Pneumoperitoneum is air or gas in the abdominal (peritoneal) cavity. It is often seen on X-ray, but small amounts are often missed, and CT is nowadays regarded as a criterion standard in the assessment of a pneumoperitoneum. CT can visualize quantities as small as 5 cm³ of air or gas. The most common cause is a perforated abdominal viscus, generally a perforated peptic ulcer, although any part of the bowel may perforate from a benign ulcer, tumor or abdominal trauma. A perforated appendix seldom causes a pneumoperitoneum.