Magnetic resonance cholangiopancreatography: the ABC of MRCP

Secretin is an endogenous hormone normally produced by the duodenum, which stimulates exocrine secretion of the pancreas. When given as a synthetic agent intravenously (1 ml/10 kg body weight), it improves the visualisation of the pancreatic duct by increasing its calibre. Pancreatic juice flows out of the major duodenal papilla to progressively fill the duodenum. We perform a thick slab MRCP in the coronal oblique plane at baseline and then at 1, 3, 5, 7 and 9 min following injection. Its effect starts almost immediately and peaks between 2 to 5 mins. By 10 min, the calibre of the main pancreatic duct should return to baseline (Fig. 1a-c) with persistent dilatation of >3 mm considered abnormal. The indications for this technique include the detection and characterisation of pancreatic ductal anomalies and strictures, evaluation of the integrity of the pancreatic duct, characterisation of any communication between the pancreatic duct and pseudocysts/pancreatic fistulas, and the assessment of pancreatic function and sphincter of Oddi dysfunction.

This involves the use of MR lipophilic paramagnetic contrast agents, which when given intravenously, show hepato-biliary excretion. Contrast agents include gadobenate dimeglumine (Gd-BOPTA, Multihance; Bracco Imaging, Milan, Italy), gadolinium ethoxybenzyldiethylenetriamine penta-acetic acid (Gd-EOB-DTPA, Primovist; Bayer-Schering Pharma, Berlin, Germany) and, historically, mangafodopir trisodium (Teslascan; GE Healthcare, Oslo, Norway). Delayed imaging in the axial and coronal plane, performed between 10-120 min following intravenous administration, normally results in hyper-intense bile on 3D T1-weighted fat-saturated GRE images. The signal-to-noise ratio is higher than conventional T2-weighted MRCP, allowing better delineation of the bile ducts. This technique can be used for similar indications as for T2-weighted MRCP and in most cases has a similar diagnostic accuracy. It is more expensive than conventional T2-weighted MRCP and only the biliary tree is depicted. For these reasons, most centres continue to use conventional T2-weighted MRCP. However, functional MRCP does have a number of advantages, as follows: (1) it better demonstrates communications between cystic lesions and draining bile ducts in the diagnosis of congenital biliary disorders (e.g. Caroli’s disease) [11], (2) it helps to distinguish true obstruction in a dilated biliary system (where delayed or no biliary excretion is demonstrated) from pseudo-obstruction [9], and (3) it can demonstrate active extravasation of contrast in suspected bile leaks [12, 13]. Another advantage is that these gadolinium-based hepatobiliary-specific contrast agents initially distribute in the extracellular fluid compartment, thus allowing for early dynamic pre-contrast and post-contrast images in the arterial, portal venous and equilibrium phase prior to the functional cholangiogram.

In cases where there is significant biliary obstruction or impaired hepatocyte function, delayed images up to 24 h can be performed until contrast is seen in the gallbladder and duodenum.

A thick slab MRCP may obscure small filling defects or strictures as the spatial resolution is degraded because of volume averaging effects. Partial volume effects also degrade spatial resolution in MIP reformats, leading to the missed filling defects (Fig. 3a, b) and over- or under-estimation of strictures. Due to respiratory motion artefact, the biliary tree may appear stenotic, dilated, disconnected or duplicated on MIP reformats. Hence it is important to always review the original thin-section data set also.

A long cystic duct running parallel to the CBD may simulate a dilated common duct, whilst a contracted choledochal sphincter may mimic an impacted stone or stricture in the distal CBD. En face visualisation of the cystic duct insertion into the bile duct may also simulate a filling defect. Performing MRCP in multiple imaging planes or carrying out repeat MRCP imaging will help resolve these problems.

Filling defects in the bile may arise, not only from bile duct calculi but also from the presence of gas, debris, haemorrhage and tumour. Aerobilia is seen as a non-dependent filling defect on the axial images (Fig. 4a, b), whilst a signal void in the central part of the bile duct is due to flow phenomenon (Fig. 4c) and may occur in dilated ducts and at the point of insertion of a large cystic duct. The presence of iodinated contrast material will also reduce the signal intensity of bile.

Pulsatile vascular compression from adjacent vessels may mimic a stricture. The commonest site of extrinsic vascular compression is the common hepatic duct, followed by the left hepatic duct, both due to the right hepatic artery crossing its posterior aspect (Fig. 5a, b). The mid portion of the CBD may also be narrowed due to the gastro-duodenal artery. Pseudo-obstruction is typically seen as a bandlike compression with minimal proximal dilatation. Susceptibility artefacts from metallic clips and gas may give rise to difficulties in interpretation, although titanium clips used nowadays for cholecystectomy are not magnetic. Overlapping of the biliary tree with other stationary fluids (i.e. from adjacent bowel, cystic collections or ascites) may also cause interpretation problems.

A normal biliary anatomy is present in only about 60% of the population. There are many cystic duct variants [15, 16], including a low insertion, a medial insertion (Fig. 6), and drainage of the cystic duct into the right hepatic duct. The most important variant of intrahepatic bile duct branching clinically is the presence of an aberrant right posterior sectoral duct draining into the common hepatic duct or cystic duct in 5% of the population. This should be recognised on MRCP (if performed), as it may be ligated or cut at the time of cholecystectomy. The most common anatomic variant of intrahepatic bile duct branching, occurring in 13-19% of the population [15, 17] is drainage of the right anterior or posterior duct into the left hepatic duct (Fig. 7). This is important to detect in patients undergoing left hepatectomy for living related transplant donation as accidental ligation/resection may lead to cirrhosis of segments 5 and 8 or segments 6 and 7 respectively. A triple confluence [15] may also be seen (11%) where the right posterior duct, right anterior duct and left hepatic duct simultaneously drain into the common hepatic duct (Fig. 8a). When ERCP is used as the ‘gold standard’, MRCP has an accuracy of between 90 to 95% in the diagnosis of such congenital anomalies [16].

ERCP is often not possible in patients with a previous biliary-enteric anastomosis. MRCP is then useful in the demonstration of post-surgical biliary anatomy and in the detection of biliary complications, with a 100% sensitivity in the diagnosis of anastomotic strictures and a 90% sensitivity for choledocholithiasis [18].

This is the most common pancreatic variant and is associated with an increased prevalence of acute pancreatitis. There is failure of fusion of the dorsal and ventral pancreatic ducts. The larger dominant dorsal pancreatic duct drains the tail, body and superior head of the pancreas into the minor duodenal papilla (Fig. 9), whilst the smaller ventral duct drains the inferior pancreatic head and uncinate process into the major duodenal papilla, along with the CBD. MRCP has a 100% accuracy in the detection of this condition [19].

Choledochal cysts are associated with anomalous union of the pancreaticobiliary duct, where the pancreatic duct and CBD unite outside the duodenal wall and form a long common channel greater than 15 mm in length. There are five different types of choledochal cyst described (Todani classification) with three main types of anomalous pancreaticobiliary junction. MRCP can assist detection of such variants when suspected clinically.

This is usually performed in patients presenting with obstructive liver function tests and with suspected gallstones and/or a dilated CBD on ultrasound. It is also carried out in patients who have persistent symptoms and abnormal liver function following cholecystectomy. A systematic review of the literature has shown that when compared with ERCP, MRCP has an aggregated sensitivity, specificity, positive predictive value and negative predictive value of 85%, 93%, 87%, and 82% respectively [20]. Stones (as small as 2 mm) appear as dependent low-signal-filling defects within the CBD (Fig. 8a, b), surrounded by high-signal-intensity bile.

These usually develop following surgical injury (95%) from procedures such as laparoscopic cholecystectomy, hepatic resection, liver transplantation and biliary enteric anastomosis. Other causes include trauma, inflammation from choledocholithiasis, ischaemia involving the hepatic artery and primary sclerosing cholangitis (PSC). Typically a benign stricture involves a short segment, with a regular margin and symmetric narrowing. MRCP can demonstrate the site and extent of the stricture with a reported sensitivity of 91-100% [21].

PSC is a fibrosing inflammatory process of the bile ducts, resulting in stenosis of both intrahepatic (80%) and extrahepatic ducts. Seventy percent of patients have inflammatory bowel disease [22] and 10% of patients with PSC will develop a cholangiocarcinoma. MRCP is not as sensitive as ERCP in the detection of early changes, but is useful for follow-up in established cases, in order to determine severity and to recognise the development of complications. On MRCP, the ducts appear beaded or have a ‘pruned tree’ appearance with multifocal strictures demonstrated, with intervening normal or slightly dilated ducts seen (Fig. 10a, b).

Cholangiocarcinoma is a tumour arising from the bile ducts. On MRCP the periductal type is seen as a biliary stricture, involving the CBD, common hepatic duct, biliary bifurcation (Klatskin tumour) or intrahepatic ducts. Proximal bile duct dilatation occurs (Fig. 11a). Suspicious features include increased wall thickness (>3 mm), increased signal intensity on T2-weighted images (Fig. 11b) and progressive enhancement of the bile ducts (due to the fibrotic component of the tumour) following intravenous administration of gadolinium. A malignant extrahepatic bile duct stricture is likely to be longer than a benign stricture, with an irregular margin and asymmetric narrowing.

Pancreatic adenocarcinoma usually appears as a focal mass, most often in the head of the pancreas, leading to encasement and obstruction of the pancreatic duct and/or CBD. Dilatation of both ducts is seen in approximately 75% of cases appearing as the ‘double duct’ sign on MRCP (Fig. 12). Although contrast-enhanced CT is conventionally used for staging local and distant spread in pancreatic cancers, MRCP may be used as an initial investigation in identifying the level of obstruction in patients presenting with painless obstructive jaundice.

Peri-ampullary carcinoma can lead to high-grade obstruction of the CBD with abrupt termination on MRCP (Fig. 13). There is usually only mild dilatation of the pancreatic duct.

Chronic inflammation of the pancreas results in parenchymal destruction with fibrosis, fat necrosis and dystrophic calcification. Strictures in the main pancreatic duct may eventually develop a ‘chain-of-lakes’ appearance with alternating stenoses and dilatation. Side branch ectasia and intraductal calculi occur (Fig. 14). In advanced cases there can be marked dilatation of both the pancreatic duct and CBD simulating the ‘double duct’ sign seen with carcinoma of the head of pancreas. The sensitivity of MRCP in the detection of early side branch changes can be increased with the use of secretin-stimulated MRCP [23].

Pseudocysts are encapsulated fluid collections seen in both acute and chronic pancreatitis (Figs. 5, 6). These often develop within the lesser sac. MRCP is more sensitive than ERCP in demonstrating these fluid collections and may show their connection with the pancreatic duct. With ERCP, less than 50% of pseudocysts opacify with contrast [24].

Cystic pancreatic tumours include serous cystadenomas, mucinous cystic neoplasms and intraductal papillary mucinous neoplasm (IPMN). Often, small cystic lesions within the pancreas are detected incidentally on CT performed for other indications. MRCP can help delineate these tumours more clearly (Fig. 15a, b). Table 2 shows the difference in epidemiology and MRCP morphology between these cystic tumours.

Following transection of the bile ducts (usually following surgery), bile accumulates within the sub-hepatic space. Fluid collections can be appreciated on MRCP with transection of the affected bile duct. If functional MRCP is used, extravasation of contrast from the biliary tree is seen. A stricture may develop following accidental ligation or transection, with upstream dilatation demonstrated on MRCP.