Annular pancreas. Etiology, classification and diagnostic imaging.

A rare type of annular pancreas, in which the tail of the annular duct is fused with the dorsal pancreatic duct to form the main duct, and its diagnostic imagings of endoscopic retrograde cholangiopancreatography, hypotonic duodenography, computed tomography and ultrasound are described. Annular pancreas is an anomaly embryologically formed by the ventral anlage of the pancreas as the duodenum rotates, mainly due to hypertrophy of the left ventral bud. According to the fusion pattern of the ventral and dorsal pancreatic ducts, annular pancreas can be divided into three types: type I, divisional annular pancreas; type II, branch annular pancreas; and type III, main duct annular pancreas. Endoscopic retrograde cholangiopancreatography, hypotonic duodenography, computed tomography and ultrasound can show the imagings of annular pancreas from different aspects, but the final diagnosis must be based on the evidence of endoscopic retrograde cholangiopancreatography or hypotonic duodenography combined with computed tomography or ultrasound.