Choledochal cyst involves congenital cystic dilatation of any portion of the extrahepatic bile ducts, and most choledochal cysts are diagnosed in childhood; however, up to 20% of cysts manifest in adults. It is classified based on the spectrum of morphologic changes in the bile ducts. The most common classification scheme is the Todani modification of the Alonso-Lej classification. Todani type I-single cystic dilatation of the extrahepatic bile duct (EBD). Todani Type II-true diverticula of the EBD. Todani Type III-choledochocele or ectasia of an intramural EBD segment. Todani Type IV-multiple and can have both intrahepatic and extrahepatic components. Todani Type V (Caroli disease)-cystic dilatation of the intrahepatic bile ducts (IBDs). Complications of choledochal cysts in adults include rupture with bile peritonitis, stone formation, cholangitis, liver abscess, and cholangiocarcinoma (CC). CC arising in a choledochal cysts appears as an intracystic soft tissue density mass or irregular thickening of the cyst wall [1]. 5.1.2 Choledocholithiasis Regardless of whether bile duct dilatation is present, the visible ducts should be scrutinized to identify and characterize filling defects, the vast majority of which represent stones [2]. Detectability of a bile duct stone on CT depends on the calcium content and requires precontrast CT. Depending on the composition, stones may show soft tissue attenuation, near-water attenuation, or fat attenuation with peripheral calcification. Stones are manifested as a signal void on T2-weighted imaging or MRCP (Fig. 5.1). On T1-weighted imaging, cholesterol stones are generally iso-or hypointense, while pigment stones are hyperintense due to the presence of metal ions. ERCP is highly accurate for the detection of small choledocholithiasis and also provides access for therapeutic intervention.
