Congenital biliary abnormalities in adults: a practical CT and MRI guide.

Congenital abnormalities of the biliary tree are increasingly identified in adulthood as incidental findings on cross-sectional imaging. They range from clinically silent anatomical variants to complex malformations with relevant clinical or surgical implications. This review provides a practical framework for radiologists who encounter these entities in adult patients, organizing them into four categories: congenital cystic biliary dilation, fibropolycystic liver disorders arising from ductal plate malformation, anatomical variants of the biliary tree, and rare developmental anomalies such as ductal duplication, ectopic biliary drainage, and gallbladder agenesis or hypoplasia. Magnetic resonance imaging with cholangiopancreatography (MRCP) is the diagnostic cornerstone, allowing non-invasive assessment of ductal communication, morphology, and pancreaticobiliary junction anatomy, while computed tomography (CT) plays a complementary role in evaluating complications. Key diagnostic CT and MRI features are discussed, including the central dot sign of Caroli disease, the diffuse non-communicating cystic pattern of polycystic liver disease, and the abnormally long common channel characteristic of pancreaticobiliary maljunction. The review also addresses the surgical relevance of biliary anatomical variants. A compact diagnostic algorithm and a structured reporting checklist are provided, together with the acquired conditions that most often mimic congenital disease in adults. From a clinical standpoint, malignant potential varies substantially across entities, ranging from negligible in polycystic liver disease to substantially increased in choledochal cysts and pancreaticobiliary maljunction, although the available estimates derive largely from heterogeneous, mostly retrospective rare-disease series and should be interpreted as orders of magnitude rather than precise risks. This variability nonetheless drives surveillance strategy. A systematic diagnostic approach, combined with close collaboration between radiologists, hepatologists, and surgeons, is essential to guide appropriate management and avoid both missed malignancies and unnecessary intervention.