Central dot sign | Radiology Signs

Answer:

The central dot sign is a small, strongly enhancing focus lying centrally within a saccularly dilated intrahepatic bile duct. The dot is an intrahepatic portal vein branch, usually accompanied by its hepatic artery, surrounded on all sides by the dilated duct: the fibrovascular bundle is engulfed rather than displaced. It is the classic sign of Caroli disease, the congenital communicating cavernous ectasia of the intrahepatic ducts, and is best appreciated on contrast-enhanced CT or post-gadolinium MRI, with MRCP demonstrating the communication of the cystic spaces with the biliary tree. On ultrasound the same structure appears as an echogenic focus or bridging fibrovascular band. The sign is highly suggestive but not pathognomonic and is reported in obstructive biliary dilatation, peribiliary cysts and periportal lymphoedema. Its diagnostic value is in separating Caroli disease from polycystic liver disease and simple cysts, which do not communicate with the biliary tree and contain no central vessel.

Why is it called so?

A single enhancing dot sits in the centre of each rounded dilated duct on axial images.

Pathophysiology

Caroli disease results from ductal plate malformation of the large intrahepatic bile ducts, producing segmental saccular ectasia that communicates with the rest of the biliary tree. Because the affected ducts run within the portal triads, they balloon around the accompanying portal vein branch and hepatic artery rather than away from them. On axial imaging the dilated duct therefore encircles the vessels, and after contrast the vessel enhances brightly against the non-enhancing bile, producing the dot. The associated congenital hepatic fibrosis in Caroli syndrome, and the bile stasis within the ectatic segments, explain the accompanying risks of recurrent cholangitis, intraductal stones and cholangiocarcinoma.

Alternative names: Central dot, dot sign of Caroli disease

Other associated named signs: String of beads and string of pearls of biliary dilatation, and the bunch of grapes appearance of biliary hamartomas

References

1. Brancatelli G, Federle MP, Vilgrain V, Vullierme MP, Marin D, Lagalla R. Fibropolycystic liver disease: CT and MR imaging findings. Radiographics. 2005;25(3):659-70. PMID: 15888616.