Mexican hat sign is seen in
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
Sigmoid volvulus
Midgut volvulus
Sessile colonic polyp
Pedunculated polyp
Gastrointestinal Radiology