Dr. Qayyum is an Assistant Professor of Radiology, and Dr. Coakley is an Associate Professor of Radiology and Chief of Abdominal Imaging, Department of Radiology, University of California, San Francisco, CA.

Magnetic resonance cholangiopancreatography (MRCP) is the partial or complete selective imaging of the biliary system and pancreatic duct, which utilizes the inherent contrast of the fluid-filled ducts to generate images. The partial or complete selectivity for ductal imaging distinguishes MRCP from other imaging modalities used for imaging the pancreatobiliary system, such as computed tomography (CT) or ultrasound. MR cholangiopancreatography has undergone considerable technical advances over the last 10 years, rendering it an alternative to endoscopic retrograde cholangiopancreatography (ERCP) in most clinical settings. This review will discuss some of the relative advantages and disadvantages of MRCP, important aspects of technique, and major clinical applications.

Advantages and disadvantages of MRCP

The major advantage of MRCP over ERCP or percutaneous transhepatic cholangiography is the noninvasive nature of the technique. Conventional cholangiography has a major complication or adverse event rate of approximately 3%. Such events include sepsis, bleeding, bile leak, and death. ^1,2 In addition, MRCP does not require sedation and is more economical. A further advantage of MRCP is the ability to partially or completely selectively depict the ductal structures. Partial selection of duct depiction enables assessment of the pancreatic paren-chyma and adjacent tissues. Conventional cholangiography depicts only the duct lumen, and periductal pathology can only be inferred indirectly from these images. ³ Because MRCP relies on signal from fluid within ducts and not on intraductal contrast injection, all fluid-filled structures within the field-of-view are displayed. This offers the advantage of visualizing excluded duct segments or cystic tumors that do not communicate with the pancreaticobiliary tree.

The major disadvantage of MRCP compared with conventional cholangiography is a lower spatial resolution, which limits evaluation of subtle small duct changes, eg, side branch changes of chronic pancreatitis and sclerosing cholangitis. Ductal distension from the contrast injection during ERCP does not occur with MRCP, which is a contributing factor to the limited assessment of small branches. A potential criticism of MRCP is that endobiliary therapeutic procedures cannot be performed. Many patients who undergo ERCP do not have therapeutic intervention. In patients who undergo ERCP for choledocholithiasis prior to cholecystectomy, 40% to 70% of examinations were negative for the presence of common bile duct stones. ⁴

Technique

MR cholangiopancreatography should be performed with surface phased-array multicoils, because of the higher signal-to-noise ratio and consequent increase in spatial resolution. ⁵ The basis of MRCP is the use of high-resolution breath-hold T2-weighted sequences to image the pancreatic and biliary ducts. Fluid is bright on T2-weighted sequences, which can be used to visualize the ductal lumen. No exogenous contrast is required, as the contrast mechanism is entirely endogenous. Single-shot rapid acquisition with relaxation enhancement (SS-RARE) has emerged as the optimal sequence for MRCP, with the combined advantage of rapid image acquisition and relatively high spatial resolution. ⁶ This protocol is an ultra-fast T2-weighted sequence that enables sub-second slice acquisition, which largely overcomes the problem of motion artifact. Imaging of the pancreatic and biliary ducts can be performed in a single breath-hold. Physiologic motion, particularly breathing, is a major contributing factor to image degradation, and has been the greatest obstacle to diagnostic quality MRCP. Breath-hold imaging is essential to reducing this source of image degradation. Section misregistration or motion artifacts severely limit assessment of small structures. Single-shot rapid acquisition with relaxation enhancement is less sensitive to magnetic susceptibility artifacts than are gradient-echo sequences. Common commercial versions of SS-RARE include single-shot fast-spin-echo (SSFSE; GE Medical Systems, Waukesha, WI) and half-Fourier acquisition single-shot turbo-spin-echo (HASTE; Siemens Medical Systems, Iselin, NJ) imaging.

The SS-RARE sequences are performed using a long echo time (TE) value (eg, 150 to 600 msec) with fat suppression, or by using an extremely long TE value (eg, 600 to 1200 msec) without fat suppression. ⁷ Signal acquisition from a single thick slice, or maximum intensity projection (MIP) postprocessing of multiple thin slices, can be used to generate images resembling a cholangiogram. However, source images may yield more information than cholangiogram-like images. In a study of 108 patients, ⁸ the reported sensitivity for detection of stones was 87% using source thin slices, 24% using MIP reconstruction, and 48% using single thick-slice MRCP. An alternative approach to MRCP is to use an intermediate TE value (eg, 100 msec) without fat suppression. This enables visualization of both the fluid-filled ducts and periductal structures, but MIP postprocessing is not possible. MR cholangiopancreatography techniques therefore usually include both sequences. Contiguous 3- to 5-mm slices result in good image quality. Thinner slices may be excessively degraded by poor signal-to-noise ratio. Smaller filling defects may be missed on thicker slices. A 256 * 192 to 256 acquisition matrix is usually adequate. In patients with limited breath-holding capacity, images can be acquired as >= two or more contiguous or overlapping stacks, rather than interleaved stacks, in order to reduce slice misregistration. In uncooperative patients, reasonable quality images may be acquired during quiet respiration.

Clinical applications

Biliary disease and strictures

Ductal dilatation is depicted accurately by MRCP with sensitivity of 95%. ^9-11 The additional benefit of MRCP is the ability to localize the site, length, and cause of obstruction. Strictures are seen as focal ductal narrowing with proximal dilatation (Figure 1). The absence of proximal dilatation may be indicative of a diffuse ductal pathology, such as sclerosing cholangitis, or of a lack of duct distensibility, such as in cases of cirrhosis. Tight strictures may appear as segments of signal loss, limiting characterization. The use of partially duct-selective MRCP enables depiction of extra-ductal anatomy, which is useful in determining the cause of obstruction. In a study of 79 patients, MRCP enabled correct diagnosis of malignant obstruction with a sensitivity and specificity of 86% and 98%, respectively. ⁹ MR cholangiopancreatography has demonstrated high accuracy in the assessment of higher grade and larger duct strictures. In a recent study of 34 post-operative patients, 6 of whom had strictures proven by direct cholangiography, MRCP showed a sensitivity of 100% and specificity of 87%. ¹² Distinguishing distal common duct obstruction secondary to ampullary stenosis, edema, or sphincter of Oddi dysfunction may be problematic with MRCP and may require direct visualization of the ampulla.

Primary sclerosing cholangitis

The diagnosis of subtle small duct changes of sclerosing cholangitis is difficult with MRCP. However, in a recent study of 136 patients (34 diagnosed with sclerosing cholangitis at ERCP), MRCP demonstrated a sensitivity of 85% to 88% and specificity of 92% to 97% for two independent readers. ¹³

Choledocholithiasis

Technologic advances and experience have led to improved sensitivity and specificity of MRCP in choledocholithiasis from 81% and 98% to 90% and 100%, respectively. ^9,14 Further, recently reported accuracy of MRCP in choledocholithiasis indicates results similar to ERCP. ^15-17 Biliary stones are depicted as round or faceted foci of reduced signal or signal void surrounded by high signal intensity bile (Figure 2). Intraluminal filling defects may also be due to pneumobilia, blood clots, intraductal tumors, and parasites. Recognized pitfalls giving rise to false-positive findings also include signal voids from surgical clips, and the right hepatic artery as it traverses the upper portion of the common duct. ^18,19 Despite the accurate depiction of biliary stones, the clinical role of MRCP in choledocholithiasis is not yet clear. Patients with strong clinical, biochemical, and sonographic or CT evidence of choledocholithiasis typically require therapeutic ERCP. MR cholangiopancreatography is valuable in confirming common bile duct stones in cases in which ERCP is higher risk. This group of patients includes those with acute pancreatitis, prior biliary-enteric anastomosis, proximal enteric strictures, and cardiopulmonary disease, as well as pregnant women.

MR cholangiopancreatography is also indicated following failed ERCP cannulation. Reported failure rates for ductal cannulation vary from 3% to 10%. ^1,20,21 Difficult cannulation often requires repeated ERCP attempts and precut papillotomy, with increased risk of complications, including hemorrhage, pancreatitis, perforation, and cholangitis. ²² Unsuccessful ductal cannulation may require antegrade cholangiography or laparotomy for common duct access. MR cholangiopancreatography increases diagnostic accuracy and enables appropriate patient selection prior to such invasive procedures.

MR cholangiopancreatography is indicated in patients with low to moderate clinical suspicion for common duct stones, because the relative risks of ERCP as a primary diagnostic modality may be difficult to justify. MR cholangio-pancreatography is useful in patients undergoing laparoscopic cholecystectomy. Common duct calculi are found in 10% to 15% of patients who undergo cholecystectomy. ²³ The high sensitivity and specificity of MRCP for choledocholithiasis may be used to limit preoperative ERCP to those patients requiring therapeutic intervention.

Chronic pancreatitis

In a study of 37 patients, the reported MRCP sensitivity was 87% to 100% for dilatation, 75% for narrowing, and 100% for ductal calculi compared with ERCP. ²⁴ In 1994, Outwater et al ²⁵ published one of the more comprehensive studies of MRCP in chronic pancreatitis. Two independent reviewers evaluated the MRCP findings in 39 patients with chronic pancreatitis, using ERCP as the standard of reference (Table 1). MR cholangiopancreatography had a greater accuracy for the diagnosis of main duct changes rather than the more subtle side branch abnormalities. These findings are expected because of the limitations of spatial resolution with MRCP (Figures 3 and 4).

Acute pancreatitis

The role of ERCP in acute pancreatitis is to remove retained stones in the common bile duct or papilla. The benefit of early ERCP in acute pancreatitis remains controversial. A benefit suggested by early studies ^26,27 was not supported in a recent controlled trial in patients without continued jaundice or cholangitis. ²⁸ MR cholangiopancreatography could limit the use of ERCP to patients requiring therapeutic intervention. ¹⁴

Neoplastic obstruction

Obstruction secondary to neoplasia presents a diagnostic and therapeutic challenge. Treatment planning requires identification of the level and cause of obstruction. Earlier studies were disappointing; for example, Wallner et al ²⁹ reported the level and cause of obstruction depicted by a gradient-echo MRCP technique in only 7 (78%) and 5 (56%) of 9 patients. Recent studies have demonstrated better results, which most likely reflect improved techniques. Schwartz et al ³⁰ reported correct identification of the level of obstruction in 27 (84%) and 28 (88%) of 32 patients by two independent observers, respectively, using breathhold SS-RARE MRCP without fat suppression and an intermediate T2 value of 100 msec. The site of underlying tumor was correctly identified in 27 (84%) and 29 (91%) of the patients, with pathologically confirmed neoplastic duct obstruction. Only the MRCP images were viewed, indicating that the results reflect the ability of the MRCP technique in isolation to provide the same information that would traditionally have been expected from conventional cholangiography (Figure 5).

Intraductal papillary mucinous tumor of the pancreas

Intraductal papillary mucinous tumors (IPMT) (mucin-hypersecreting tumor, duct ectatic mucinous cystic tumor) arise from the epithelium of the main pancreatic duct or its branches. Intraductal papillary mucinous tumor of the pancreas has a primarily intraductal, papillomatous growth pattern associated with excessive mucin secretion, resulting in progressive ductal dilatation. ³¹ Tumors may arise from the main duct or side branches with a predilection for the pancreatic head. Segmental or diffuse main duct tumors are seen as ductal dilatation, which may be associated with parenchymal atrophy, ³² and may be difficult to distinguish from chronic pancreatitis. Diagnosis is confirmed on ERCP in the presence of a bulging, dilated papilla with large amounts of mucin leaking from the ampulla of Vater. ³³ MR cholangiopancreatography can depict a bulging or gaping papilla, which is suggestive of IPMT when it measures >1 cm. ³³ Pancreatic cystic dilated tubular structures, which may or may not be seen to communicate with the main duct, is suggestive of IPMT (Figures 6 and 7). Malignant transformation of IPMT is recognized, and resection is recommended where possible. ³¹

Anatomic variants

Pancreas divisum is the most common developmental anomaly of the pancreatic duct and is due to nonunion of the duct of Wirsung and the duct of Santorini, with main drainage through the duct of Santorini to the minor papilla rather than to the ampulla of Vater. The frequency of pancreas divisum varies from 4% to 14% at autopsy. The key diagnostic findings are the passage of the main pancreatic duct to the minor papilla with the ventral duct being smaller than the dorsal duct, or not visible (Figure 8). In a study of 108 patients, (6 with pancreas divisum by ERCP), the sensitivity and specificity of MRCP was reported as 100%. ³⁴ The clinical importance of pancreas divisum is related to the following factors: incomplete pancreatic duct visualization from the standard major papilla cannulation during ERCP; misinterpretation of the short ventral duct for other causes of main duct cut-off, such as adenocarcinoma; and possible functional obstruction from the minor papilla to pancreatic duct drainage in some people with consequent elevation in duct pressure and pancreatitis. ³⁵

Several clinically significant congenital anomalies of the biliary system have been described. These anatomical variants are important because of the associated risk of bile duct injury during laparoscopic cholecystectomy, or because they may represent relative contrary indications to right hepatectomy in living related liver donors. Prior to consideration of laparoscopic cholecystectomy, it is important to recognize a low cystic ductal insertion, a medial cystic ductal insertion, a long, paralleled course of the cystic duct with the common duct, or a short cystic duct. Aberrant drainage of the right posterior segmental duct to the common duct confluence or left hepatic duct may represent a relative contraindication to right hepatectomy in living related liver donors (Figure 9). The variation in biliary ductal anatomy may be depicted by MRCP. In a series of 171 patients, using direct cholangiography as a standard of reference, MRCP showed a sensitivity and specificity of 86% and 100%, respectively, for the diagnosis of variant cystic duct anatomy. For the depiction of an aberrant right posterior segmental duct, the reported sensitivity and specificity were 71% and 100%, respectively. ³⁶

Postcholecystectomy complications

Postcholecystectomy disorders are rare but are increasingly frequent as a result of the rapid acceptance of laparoscopic cholecystectomy. Such complications include bile duct injury, retained bile duct stones, biliary leak, and biliary fistula. ³⁷ Postcholecystectomy biliary complications have traditionally been imaged with ultrasound or CT, followed by ERCP or percutaneous transhepatic cholangiography. MR cholangiopancreatography has also been used to demonstrate post-surgical complications. In a recent study of 17 patients referred for evaluation of suspected postcholecystectomy biliary disorders, investigators used breath-hold SS-RARE MRCP to establish a diagnosis, which was confirmed at surgery or by follow-up imaging. ³⁸ Complications included biliary occlusion, peribiliary lesions, nonspecific biliary dilatation, bile duct stones, and biliocolic fistula. Two independent readers correctly categorized the complications in 15 (88%) and 13 (76%) cases, respectively, with excellent interobserver agreement (kappa = 0.82). Biliary strictures and transections were not distinguished at MRCP, but grouped together as occlusion.

Conclusion

MR cholangiopancreatography continues to evolve as a noninvasive imaging technique of the biliary and pancreatic ductal system, with results comparable to ERCP, but without the intrinsic risks. Common current indications for MRCP include unsuccessful or contraindicated ERCP. ^39,40 Patient preference for noninvasive imaging may also be a consideration. MR cholangiopancreatography should be considered if there is a low index of suspicion for pancreatic or biliary disease and therapeutic ERCP is considered unlikely, as well as in patients with suspected neoplastic pancreatic or biliary obstruction prior to surgical or interventional planning.

An unresolved issue is whether MRCP should be performed alone, with a limited charge, or as part of a full abdominal study. This was recently addressed in a study of 62 patients with biliary dilatation of known cause. ⁴¹ The diagnostic accuracy in the differentiation of benign from malignant causes of obstruction was greater using combined MRCP images, standard T1, T2, and postgadolinium images for both readers. The possibility of missing tumors or other significant pathology with the use of only MRCP images argues for mandatory imaging with a full study. However, if imaging is requested to answer a specific question, such as the evaluation of the common duct for stones, it may be reasonable to perform a limited study, provided the clinician is aware of the ramifications.

Currently recognized limitations of MRCP include lack of dynamic information and limited resolution. Ongoing technical refinements are likely to result in diagnostic improvements. For example, stimulation of pancreatic exocrine secretion using secretin infusion has demonstrated reduced duodenal filling and persistent ductal distention secondary to papillary stenosis in patients with suspected pancreatic disease. ⁴² The recent use of manganese as an intravenous biliary MR contrast agent allows depiction of a functional component to MRCP, rather like cholescintigraphy. Manganese facilitates T1 relaxation within hepatocytes and was developed initially as a hepatic parenchyma contrast agent. Intravenous manganese undergoes hepatobiliary excretion, and on delayed images the biliary tree is depicted as high signal intensity on T1-weighted images ⁴³ (Figure 10). Such advances, in conjunction with improved technology, should overcome concerns about the utility of MRCP. The role of MRCP is to provide a noninvasive imaging alternative to ERCP for diagnosis, ultimately limiting the use of invasive techniques to therapeutic procedures. AR