when a large volume of free air occupies the oval-shaped peritoneal cavity.