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The sonographic appearance of intraperitoneal air collection has been studied in 46 patients. In 30 patients (group I), a pneumoperitoneum had been iatrogenically induced either during aspiration of ascitic fluid or during laparoscopy. Three normal volunteers (group II) had been subjected to graded intraperitoneal air injection to quantify the smallest amount of air detectable by ultrasound (US). In eight patients (group III) the sonographic demonstration of free intraperitoneal air led to a diagnosis of hollow visceral perforation; whereas in another five patients (group IV) the sonographic findings reinforced the clinical suspicion of a 'sealed off' perforation in the presence of negative roentgenograms. In all patients intraperitoneal air was seen as an echogenic line with a posterior reverberation or ring down artefact. In patients with free air, this was best seen in the perihepatic spaces with the patient in the supine, left lateral decubitus or prone position. As little as 5 mL of air could be consistently detected in all three volunteers (group II). Artefacts leading to a pseudopneumoperitoneum on US included; (i) the artefacts distal to an overlying rib; (ii) ring-down artefact from air in the adjacent lungs; and (iii) hepatodiaphragmatic interposition of colon. With proper sonographic technique and principles of interpretation these can be distinguished from true intraperitoneal air. Although sonography may be more informative than conventional radiology in patients with hollow visceral perforation, we did not find it more sensitive than conventional roentgenograms in detecting free intraperitoneal air. Sonography, however, is distinctly superior in patients with a sealed off perforation in whom conventional roentgenograms are frequently negative.

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