When small or large bowel is distended with fluid, certain characteristic features can be seen on ultrasound which cannot be visualized when the bowel is collapsed. The proximal and distal portions of the small bowel can frequently be differentiated both in vitro and in vivo by the echoes emanating from the valvulae conniventes ("keyboard sign"). This depends upon the angle at which the bowel segment is scanned in relation to its long axis, the relative amount of intraluminal fluid and gas, the diameter of the bowel in relation to the beam width of the transducer and the intrinsic resolution of the scanning system. The ascending and transverse colon can be recognized by haustral sacculations, the distal rectosigmoid colon and rectum by the rectal valves. The tubular shape, characteristic arrangement and location and peristaltic motion of most fluid-filled bowel segments are helpful in differentiating loops of bowel from cystic masses in the abdomen or pelvis.