The telltale triangle: use of the supine cross table lateral radiograph of the abdomen in early detection of pneumoperitoneum.

The most sensitive method for detecting a small pneumoperitoneum depends upon directing a horizontal x-ray beam tangential to the collection of free air. We have observed in the detection of small amounts of free intraabdominal air that the supine cross table lateral abdominal view is most valuable. Such small amounts of free intraperitoneal air collect between loops of bowel at the highest points in the peritoneal space forming typical small triangular lucencies with their bases at the peritoneal reflection.