In this radiology lecture, we discuss the CT and MRI appearance of perihilar cholangiocarcinoma. Key points include: * Perihilar cholangiocarcinoma (AKA Klatskin tumor) occurs at bifurcation of the hepatic duct. * Cholangiocarcinoma (CC) is a primary malignant tumor of bile duct epithelium, usually adenocarcinoma. * CC is the most common primary hepatic malignancy after hepatocellular carcinoma (HCC), and most are extrahepatic (as opposed to intrahepatic). * Appearance of CC is based on growth pattern: Mass-forming, periductal infiltrating, and intraductal growing. * Risk factors: Parasite infection, choledochal cyst, primary sclerosing cholangitis, recurrent pyogenic cholangitis, and inflammatory bowel disease (ulcerative colitis). * Patients are usually 65 or older. * On CT and MRI, perihilar CC appears as a biliary stricture with shouldering/abrupt tapering. * If a mass is visible, will typically have rimlike enhancement with gradual centripetal enhancement on delayed images, be T2 bright (but not as homogeneous or as bright as hemangioma), and may have a targetlike appearance on DWI (favors CC over HCC). Click the YouTube Community tab or follow on social media for bonus teaching material posted throughout the week! Instagram: https://www.instagram.com/radiologistHQ/ Facebook: https://www.facebook.com/radiologistHeadQuarters/ Twitter: https://twitter.com/radiologistHQ