Pneumoperitoneum

(A) This image demonstrates multiple A lines with the probe placed on the anterior abdominal wall. A lines are one or more horizontal hyperechoic lines that are equidistant from one another and a multiplicative of the distance between the skin surface and the peritoneal line. A lines are a familiar feature of lung ultrasonography where they indicate a normal aeration pattern at the site of probe placement, if there is associated lung sliding. In the abdomen, they indicate the presence of air within the gut lumen (normal), or air that is external to the gut lumen (free peritoneal air). The presence of gut sliding with A lines indicates that the air collection is intraluminal while A lines in the absence of gut sliding is consistent with free peritoneal air. Gut sliding is identified as a mobile linear interface between the surface of the gut and the inside of the abdominal wall.
(B) This image demonstrates enhancement of the peritoneal line with a normal attenuated peritoneal line on either side of enhanced line, when the image is obtained with the probe placed on the anterior abdominal wall. Free air within the abdomen is associated with an enhanced peritoneal line. In this case, free air is detected at the site of peritoneal line enhancement, but it is absent in the adjacent areas. There is a comet tail artifact deep to the region of peritoneal enhancement. The comet tail artifact, also known as a ring down artifact, is vertical in orientation; whereas A lines are horizontal in orientation.