Complex sentence: Pneumoperitoneum is air or gas in the abdominal (peritoneal) cavity, 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.
Split into simpler sentences: 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.