Biliary dilation refers to enlargement of the intrahepatic or extrahepatic bile ducts, indicating obstruction of bile flow. The common bile duct (CBD) normally measures up to 6 mm in diameter; measurements up to 8–10 mm may be acceptable in elderly patients or post-cholecystectomy. Dilation is identified on ultrasound as tubular anechoic structures in the liver that parallel the portal veins (the “parallel channel” or “shotgun” sign). Common causes of biliary obstruction include choledocholithiasis, stricture, and pancreatic or bile duct malignancy.