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.