Gastrointestinal Imaging

Pneumoperitoneum

presence of air in the peritoneal cavity. The most common cause is spontaneous or traumatic perforation of bowel, particularly perforation of peptic ulcer. Less common are perforation of malignant tumours or of diverticulitis whereas perforation of the appendix will almost never cause pneumoperitoneum. After laparotomy gas usually disappears within 3 - 6 days but may be present up to 24 days after operation. The most common cause for spontaneous pneumoperitoneum is suction of air through the female genital tract.

Plain films of the abdomen are very suited for the detection of pneumoperitoneum. Upright and supine films are routinely performed. Intraperitoneal free air is best visualized under the right diaphragm ("cupola sign") in upright position and in the right side up decubitus position between the liver and the right lateral peritoneum. In the supine position free abdominal gas may also appear as a linear or triangular subhepatic collection overlying the right kidney in the subhepatic space and Morrisons pouch. Due to the presence of gas outside the bowel wall it may occur that both the mucosal and the serosal surface of the bowel wall is depicted ("Riglers sign") (Fig.1). This feature usually indicates large amounts of intraperitoneal air and fluid (Fig.2). Intra-abdominal linear structures such as the falciform or umbilical ligament may become visible in patients with pneumoperitoneum.

 

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Fig.2

Pneumoperitoneum. Radiograph of the abdomen in lateral decubitus position with right side up. A very large amount of air is visible between the right lateral abdominal wall and the lateral border of the caecum and ascending colon.
Pneumoperitoneum, Fig.1
Pneumoperitoneum, Fig.2