Mexican hat sign is seen in
Sigmoid volvulus
Midgut volvulus
Sessile colonic polyp
Pedunculated polyp
Pedunculated colonic polyps form the "Mexican hat sign." The Mexican hat sign is formed by the appearance of 2 concentric rings; the outer ring represents the "en face" visualization of barium coating the surface of the head of a pedunculated polyp, while the inner ring represents a meniscus of barium surrounding the stalk of the polyp visualized through the head
Gastrointestinal Radiology