There are various anatomic variants of the gall bladder (GB), including agenesis, duplication, Phrygian cap, diverticulum, ectopic GB, and wandering GB [3, 4]. Among them, Phrygian cap is the most common abnormality of the GB shape, with an incidence of 1-6 % of the population [3, 4]. The GB fundus is frequently folded upon itself and partially septate. When a fold or septum of the GB between the body and fundus is identified, Phrygian cap is diagnosed. Phrygian cap may be a source of confusion on imaging, but it has little clinical significance [3].

Recognizing congenital anomalies and anatomic variants of the biliary tree is important in clinical practice. Laparoscopic cholecystectomy and living-donor liver transplantation are frequently performed. The correct diagnosis of anatomic variant that may increase the risk of the bile duct injury aids in surgical planning and can prevent inadvertent complications, such as ductal ligation or structure and biliary leaks. Furthermore, congenital anomalies of the biliary tree may be undetected until adulthood, and an extensive workup is undertaken before correct diagnosis is made because symptoms and signs of the congenital anomaly are often nonspecific. Thus, in case of persistent and unexplained symptoms and signs, including cholangitis, pancreatitis, jaundice, recurrent abdominal pain, nausea, and vomiting, underlying congenital anomalies of the biliary tree should be considered.

Various kinds of congenital anomalies and anatomic variants involve the biliary tree, including aberrant or accessory biliary ducts, aberrant cystic duct insertion, choledochal cyst, and anomalous pancreaticobiliary ductal union (APBDU). Therefore, radiologists need to be familiar with these disease entities and their image features.

A choledochal cyst is a relatively rare congenital cystic dilatation of the biliary tree. Women are affected four times more often than men, and 7% of choledochal cysts are found during pregnancy. The majority of choledochal cysts are diagnosed in childhood, with classic triads of obstructive jaundice, right-upper-quadrant pain, and palpable mass. However, up to 20% of choledochal cysts are recognized in adults with increasing frequency. Manifestation in adults is nonspecific, which often leads to a delay in diagnosis. The most common symptoms in adults are chronic relapsing cholangitis and jaundice. Acute pancreatitis, cholecystitis, and biliary calculi are commonly associated, and complications such as biliary obstruction, biliary cirrhosis, cyst rupture, portal vein thrombosis, and hepatic abscess wall can occur. Furthermore, tumorous conditions, including cholangiocarcinoma (CC) and GB carcinoma, have been reported frequently in a choledochal cyst [3].

There are five types of choledochal cyst [3–6]:

APBDU is characterized by union of the distal common bile duct and pancreatic duct proximal to the duodenum, with formation of a long common channel (>15 mm). Prevalence varies from 1.5% to 3.2% [3]. According to the fusion style of the pancreatic duct and common bile duct, there are two subtypes of APBDU: P-C type and CP type. The presence of a long common channel may allow reflux of pancreatic secretions into the biliary system and can result in the formation of a choledochal cyst. Conversely, reflux of bile into the pancreatic duct can cause pancreatitis. Associated conditions, including CC, GB cancer, choledocholithiasis, and chronic pancreatitis, may present. An anomalous junction should be diagnosed and surgically treated before pancreaticobiliary complications occur [3, 6].

PSC is a chronic cholestatic liver disease of unknown cause characterized by inflammation, fibrosis, and destruction of intrahepatic and extrahepatic bile ducts. It affects male patients more commonly than female patients. PSC is commonly associated with inflammatory bowel disease (IBD), particularly ulcerative colitis, in more than half of the cases [7–9].

On direct cholangiography, multiple segmental strictures involving both the intrahepatic and the extrahepatic bile ducts are the characteristic finding of PSC, and alternating strictures and dilatations result in the beaded appearance. Ductal dilatation is usually mild and proportionately less than the degree of stricture, because periductal inflammation and fibrosis impair ductal dilatation. As fibrosis progresses, obliteration of peripheral intrahepatic ducts may give the biliary tree a prunedtree appearance, which usually indicates the development of cirrhosis. CT findings of PSC are similar to cholangiography findings and include segmental intrahepatic and extrahepatic bile duct dilatations with multifocal strictures. Ductal-wall enhancement and periductal patchy enhancement can be seen. Intrahepatic duct stones are rarely seen. MRCP findings are similar to direct cholangiography findings and include multifocal segmental strictures alternating with mild bile duct dilatation. Ductal-wall irregularities, diverticula, and stones can be seen. On MRI, hepatic parenchymal changes may be present. T2-weighted images may reveal increased signal intensity in a peripheral wedge-shaped or fine reticular pattern, which is thought to be caused by extension of periductal inflammation to involve vascular and lymphatic channels [7–9].

RPC is characterized by intrahepatic ductal pigmented stones commonly accompanied by recurrent gram-negative bacterial infections. RPC is also known as oriental cholangiohepatitis because this disease has a propensity to occur in Asian populations. The cause of RPC is unclear, but associations with parasitic infestation such as clonorchiasis and ascariasis, portal bacteremia, and malnutrition have been suggested. RPC increases the risk of CC, with reported incidence rate of 5–6%. Chronic bile duct irritations by intrahepatic calculi, bile stasis, and bacterial infections have been proposed as responsible carcinogenic factors [7, 10, 11].

Cross-sectional imaging usually demonstrates dilated ducts containing stones and sludge. The lateral segment of the left hepatic lobe and the posterior segment of the right hepatic lobe are more commonly involved than other hepatic segments. Parenchymal atrophy of involved hepatic segments, ductal-wall enhancement and thickening, biliary strictures, hepatic abscess and biloma, segmental hepatic parenchymal enhancement, and pneumobilia are findings of RPC. As the disease progresses to cirrhosis, atrophy of left lateral and right posterior hepatic segment, along with hypertrophy of caudate lobe and left medial segment, result in round liver configuration. Cholangiography findings of RPC include intrahepatic or extrahepatic duct stones as filling defect, dilatation of extrahepatic duct and central intrahepatic duct, ductal rigidity and straightening, increased or right-angle branching pattern, decrease in arborization, acute tapering of peripheral duct, luminal irregularity, and focal strictures. MRCP findings of RPC are similar to those of direct cholangiography but are advantageous in the case of a missing duct due to severe stricture. When CC occurs in patients with hepatolithiasis, periductal soft tissue density, higher ductal enhancement than normal duct on portal- venous phase, ductal wall thickening, portal vein obliteration, and lymph node enlargement on CT images are significant findings for differentiating cancer from periductal fibrosis [7, 10, 11].

Complete or partial bile duct obstruction leads to bile stasis and predisposes to ascending cholangitis by bacterial infection. Patients present with right-upper-quadrant pain, fever, chills, and jaundice. Benign causes of biliary obstruction are choledocholithiasis, postoperative biliary stricture, or papillary stenosis; malignant cases are CC, pancreatic carcinoma, and malignant hilar lymphadenopathy. The role of imaging is to detect biliary dilatation and determine the cause and level of obstruction. On crosssectional imaging with contrast enhancement, ductal wall thickening and enhancement, upstream ductal dilatation, patchy periductal enhancement, and obstructive lesions per se indicate OC [7, 10].

Clonorchiasis is a trematodiasis caused by chronic infestation by liver flukes, Clonorchis sinensis. C. sinensis eggs are passed in human feces, and infestation occurs by ingestion of raw fish. The worms cause obstruction of bile flow within the biliary tree and trigger inflammatory cell infiltration. As disease progresses, periductal fibrosis, ductal epithelial hyperplasia, and CC can develop. On cholangiography, C. sinensis appears as an ellipsoid filling defect in the intrahepatic duct measuring several millimeters. CT and US reveal diffuse and mild dilatation of intrahepatic ducts, particularly in the liver periphery, without extrahepatic duct dilatation. Mild, uniform, and disproportionate dilatation of the biliary tree from the extrahepatic duct to far peripheral ducts is the typical finding of clonorchiasis [7, 12]. Clonorchiasis increases the risk of CC [13].

Ascaris lumbricoides is the most prevalent human helminth worldwide. The disease is transmitted by the ingestion of food contaminated with parasitic eggs, which eventually develop into adult worms within the gastrointestinal tract [14]. The adult worms may remain within the intestines and occasionally enter the biliary system through the ampulla of Vater. Biliary colic and cholangitis may develop when worms obstruct the common duct. Worms are seen as elongated, longitudinal filling defects on cholangiography or as thin, tubular, echogenic intramural lesions on US. MRCP shows intraductal worms as a filling defect of linear low signal intensity [15].

Acute cholecystitis is usually caused by GB outlet obstruction. Acute calculous cholecystitis indicates the obstruction is caused by gallstones. Inflammation of the GB wall starts from the mucosa and progresses to all layers unless it is treated. Typical clinical manifestation includes fever, right-upper-quadrant pain, leukocytosis, and Murphy sign [17]. Imaging is usually necessary to make a confident diagnosis. Acute cholecystitis should be differentiated from other diseases, such as peptic ulcer, liver abscess, pancreatitis, renal colic, and retrocolic appendicitis. CT features include distended GB with thickened wall, pericholecystic inflammatory change, pericholecystic fluid, and transient hepatic attenuation difference in the GB bed of the liver caused by reactive hyperemia [18]. The complication of acute cholecystitis includes gangrenous cholecystitis, hemorrhagic cholecystitis, emphysematous cholecystitis, and perforation [19, 20].

Chronic cholecystitis results from wall damage by repeated attacks of inflammation for an extended period usually caused by gallstones, and it usually manifests as a thickened wall and luminal contraction by fibrosis and scarring. Chronic-cholecystitis-related complications include fistula, xanthogranulomatous cholecystitis, and porcelain GB [21, 22].

Adenomyomatosis, also termed adenomyomatosis hyperplasia, is a benign hyperplastic disease. It is relatively common, found in 2–5% of all surgical specimens of the GB [16]. Adenomyomatosis is defined as epithelial proliferation and muscular hyperplasia with Rokitansky- Aschoff sinuses. Adenomyomatosis of the GB is morphologically classified into three types: diffuse, segmental, and fundal. It may simulate GB cancer because it appears as a diffuse or localized wall thickening. Typical imaging findings are wall thickening with intramural cysts and echogenic intramural foci on US, enhancing epithelium within intramural diverticula surrounded by the relatively unenhanced hypertrophied GB muscularis (Rosary sign) on CT, and the pearl necklace sign on MRCP images [23].