Bowler-hat sign: a simple principle for differentiating polyps from diverticula.

The bowler-hat sign has been described both with colonic polyps and with diverticula. The authors describe a simple principle for evaluating a bowler-hat sign in order to determine whether it is caused by a polyp or a diverticulum. If the bowler hat points toward the center of the long axis of the bowel, it represents an intraluminal structure (ie, a polyp). If, however, it points away from the center of the long axis of the bowel, it represents an extraluminal structure (ie, a diverticulum). Only if the bowler hat is located in the midline or is directly parallel to the long axis of the bowel is it impossible to classify the abnormality as a polyp or a diverticulum. The value of this principle was confirmed with both a radiographic model and a blinded review of radiographs from 37 cases demonstrating the bowler-hat sign. When a bowler hat is present on a double-contrast barium enema examination, use of this principle provides a simple and objective means of differentiating a polyp from a diverticulum.