Causes of Free Air

The most common imaging modality used to rule out free air is the upright abdominal radiograph. The most sensitive imaging modality to detect free air is the CT. Perforation or rupture of an air-containing bowel loop is the most common cause of free intraperitoneal air. The three radiologic signs of free air are air beneath the diaphragm, visualization of both sides of the bowel wall, and visualization of the falciform ligament. To see air below the diaphragm, it is important to place the patient in the upright or left lateral decubitus (if the patient is unable to stand) position. To visualize the falciform ligament in a case of free air, the patient should be supine.