Windsock sign (duodenal web)

The windsock sign is a typical appearance of a duodenal web (intraluminal duodenal diverticulum) on upper gastrointestinal contrast series which consists of an intraduodenal barium contrast-filled sac that is surrounded by a narrow lucent line (web or intraluminal mucosal diaphragm) which is well demonstrated as the barium in the duodenum passes distal to the diverticulum.

This should not be confused with the windsock-like appearance seen in thoracic aortic dissection.