Bowler hat sign | Radiology Signs

Answer:

Bowler hat sign: Biconvex filling defect seen en face on double-contrast barium enema, most often representing a small sessile colonic polyp, but the identical appearance can also be produced by a diverticulum. The direction the dome of the “hat” points distinguishes the two: pointing toward the center of the bowel lumen indicates an intraluminal polyp, while pointing away from the lumen indicates an extraluminal diverticulum.

Why is it called so?

Resembles the rounded dome and brim of a traditional bowler hat due to the biconvex shape of the filling defect with barium coating the edges.

Pathophysiology:

In double-contrast barium enema, a small sessile polyp projecting into the lumen, or a diverticulum pouching outward from the wall, is viewed en face, creating a central area of trapped barium surrounded by a biconvex rim resembling a Bowler hat sign. Because both lesions can produce this identical biconvex appearance, the orientation of the dome relative to the long axis of the bowel lumen is used as a simple, objective principle to differentiate an intraluminal polyp (dome points toward the lumen) from an extraluminal diverticulum (dome points away from the lumen).

Alternative names: None

Other associated named signs: Mexican hat sign (pedunculated colonic polyp, seen as a pair of concentric rings representing the head and stalk of the polyp)

References

1. Miller WT, Levine MS, Rubesin SE, Laufer I. Bowler-hat sign: a simple principle for differentiating polyps from diverticula. Radiology. 1989.