MR Imaging Findings of Clonorchiasis explained on the basis of fluke size alone as the flukes are flat and 1.5 5 mm thick. Adenomatous hyperplasia, mucus in the bile ducts caused by cholangitis, periductal fibrosis and stricture may have an additional role in the occlusion of ducts and the resultant dilation of the proximal intrahepatic ducts (3, 6, 20 22). Lim et al. (10) reported that “too many intrahepatic ducts” sign in 22 (76%) of their 29 patients with clononorchiasis on direct cholangiography. In our study, MR cholangiography found “too many intrahepatic ducts” sign in 16 (62%) of the 26 patients. A slightly lower percentage of our study showed “too many intrahepatic ducts” sign than the previous study. We think this was caused by the different selection criteria. The inclusion data of the previous study did not included mild infestation of clonorchiasis, as many of the patients with clonorchiasis had not undergone direct cholangiography for a mild infestation. Our study included a mild infestation of clonorchiasis. Ultrasonography showed an increased echogenicity of the ductal wall, reflecting cholangitis and periductal fibrosis (17). Choi et al. (8) reported that CT failed to show thickening of the bile duct wall, which might be evidence of periductal fibrosis caused by the worms. On the contrary, Kim et al. (9) showed periductal contrast enhancement in 45% of clonorchiasis patients, thereby suggesting periductal fibrosis. Periductal enhancement was frequently observed in 21 (81%) of our cases. This was due to the increased spatial resolution of the soft tissue on MR compared to CT. The filamentous wavy and/or elliptical filling defects within the bile ducts, representing adult flukes, are pathog- nomonic findings on direct cholangiography, but the defects can be obscured by the contrast medium (7, 20). The filling defects of the worm are usually small and irregular, with a uniform size on direct cholangiography (6, 10, 20). The criterion for the diagnosis of stones is known to be the presence of a round, oval or multiangular signal void in the lumen of the bile duct on MRC (11, 14). In our study, the filling defects within the dilated intrahepatic bile ducts were shown in 6 (24%) of the 26 patients. The filling defects were elliptical, irregular shaped, hypointense lesions on the T2-weighted image and MRC. Although not all the patients underwent direct cholangiography or imaging-pathologic correlation, the filling defects of MR image may be regarded as fluke aggregates. C. sinensis has a high prevalence of coexistent with a hepatobiliary malignancy (3, 5, 23). Chung et al. (23) found that C. sinensis infection was associated with a cholangiocarcinoma about five times more often than with a hepatocellular carcinoma. Experimental study has shown Korean J Radiol 5(1), March 2004 that prolonged C. sinensis infection produces adenomatous hyperplasia and bile duct carcinomas (24). Our study showed that 9 (35%) of the 26 patients had hepatobiliary malignancy. In these patients, 7 had cholangiocarcinoma. There were no patients with other causes predisposing to a cholangiocarcinoma, such as ulcerative colitis, gallstones or recurrent pyogenic cholangitis. Two patients with hepato- cellular carcinomas had liver cirrhosis caused by hepatitis B infection, a well-known cause of hepatocellular carcino- mas. The differential diagnosis of clonorchiasis includes primary sclerosing cholangitis, recurrent pyogenic cholan- gitis and cholangiocarcinoma. Primary sclerosing cholangi- tis has stenosis, beading and pruning of the dilated intrahepatic bile duct; however, the ducts are much more irregular and serpiginous, which are unlikely in clonorchia- sis (19). Recurrent pyogenic cholangitis is characterized by decreased branching and abrupt tapering (“arrowhead” appearance) of the peripheral hepatic ducts as a result of stenosis of the peripheral ducts (25). Dilatation of the extrahepatic bile duct and a stone on recurrent pyogenic cholangitis are more common than clonorchiasis. The pattern of intrahepatic bile duct dilation in clonorchiasis is diffuse, mild dilation of the intrahepatic bile duct without abrupt tapering of the peripheral hepatic ducts. With a cholangiocarcinoma, the obstruction lesion is usually detected, and the entire tree proximal to the mass is proportionally dilated, not just the peripheral hepatic bile ducts, as in clonorchiasis (7). There were two limitations to this study; firstly, the MR sequences were variable due to the study being retrospec- tive. A prospective study will be necessary to evaluate the roles of the various MR sequences. Secondly, not all patients underwent direct cholangiography, which is regarded as the gold standard for depicting bile duct abnormalities, including ductal dilatation, stricture and filling defects. However, direct cholangiography is an invasive technique and does not depict intrahepatic bile duct dilatation on the proximal level of obstruction. Several reports have shown MRC to be a useful imaging technique that demonstrates the ductal changes in the biliary system (11 13, 16, 26). In conclusion, MR imaging demonstrates various findings of clonorchiasis, including mild diffuse dilatation, wall thickening and enhancement, and filling defects in the intrahepatic bile ducts and an associated hepatobiliary malignancy. The most common finding of clonorchiasis on MR imaging is diffuse, mild dilatation of the small intrahep- atic bile ducts without dilatation of the extrahepatic bile duct. 29