What is “pneumoperitoneum” and how is it identified on ultrasound?

Pneumoperitoneum refers to the presence of air in the peritoneal cavity, which can be a critical sign of abdominal pathology, often related to perforation of a hollow organ or following certain surgical procedures. On ultrasound, it can be identified as echogenic areas without any associated structures, which means that the air does not produce a solid mass or fluid collection.

When performing an abdominal ultrasound, the presence of air creates highly reflective surfaces that appear as bright (echogenic) spots against the darker background of surrounding tissues. These echogenic areas represent the air bubbles or pockets and may cause "ring-down" artifacts due to the way ultrasound waves interact with the air. Hence, the identification of these specific patterns allows sonographers to diagnose pneumoperitoneum effectively.

In contrast, fluid accumulation in the peritoneal cavity would present as anechoic or hypoechoic areas rather than echogenic, while a mass in the abdominal cavity would typically have a definitive structure and associated features on ultrasound. Likewise, swelling of the abdominal wall pertains to a different category of pathology that does not involve air within the peritoneum.