MR Imaging

MR cholangiopancreatography (MRCP) is based on the principle that stationary fluids are signal intense on heavily T2-weighted images as opposed to soft tissue structures, solid materials such as calculi, or blood vessels that contain flowing blood. MRCP thus enables the visualization of bile ducts, intraductal calculi, stenoses, and anatomical variants of the biliary system, without the need for contrast material and independent of excretory function. Modern MR techniques enable image acquisition within very short breath-hold intervals (Fig. 36). MRCP has rapidly become recognized as an attractive alternative to the established noninvasive imaging techniques for imaging of the biliary system because it has no known side effects and because it has yielded excellent diagnostic results for both bile duct calculi and stenoses. MRCP may now be considered as a very accurate technique for the diagnosis of bile duct calculi and stenoses, provided that the imaging parameters are chosen correctly and that image interpretation is based on both coronal and transverse images. With modern equipment, MR images are acquired in short breath-hold intervals with very consistent quality and excellent resolution. Hepatobiliary contrast materials such as Mangafodipir (Mn-DPDP) are excreted into the bile ducts and allow direct visualization of the biliary tree on

Fig.3. Choledochal cyst. T2- weighted 'EXPRESS' image in the coronal projection (a) and transverse plane (b) delineates the morphology and extent of the cystic malformation, making direct cholangiography unnecessary prior to surgery

Table 1. Diagnostic key elements

T1-weighted images. There are some specific clinical indications in the context of bile duct imaging, such as post-operative or post-traumatic bile leaks [1].

Radiologic Diagnosis of Selected Bile Duct Pathologies

Some diagnostic key questions are summarized in Table 1.

Choledocholithiasis is present in approximately 15% of patients undergoing cholecystectomy. The sensitivity of US for bile duct calculi varies widely, from less than 30% to over 70%, but there is a general consensus that US is not sufficiently reliable to rule out bile duct calculi. Although the reported sensitivity of CT is between 80% and 90%, MRCP is superior to CT and may currently be considered the most sensitive noninvasive technique for the detection of bile duct stones; several studies have confirmed that appropriate, modern MRCP technique enables the detection of more than 95% of calculi [2, 3].

Primary sclerosing cholangitis (PSC) is a chronic, nonin-fectious inflammatory disorder that results in progressive,

Table 1. Diagnostic key elements

• Stone disease (cholecystolithiasis, choledocholithiasis, intrahepatic lithiasis, Mirizzi syndrome)

• Postoperative situations (previous cholecystectomy, sphincterotomy or bilio-digestive anastomosis resulting in enlarged bile ducts, pneumobilia, acute bile leakage, etc.)

• Presence of dilatation of intra- or extrahepatic bile ducts

• Level of bile duct dilatation (intrahepatic/bifurcation/common hepatic, ampulla)

• Cause of bile duct dilatation (stone, solid or cystic mass, stenosis, functional disorder)

• Vascular changes (infiltration/ thrombosis of portal vessels, hepatic arteries)

Fig. 5. Cholangiocar-cinoma at the level of the common hepatic duct. a Breath-hold coronary projection (T2-weighted 'EXPRESS') image showing a stenosis of the common hepatic duct, involving the hepatic bifurcation. Dynamic Gadolinium-enhanced T1-weighted GRE images in the arterial phase (b), portal venous phase (c) and equilibrium phase (d) show the tumor as an enhancing, hy-perintense mass

Fig.6. Adenocarcinoma of the ampullary region. T2-weighted 'EXPRESS' image shows a large mass displacing the duodenum and an intraductal component of the tumor, causing obstruction

multiple stenoses of the intra- and extrahepatic bile ducts, eventually resulting in obliteration of intrahepatic ducts and asymmetrical hepatic atrophy. The etiology is unclear, but there is an association with inflammatory bowel disease and other conditions. Initially, there is edema and proliferation of connective tissue in the periportal spaces and around the bile ducts. Progressive extension of fi-brotic proliferation into the hepatic parenchyma eventually results in biliary cirrhosis. Although stenoses of the bile ducts may be solitary, typical cholangiographic features include multifocal stenoses of the bile ducts, often with bilateral intrahepatic involvement. However, dilatation of the peripheral ducts is usually less prominent than with stenosis of other origin because the bile duct walls become thickened and rigid due to the inflammatory process and therefore are less prone to passive dilatation (Fig. 2). PSC significantly predisposes to transformation into cholangiocarcinoma and should therefore be considered a premalignant condition. The distinction between PSC and cholangiocarcinoma may be impossible with imaging tests alone. In the presence of PSC, adenopathy of the he-patoduodenal ligament or in the celiac or pancreaticoduo-denal region is often due to reactive inflammation and should not by itself be taken as a strong indicator of malignant degeneration. Cross-sectional imaging techniques are now being primarily used. Although the diagnosis of PSC may be straightforward in the presence of mild, irregular, multisegmental intrahepatic bile duct dilatation without demonstration of a distinct mass, it must be kept in mind that early stages of the disease may be very difficult to detect and that correlation with hepatic biopsy may be required. Besides irregular, multisegmental intrahepat-ic bile duct dilatation, US may demonstrate focal or generalized thickening and hyperechogenicity of the walls of the intra- and extrahepatic bile ducts. The CT signs of PSC include nodularity and enhancement of the wall of intra- or extrahepatic bile ducts, mild multifocal intra- or extrahepatic biliary dilatation leading to patterns such as skip lesions, pruning, and beading. The typical findings of PSC are also readily recognized with MRCP, and some authors have reported favorable results [4]. Due to its superior resolution, direct cholangiography remains the method of choice, showing, besides typical strictures, an irregular aspect of the biliary mucosa with small out-pouchings or microdiverticula.

Bile duct cysts must be distinguished from multiple intra-hepatic cysts without connection to the biliary tree. In typical cases, the diagnosis may be readily made with noninvasive imaging studies [5]. Today MRCP is the method of choice for noninvasive delineation of benign cystic biliary disorders (Fig. 3, 4). The absence of ionizing radiation is a major advantage in this benign disorder, which often affects young adults and children. Congenital cystic malformations of the bile ducts may, however, be associated with adult polycystic disease of the liver and kidney. This may sometimes represent a diagnostic challenge, and a complete radiologic workup may then require a cholangio-graphic study in addition to a standard US or CT examination. Correct diagnosis of cystic biliary malformations is also important because these lesions may predispose to the development of cholangiocarcinoma.

Choledochal cysts may be congenital or acquired, and appear as large cystic masses within the hepatoduodenal ligament, usually along with an abrupt caliber change. They may extend to the intrahepatic level, either unilaterally or bilaterally. Choledochal cysts may be classified according to their location and their concentric or asymmetrical appearance.

Choledochocele is a cyst-like dilatation of the distal portion of the common bile duct with protrusion of the am-pullary portion into the duodenal lumen. It may be congenital or acquired, probably due to stone passage. The imaging features of choledochocele include a smooth, club-like cystic mass that protrudes into the duodenal lu men, usually along with moderate dilatation of the extra-hepatic common bile duct.

Caroli's disease is a congenital ectasia of the intrahepat-ic bile ducts. Although it is a benign condition, it is associated with a poor prognosis because it predisposes to intrahepatic sludge and lithiasis with subsequent cholan-gitis and abscess formation. The typical radiologic features of Caroli's disease include a saccular, cyst-like appearance of the intrahepatic bile ducts along with sludge and/or small calculi. Dilatation of the extrahepatic bile ducts may be primarily associated, but may also develop secondarily due to the passage of calculi.

Malignant neoplasms involving the bile ducts typically present with painless obstructive jaundice [6]. Primary tumors originating from the bile ducts are referred to as cholangiocarcinoma. The so-called Klatskin tumor is a scirrhous form of cholangiocarcinoma, which spreads within the ductal tissue without a major mass lesion and is commonly located at the hepatic bifurcation (Fig. 5). The polypoid, intraductal form and the intrahepatic masslike form of cholangiocarcinoma are much less common. Since advanced cholangiocarcinoma often contains abundant fibrotic components, it may display an increased contrast uptake on delayed, contrast-enhanced CT or MR images. Tumors of the distal bile duct, such as ampullary carcinoma, need to be distinguished from pancreatic carcinoma, since prognosis and treatment may be different (Fig. 6) [7]. A wide variety of secondary neoplasms may cause obstructive jaundice due to mechanical compression of the intra- and extrahepatic bile ducts. Carcinoma of the pancreas, gallbladder, stomach, colon and breast, and malignant lymphoma need to be considered in this context.

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