Central dot sign
The central dot sign represents the telltale appearance of intrahepatic portal vein branches surrounded by severely dilated bile ducts, which can be visualized by either CT, MRI, or US.
The central dot sign is primarily encountered in congenital intrahepatic bile duct dilatation (Caroli syndrome) . While initially considered specific for this disease, later results have shown that the central dot sign sporadically also occurs in other conditions (e.g. peribiliary cysts, periportal lymphedema, and jaundice due to biliary obstruction) . Nevertheless, the sign aids differentiating Caroli syndrome from other conditions resulting in intrahepatic bile duct dilatation such as primary sclerosing cholangitis or recurrent pyogenic cholangitis .
On ultrasonography, the central dot sign manifests as echogenic foci or bundles (depending on the imaging plane) surrounded by the hypoechoic cystic bile duct dilatations .
When using cross-sectional imaging the sign is usually best seen in the axial imaging plane. The "dot" represents the cross-section of a portal vein branch, completely surrounded by the abnormally dilated bile ducts.
If intravenous contrast is used, the central portal vein branches show profound enhancement making the appearance even more striking .