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Magnetic resonance cholangiopancreatography
I. ADVANCES
IN IMAGING OF THE
HEPATOBILIARY TREE
Magnetic resonance cholangiopancreatography Fergus V. Coakley, MD, Aliya Qayyum San Francisco, California
Magnetic resonance cholangiopancreatography (MRCP) refers to selective or partially selective MR imaging of the pancreatic and biliary ducts. Complete or partial selectivity for ductal imaging is an important component of this definition, because this selectivity distinguishes MRCP from other cross-sectional modalities such as CT and ultrasound, which can also be used to assess pancreatic and biliary ductal disease. MRCP was first described in the 1980s. Since then MRCP has undergone considerable technical evolution and maturation and has reached the stage at which MRCP is a viable alternative to diagnostic endoscopic retrograde cholangiopancreatography (ERCP) in most settings (Fig. 1). This article discusses some of the relative advantages and disadvantages of MRCP, important aspects of technique, and major clinical applications. Emphasis is placed on the scientific data supporting the contention that MRCP is competitive with ERCP. It should be noted that many clinical studies of MRCP use ERCP as the standard of reference. This carries the implicit and unstated assumption that ERCP results are absolute and objective. It is true that in practice ERCP is often considered the definitive test for structural disease of the pancreatic and biliary ducts. However, as with any other imaging test, ERCP is subject to error and variability, as illustrated by studies in which clinical or surgical results are used as the standard of reference. For example, in a study of 124 patients with suspected pancreatic cancer in which the standard of reference was biopsy or one year of clinical followup, the sensitivity and specificity of MRCP for the diagnosis of pancreatic cancer were 84% and 97%, respectively, compared with a sensitivity and specificity of 70% and 94% for ERCP.1 Similarly, in a study of 72 patients undergoing cholecystectomy, preoperative ERCP had a sensitivity and specificity of 91% and 100%, respectively, for the diagnosis of From the Department of Radiology, Abnormal Imaging, University of California–San Francisco, California. Reprint requests: Fergus V. Coakley, MD, Radiology Department, USCF Medical Center, San Francisco, CA 94122. Copyright © 2002 by the American Society for Gastrointestinal Endoscopy 0016-5107/2002/$35.00 + 0 37/0/124751 doi:10.1067/mge.2002.124751 S2
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A
B Figure 1. Comparison of an ERCP (A) and a contemporaneous MRCP (B) in a patient with an unremarkable biliary duct system. As commonly performed, MRCP uses heavily T2-weighted sequences to depict the fluid within the ducts as high signal intensity. State of the art MRCP now provides very similar images to ERCP.
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common duct stones, using intraoperative cholangiography and bile duct exploration as a standard of reference.2 ADVANTAGES AND DISADVANTAGES OF MRCP MRCP is less expensive than ERCP and does not require deep sedation. However, the major advantage of MRCP over direct (or conventional) cholangiography such as ERCP or percutaneous transhepatic cholangiography is the noninvasive nature of the procedure. This is not a trivial consideration, because the major complication rate of convention cholangiography is approximately 3%. Such complications include sepsis, bleeding, bile leak, and death.3,4 The major disadvantage of MRCP compared with conventional cholangiography is a somewhat lower spatial resolution, such that MRCP continues to be partially limited in the assessment in fine detail (Fig. 2), such as subtle small duct changes of sclerosing cholangitis and side branch changes of chronic pancreatitis. The suboptimal depiction of small duct disease is a result not only of current technical limitations of spatial resolution but also of the lack of duct distention during MRCP compared with the direct introduction of contrast during conventional cholangiography. Another important advantage of MRCP is that ductal depiction can be entirely selective, so only the ductal content is depicted, or semi-selective, so the ductal content is depicted with the surrounding tissue. This latter technique can be especially useful in neoplastic disease of the pancreatic or biliary ducts.5 In contrast, conventional cholangiography directly depicts only the duct lumen, and periductal pathology is only indirectly inferred from these images.6 Furthermore, when MRCP is performed as part of a full abdominal MRI study, the examination can provide a one-stop evaluation of the nature and site of ductal disease and the extent and stage of any underlying tumor, including detection of associated adenopathy or liver metastases.7 Finally, MRCP also has the advantage of displaying all fluid-filled structures within the field of view, not simply those structures that have been made opacified by contrast injection. This can be particularly useful for the depiction of excluded duct segments or cystic tumors that do not communicate with the pancreaticobiliary tree (Fig. 3). A potential criticism of MRCP is that endobiliary therapy cannot be performed as part of the procedure. This may not always be a disadvantage because MRCP results can be utilized for interdisciplinary planning of definitive therapy without the potentially negative effects of endobiliary intervention. For example, two separate studies of periampullary tumors and proximal cholangiocarcinoma performed VOLUME 55, NO. 7 (SUPPL), 2002
A
B Figure 2. ERCP and MRCP in a 45-year-old patient with primary sclerosing cholangitis and a previous right hepatectomy for cholangiocarcinoma. A, Multifocal ductal irregularity and stricturing are better appreciated on ERCP. B, MRCP remains somewhat limited in the assessment of small duct disease, because of a somewhat lower special resolution and the lack of duct distention by exogenous contrast during MRCP.
at Memorial Sloan-Kettering Cancer Center have shown that preoperative endobiliary stent placement is associated with increased bacterial contamination of the biliary system, wound and intra-abdominal complications, and postoperative infections.8 In addition, many patients undergoing ERCP have only a diagnostic study and no therapeutic interventions are performed. Such patients would be ideal candidates for MRCP, provided they could be identified with reasonable accuracy before imaging evaluation. TECHNIQUE As routinely performed, MRCP utilizes high-resolution breathhold T2-weighted sequences to obtain GASTROINTESTINAL ENDOSCOPY
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A
A
B
B Figure 3. ERCP and MRCP in a 75-year-old man with vague upper abdominal symptoms. A, ERCP shows dilatation of the gallbladder and biliary system to the level of the ampulla of Vater where there is a gradual tapered narrowing. The cause of this narrowing is not clear. B, MRCP shows a large noncommunicating cystic tumor is the cause of obstruction. Pathologic analysis showed a mucinous cystadenoma with foci of carcinoma in situ. One of the advantages of MRCP is the ability to depict noncommunicating cystic pathology and excluded duct segments.
diagnostic quality images of the pancreatic and biliary ducts. Fluid is bright on T2-weighted sequences, and, therefore, such sequences can be used to depict the fluid within the ductal lumen. That is, MRCP utilizes the intrinsic brightness of fluid on T2weighted sequences; this contrast mechanism is S4
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Figure 4. Gradient echo T1-weighted coronal image and SSRARE T2-weighted coronal image in a patient who has had a cholecystectomy. A, A large signal void is seen in the gallbladder fossa on the gradient echo sequence as a result of magnetic susceptibility artifact induced by the cholecystectomy clips. B, The SS-RARE sequence is much less susceptible to artifact, and no signal void is evident on this sequence. Low susceptibility to artifact is one of the advantages of the SS-RARE sequence, which is currently the most widely used sequence for MRCP.
entirely endogenous and no exogenous contrast is required. As described it would appear to be a simple and straightforward procedure, but the requirement for high quality T2-weighted imaging that is also rapid enough to be performed during breathholding has been one of the major challenges in abdominal MRI. When first described, MRCP was usually performed with gradient echo-based sequences, because at the time these represented the fastest available T2-weighted sequences with reasonable spatial resolution. Later, fast spin-echo sequences were developed, which gradually replaced gradient echo-based MRCP. More recently, singleshot rapid acquisition with refocused echoes (SSRARE) has emerged as the optimal sequence for VOLUME 55, NO. 7 (SUPPL), 2002
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A
B
C
D
Figure 5. Comparison of MRCP techniques in a 64-year-old patient with choledocholithiasis. A and B, Thin section heavily fluid weighted images can be combined using maximum intensity projection post processing to create a 3-dimensional cholangiographic image (B). C, A cholangiogram-like image can also be obtained by using a single thick (e.g., 4 to 6 cm) slab which includes all of the pancreaticobiliary system in one image. While this is similar to the MIP image, it is not three-dimensional and cannot be rotated. D, Another approach uses a less heavily T2-weighted sequence to preserve periductal tissue detail. This mixed fluid and tissue weighting has the advantage of demonstrating periductal anatomy but cannot be used to generate cholangiographic type images.
MRCP and has the dual advantages of rapid image acquisition with relatively high spatial resolution.10 SS-RARE is an ultra-fast T2-weighted sequence, which allows subsecond slice acquisition. This largely overcomes the problem of motion artifact because physiologic motion is “frozen” and imaging of the pancreatic and biliary ducts can be performed in a single breathhold. Breathhold imaging is crucially important, since image degradation by physiologic motion, particularly breathing, has been the greatest barrier to uniformly successful MRCP. Section misregistration or motion artifacts severely limit assessment of small structures. SS-RARE is less sensitive to magnetic susceptibility artifacts than gradient-echo sequences (Fig. 4). SS-RARE is available as a software upgrade from major manufacturers, at an approximate cost of $20,000. Hardware VOLUME 55, NO. 7 (SUPPL), 2002
requirements include a medium to high field strength (≥0.5T) and high performance gradients.11 Common commercial versions of SS-RARE include single-shot fast spin-echo (SSFSE; General Electric, Florence, South Carolina) and half-Fourier acquisition single-shot turbo spin echo (HASTE; Siemens, Munich, Germany). In addition to using the optimal sequence, MRCP should be performed with surface phased-array multicoils because the resulting improvement in signal-to-noise ratio increases spatial resolution.12 Typically, MRCP is performed using parameters in which fluid is the main signal source. In SS-RARE this is achieved by using a long echo time (TE) value (e.g. 150-600 msec) with fat suppression or by using an extremely long TE value (e.g. 600-1200 msec) without fat suppression.13 Cholangiogram-like GASTROINTESTINAL ENDOSCOPY
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Table 1. UCSF magnetic resonance cholangiopancreatography protocol
Plane Axial
Figure 6. Maximum intensity projection image of the biliary system using a T1-weighted sequence after intravenous injection of manganese. The clinical utility of such T1-based excretory MRCP remains under investigation, but has the potential to add a functional element to conventional T2-based MRCP.
images can be produced, either by acquiring signal from a single thick slice or by maximum intensity projection (MIP) postprocessing of multiple thin slices (Fig. 5). The conceptual basis for such “fluidonly” techniques is an implicit assumption that MRCP images must closely resemble conventional cholangiograms. However, such cholangiogram-like images may convey less diagnostic information than the source images. In a study of 108 patients with a variety of biliary and pancreatic disease, bile duct stenoses, dilatation, and stones were all better seen on source thin sections than on either MIP reconstructions or single thick slice MRCP.14 For example, the sensitivity for 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 (e.g. 100 msec) without fat suppression. These parameters result in images where fluid is bright, but periductal structures are also well depicted and are not deliberately excluded from the final image (Fig. 5). This allows assessment of extra luminal detail, which may be of critical importance, such as in the assessment of neoplastic duct obstruction. Another advantage of this technique is that the duodenal lumen is clearly distinguishable, which facilitates identification of the ampulla of Vater, irrespective of whether the duodenum contains air (dark) or fluid (bright). Therefore, administration of oral fluid, which has been recommended to highlight the duodenum on “fluid-only” sequences, is unnecessary.15 A disadvantage of this technique is that MIP postprocessing is not possible. However, these two approaches to MRCP are not mutually exclusive because of the very short acquisition times with SS-RARE. Both “fluid-only” and S6
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Slice thickness and gap (mm) TE (ms)
Matrix
4/0
80-100
256/160
Coronal (+/– 4/0 oblique) Coronal 4/0
80-100
256/160-192
Coronal slab
40-60
Comment Thin section mixed fluid and tissue sequence. Useful for evaluation of periductal tissues.
600-1200 256/160-192 Thin section “fluidonly” sequence; can be used for MIP 600-1200 320/320 Single thick slice (slab) “fluid-only”
UCSF, University of California–San Francisco; TE, echo time; MIP, maximum intensity projection.
mixed intensity imaging can be performed on every patient with only a minimal overall increase in total imaging time. Based on these considerations, a practical approach to MRCP would be to perform axial and coronal SS-RARE imaging of the pancreatic and biliary ducts with additional planes, including oblique planes as indicated. With careful selection of imaging volume, acquisition time is usually short enough to perform each series in a single breathhold. In patients with limited breath-holding capacity, images can be acquired as two or more contiguous or overlapping stacks rather than interleaved stacks to reduce slice misregistration. In uncooperative patients, images acquired during quiet respiration are often surprisingly cooperative. Contiguous 3- to 5mm 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 256 192-256 acquisition matrix is usually adequate. Higher resolution matrix (e.g. 512 or 1024) can be used but may result in grainy images, at least with current technology. Mixed fluid and tissue images can be acquired with a TE of 100 msec without fat suppression. “Fluid-only” images can be acquired with a longer TE (150-300 msec) with fat suppression, or extremely long TE (e.g. 1200 msec) without fat suppression. The current protocol for MRCP at our institution is shown in Table 1. The description of MRCP in the previous paragraphs is based on T2-weighted contrast, which is the usual basis for MRCP in practice. A recent development has been the emergence of manganese as a biliary contrast agent (Fig. 6). Manganese is bright on T1-weighted images and was initially developed as an agent for hepatic parenchymal enhancement. However, manganese is predominantly excreted by VOLUME 55, NO. 7 (SUPPL), 2002
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A A
B
B
C Figure 7. ERCP performed in a 72-year-old man with recurrent right upper quadrant pain and a remote history of cholecystectomy. A, Multiple stones are seen in the common duct, and were initially presumed to be the cause of symptoms. B and C, An MRCP performed two months later confirms the presence of multiple stones in the common duct but also demonstrates a constricting pancreatic carcinoma surrounding the distal common duct. These sequences were obtained with a mixed fluid and tissue weighting, and illustrate the importance of assessing the periductal anatomy in addition to assessment of the luminal content. VOLUME 55, NO. 7 (SUPPL), 2002
Figure 8. ERCP (A) and MRCP (B) in a patient with a cholangiocarcinoma of the middle common duct. Biliary dilatation is less pronounced on the MRCP image because MRI was performed after endobiliary stenting. Nonetheless, the stricture is well seen.
the biliary route, and on delayed images the biliary tree is depicted as high signal intensity on T1-weighted images.16 The utility of such excretory T1-based MRCP remains under investigation, but does add an element of functional assessment that may become a useful clinical application. MAJOR CLINICAL APPLICATIONS It would be impractical to attempt an encyclopedic review of all of the reported applications of GASTROINTESTINAL ENDOSCOPY
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Table 2. Results of recent studies investigating the accuracy of MRCP in choledocholithiasis.19-21 Total patients
Number with stones
Sensitivity (%)
191 50 278
34 32 76
91 88 92
MRCP, and this review concentrates on the major clinical applications of MRCP. These major applications include the evaluation of common causes of duct obstruction (stones, tumors, and strictures), primary ductal disease (including chronic pancreatitis, sclerosing cholangitis, and mucinous ductal ectasia), important anatomic variants of ductal anatomy (including pancreas divisum and surgically important biliary variants), and postcholecystectomy biliary disorders. Choledocholithiasis An early study of MRCP in choledocholithiasis demonstrated a sensitivity and specificity of 81% and 98%.17 With additional experience and improvements in technique, the same group subsequently reported a sensitivity and specificity of 90% and 100%.18 These latter figures are similar to the reported accuracy of ERCP and illustrate the study improvement in MRCP of choledocholithiasis (Fig. 5). Other more recent studies19-21 have again confirmed that MRCP is of comparable accuracy to ERCP in this condition (Table 2). Potential causes of signal voids in or adjacent to the common duct, which may result in a false positive diagnosis of choledocholithiasis, include air bubbles, surgical clips, and the appearance of the right hepatic artery as it traverses the upper portion of the common duct.22,23 Neoplastic obstruction Tumors obstructing the pancreatic and biliary ducts present a major diagnostic and therapeutic challenge. These tumors may arise directly from the ducts, or involve the ducts by extension from primary or metastatic tumors of the liver, gallbladder, pancreas, or adjacent lymph nodes. Identification of the level and cause of obstruction is critical to treatment planning. Earlier reports of MRCP in the assessment of neoplastic pancreatic and biliary ductal obstruction were not encouraging. For example, level and cause of obstruction were depicted by a gradientecho MRCP technique in only 7 (78%) and 5 (56%) of 9 patients with malignant obstructive jaundice.24 More recent studies have shown better results, most likely reflecting improvements in MRCP technology. S8
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Specificity (%)
Positive predictive value (%)
Negative predictive value (%)
98 94 97
91 97 92
98 81 97
Table 3. Accuracy of MRCP in chronic pancreatitis26 Main duct dilatation Main duct narrowing Side branch dilatation Filling defects
Sensitivity (%)
Specificity (%)
54-75 69 38-62 43-83
94-98 96-97 89 98
In a study of breathhold SS-RARE MRCP without fat suppression and an intermediate T2 value of 100 msec in 32 patients with pathologically confirmed neoplastic duct obstruction, the level of obstruction was correctly identified in 27 (84%) and 28 (88%) of the 32 cases by two independent observers, respectively, and the site of underlying tumor was correctly identified in 27 (84%) and 29 (91%) cases.5 Of particular note, in this study the readers looked only at the MRCP images and did not review the additional images performed as part of the standard abdominal MRI examination. That is, these results reflect the ability of the MRCP technique in isolation to provide the same information that would traditionally have been expected from conventional cholangiography (Fig. 7). Strictures MRCP remains limited in the ability to detect subtle strictures of smaller ducts because of a combination of limited spatial resolution and lack of distention by exogenous contrast. However, MRCP is of high accuracy in the evaluation of higher grade strictures and strictures in larger ducts (Fig. 8). For example, in a recent study of 34 postoperative patients, 6 with strictures as proven by direct cholangiography, MRCP showed a sensitivity and specificity of 100% and 86% to 87%, respectively.25 Chronic pancreatitis One of the more comprehensive studies of MRCP in chronic pancreatitis was published several years ago. In this study, 2 independent reviewers evaluated the MRCP images in 39 patients with chronic pancreatitis for a variety of ductal findings, using ERCP as the standard of reference. The sensitivity VOLUME 55, NO. 7 (SUPPL), 2002
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Figure 9. MRCP image in a 57-year-old patient with marked changes second to chronic pancreatitis. The grossly dilated main pancreatic duct and multiple dilated side branches are well demonstrated.
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and specificity of MRCP for these ductal changes are shown in Table 3.26 As might be expected, the results indicate a greater accuracy for MRCP in the diagnosis of main duct changes rather than more subtle side branch findings (Fig. 9). Primary sclerosing cholangitis As with the side branch changes of chronic pancreatitis, the confident MRCP diagnosis of subtle small duct changes in primary sclerosing cholangitis is somewhat problematic (Fig. 2). However, a recent study suggests that MRCP is becoming increasingly accurate.27 Two independent readers examined the MRCP images of 136 patients, 34 of whom had the diagnosis of primary sclerosing cholangitis established by ERCP. MRCP showed a sensitivity of 85% to 88% and a specificity of 92% to 97% with the diagnosis of primary sclerosing cholangitis and had a very reasonable kappa value for interobserver agreement of 0.62.
B
Mucinous ductal ectasia
Figure 10. ERCP and MRCP in a 52-year-old patient with mucinous ductal ectasia (intraductal papillary mucinous tumor). A and B, Focal cystic dilatation in the pancreatic head that communicates with the pancreatic duct system is demonstrated by both modalities.
Mucinous ductal ectasia refers to a condition in which the pancreatic ducts are distended by excessive amounts of mucin as a result of the presence of small intraductal papillary mucinous tumors. In addition to focal, multi-focal, or diffuse pancreatic ductal dilatation, ERCP demonstrates the characteristic finding of large amounts of jelly-like mucin leaking from the ampulla of Vater. This is a relatively recently described condition, and in the past many cases may have been misinterpreted as chronic pancreatitis. The preferred pathologic term is intraductal papillary mucinous tumor. However, the tumor nodules themselves are rarely visualized by imaging, and, therefore, the term mucinous ductal ectasia is more descriptive of the endoscopic and
radiologic appearances. The condition is usually found in patients over the age of 60 years, with a male predominance. The condition clinically mimics acute or chronic pancreatitis. This diagnosis should be suggested when MRCP demonstrates focal, multi-focal, or diffuse dilatation of the pancreatic duct (Fig. 10). The radiological distinction from chronic pancreatitis can be difficult, and ERCP may be required to confirm the diagnosis.28,29 Both side branch and main duct forms of mucinous ductal ectasia have been described. The condition is a predilection for the head of the pancreas. The presence of cystic dilated tubular structures, which may or may not have been seen to communicate with the main duct, is suggestive of the diagno-
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duct, a short cystic duct, and aberrant drainage of the right posterior segmental duct to the common duct confluence or left hepatic duct. The ability of MRCP to depict these variants has been studied in a series of 171 patients, using direct cholangiography as a standard of reference.31 MRCP showed a sensitivity and specificity of 86% and 100%, respectively, for the diagnosis of variance cystic duct anatomy and a sensitivity and specificity of 71% and 100%, respectively, for the depiction of an aberrant right posterior segmental duct (Fig. 11). Postcholecystectomy biliary disorders
Figure 11. MRCP image in a 28-year-old patient being assessed as a potential right hepatic lobe living donor. An aberrant drainage of the right posterior segmental duct into the commencement of the common duct is evident. This is the trifurcation pattern.
sis. The natural history of the condition has not been well established, although malignant transformation is recognized and surgery has been suggestive as the definitive treatment in patients who are good operative candidates. Anatomic variants Pancreas divisum is the major congenital anatomic variant that should be considered when assessing MRCP images of the pancreatic duct. The reported autopsy frequency of pancreas divisum varies from 4% to 14%. Pancreas divisum is characterized by drainage of the main pancreatic duct to the minor papilla, rather than by the conventional anatomic path of drainage to the ampulla of Vater. 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. A study of 108 patients, of whom 6 were found to have pancreas divisum by ERCP, showed MRCP to have a sensitivity and specificity of 100%.30 Although limited by the small number of patients with pancreas divisum, the study indicates that MRCP is of high accuracy in this diagnosis. Several important congenital variants of biliary anatomy have been described. These are of clinical relevance because they predispose to bile duct injury during laparoscopic cholecystectomy, or because they may represent relative contrary indications to right hepatectomy in living related liver donors. These variants include a low cystic ductal insertion, a medial cystic ductal insertion, a long paralleled course of the cystic duct with the common S10
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Biliary complications of cholecystectomy include bile duct injury, retained bile duct stones, biliary leak, and biliary fistula.32 Such postcholecystectomy disorders are rare, but the rapid acceptance of laparoscopic cholecystectomy by surgeons and patients has resulted in an absolute increase in the number of biliary complications encountered. The traditional algorithm for imaging of postcholecystectomy biliary complications has been ultrasound or CT, followed by ERCP or percutaneous transhepatic cholangiography. A recent study investigated the use of breath-hold SS-RARE MRCP in 17 consecutive patients referred for specialist surgical evaluation of suspected postcholecystectomy biliary disorders. 33 Final diagnoses established by surgery or other imaging were biliary occlusion, peribiliary lesions, nonspecific biliary dilatation, bile duct stones, and biliocolic fistula. Two independent readers correctly categorized these diagnoses in 15 (88%) and 13 (76%) cases, respectively, with excellent interobserver agreement (kappa = 0.82). An important limitation of the study, which illustrates a general problem with MRCP, is that biliary strictures and transections were not distinguished at MRCP, but grouped together as occlusion. The lack of distention and limitation in fine spatial resolution at MRCP make it difficult to visualize nondilated bile ducts below a stricture; abrupt termination of a dilated duct may be the result of stricture or transection. CONCLUSION MRCP is a technique which remains in evolution, but has reached sufficient maturity to be clinically useful and of comparable accuracy to ERCP. Common current indications for MRCP include unsuccessful or contraindicated ERCP.34,35 Patient preference for noninvasive imaging may also be a consideration. Other possible indications for MRCP include patients with a low index of suspicion for pancreatic or biliary disease, patients in whom therapeutic ERCP is considered unlikely, and patients with susVOLUME 55, NO. 7 (SUPPL), 2002
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pected neoplastic, pancreatic, or biliary obstruction. An unresolved issue is whether MRCP should be performed alone, with a limited charge, or as part of a full abdominal study. There is a case that a full study should always be performed, because tumors or other significant pathology may be missed on MRCP images alone. However, if the referring clinician has 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. One recent study has addressed this issue.36 Two independent readers reviewed the MRI studies of 62 patients with biliary dilation of known cause, looking first at the MRCP images alone and then at the MRCP images in combination with the standard T1, T2, and postgadolinium images. The combined approach was found to improve diagnostic accuracy in the differentiation of benign from malignant causes of obstruction for both readers (areas under the receiver operator characteristic curve of 0.95 and 0.86 for combined images versus 0.81 and 0.81 for MRCP alone, respectively). Acknowledged current limitations of MRCP include the limited resolution for smaller ducts disease and the lack of dynamic information. Both limitations may gradually recede because of continuing technical advances. For example, secretin stimulation of pancreatic exocrine secretion has been used to demonstrate reduced duodenal filling and persistent ductal distention as a result of papillary stenosis in patients with suspected pancreatic disease15; manganese may be used as a biliary contrast agent, providing a functional component to MRCP (much like cholescintigraphy). These and other advances should help overcome any residual concerns about the utility of MRCP. In the coming years, it seems likely that the role of therapeutic ERCP will remain unchallenged, but diagnostic ERCP will gradually be replaced by MRCP. REFERENCES 1. Adamek HE, Albert J, Breer H, Weitz M, Schilling D, Riemann JF. Pancreatic cancer detection with magnetic resonance cholangiopancreatography and endoscopic retrograde cholangiopancreatography: a prospective controlled study. Lancet 2000;356:190-3. 2. Frey CF, Burbige EJ, Meinke WB, Pullos TG, Wong HN, Hickman DM, et al. Endoscopic retrograde cholangiopancreatography. Am J Surg 1982;144:109-13. 3. Bilbao MK, Dotter CT, Lee TG, Katon RM. Complications of retrograde cholangiography (ERCP): a study of 10,000 cases. Gastroenterology 1976;70:314-20. 4. Harbin WP, Meuller P, Ferrucci JT. Transhepatic cholangiography: complications and use pattern of the fine-needle technique. Radiology 1980;135:15-22. 5. Schwartz LH, Coakley FV, Sun Y, Blumgart LH, Fong Y, Panicek DM. Neoplastic pancreaticobiliary duct obstruction: evaluation with breath-hold MR cholangiopancreatography. Am J Roentgenol 1998;170:1491-5. VOLUME 55, NO. 7 (SUPPL), 2002
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6. Nesbit GM, Johnson CD, James EM, MacCarty RL, Nagorney DM, Bender CE. Cholangiocarcinoma: diagnosis and evaluation of resectability by CT and sonography as procedures complementary to cholangiography. Am J Roentgenol 1988; 151:933-8. 7. Kim MJ, Mitchell DG, Ito K, Outwater EK. Biliary dilatation: differentiation of benign from malignant causes—value of adding conventional MR imaging to MR cholangiopancreatography. Radiology 2000;214:173-81. 8. Heslin MJ, Brooks AD, Hochwald SN, Harrison LE, Blumgart LH, Brennan MF. A preoperative biliary stent is associated with increased complications after pancreaticoduodenectomy. Arch Surg 1998;133:149-54. 9. Hochwald SN, Burke EC, Jarnagin WR, Fong Y, Blumgart LH. Association of preoperative biliary stenting with increased postoperative infectious complications in proximal cholangiocarcinoma. Arch Surg 1999;134:261-6. 10. Irie H, Hoda H, Tajima T, Kuroiwa T, Yoshimitsu K, Makisumi K, et al. Optimal MR cholangiopancreatographic sequence and its clinical application. Radiology 1998;206:379-87. 11. Coakley FV, Schwartz LH. Magnetic resonance cholangiopancreatography. J Magn Reson Imaging 1999;9:157-62. 12. Campeau NG, Johnson CD, Felmlee JP, et al. MR imaging of the abdomen with a phased-array multicoil: prospective clinical evaluation. Radiology 1995;195:769-76. 13. Barish MA, Soto JA. MR cholangiopancreatography: techniques and clinical applications. Am J Roentgenol 1997;169: 1295-303. 14. Yamashita Y, Abe Y, Tang Y, Urata J, Sumi S, Takahashi M. In vitro and clinical studies of image acquisition in breath-hold MR cholangiopancreatography: single-shot projection technique versus multi-slice technique. Am J Roentgenol 1997; 168:1449-54. 15. Matos C, Metens T, Deviere J, Nicaise N, Braude P, van Yperen G, et al. Pancreatic duct; morphologic and functional evaluation with dynamic MR pancreatography after secretin stimulation. Radiology 1997;203:435-41. 16. Lee VS, Rofsky NM, Morgan GR, Teperman LW, Krinsky GA, Berman P, et al. Volumetric mangafodipir trisodiumenhanced cholangiography to define intrahepatic biliary anatomy. Am J Roentgenol 2001;176:906-8. 17. Guibaud L, Bret PM, Reinhold C, Atri M, Barkun AN. Bile duct obstruction and choledocholithiasis: diagnosis with MR cholangiography. Radiology 1995;197:109-15. 18. Reinhold C, Bret PM. Current status of MR cholangiography. Am J Roentgenol 1996;166:1285-95. 19. Stiris MG, Tennow B, Aadland E, Lunde OC. MR cholangiopancreaticography and endoscopic retrograde cholangiopancreaticography in patients with suspected common bile duct stones. Acta Radiol 2000;41:269-72. 20. Varghese JC, Liddell RP, Farrell MA, Murray FE, Osborne DH, Lee MJ. Diagnostic accuracy of magnetic resonance cholangiopancreatography and ultrasound compared with direct cholangiography in the detection of choledocholithiasis. Clin Radiol 2000;55:25-35. 21. Boraschi P, Neri E, Braccini G, Gigoni R, Caramella D, Perri G, et al. Choledocolithiasis: diagnostic accuracy of MR cholangiopancreatography. Three-year experience. Magn Reson Imaging 1999;17:1245-53. 22. Regan F, Fradin J, Khazan R, Bohlman M, Magnuson T. Choledocholithiasis: Evaluation with MR cholangiography. Am J Roentgenol 1996;167:1441-5. 23. Becker CD, Grossholz M, Becker M, Mentha G, dePeyer R, Terries F. Choledocholithiasis and bile duct stenosis: diagnostic accuracy of MR cholangiopancreatography. Radiology 1997;205:523-30. GASTROINTESTINAL ENDOSCOPY
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24. Wallner BK, Schumacher KA, Weidenmaier W, Friedrich JM. Dilated biliary tract: evaluation with MR cholangiography with a T2-weighted contrast enhanced fast sequence. Radiology 1991;181:805-8. 25. Tang Y, Yamashita Y, Arakawa A, Namimoto T, Mitsuzaki K, Abe Y, et al. Pancreaticobiliary ductal system: value of halfFourier rapid acquisition with relaxation enhancement MR cholangiopancreatography for postoperative evaluation. Radiology 2000;215:81-8. 26. Outwater EK, Gordon SJ. Imaging the pancreatic and biliary ducts with MR. Radiology 1994;192:19-21. 27. Fulcher AS, Turner MA, Franklin KJ, Shiffman JL, Sterling RK, Luketic VA, et al. Primary sclerosing cholangitis: evaluation with MR cholangiography—a case-control study. Radiology 2000;215:71-80. 28. Grogan JR, Saeian K, Taylor AJ, Quiroz F, Demeure MJ, Komorowski RA. Making sense of mucin-producing pancreatic tumors. AJR Am J Roentgenol 2001;176:921-9. 29. Procacci C, Megibow AJ, Carbognin G, Guarise A, Spoto E, Biasiutti C, et al. Intraductal papillary mucinous tumor of the pancreas: a pictorial essay. Radiographics 1999;19:1447-63. 30. Bret PM, Reinhold C, Taourel P, Guibaud L, Atri M, Barkun
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31.
32.
33.
34.
35.
36.
AN. Pancreas divisum: evaluation with MR cholangiopancreatography. Radiology 1996;199:99-103. Taourel P, Bret PM, Reinhold C, Barkun AN, Atri M. Anatomic variants of the biliary tree: diagnosis with MR cholangiopancreatography. Radiology 1996;199:521-7. Slanetz PJ, Boland G, Mueller PR. Imaging and interventional radiology in laparoscopic injuries to the gallbladder and biliary system. Radiology 1996;201:595-603. Coakley FV, Schwartz LH, Blumgart LH, Fong Y, Jarnagin WR, Panicek DM. Complex post-cholecystectomy biliary disorders: Preliminary experience with evaluation by means of breath-hold MR cholangiography. Radiology 1998;209:141-6. Soto JA, Yucel EK, Barish MA, Chuttani R, Ferrucci JT. MR cholangiopancreatography after unsuccessful or incomplete ERCP. Radiology 1996;199:91-8. Pavone P, Laghi A, Catalano C, Broglia L, Panefianco V, Messina A, et al. MR cholangiography in the evaluation of patients with biliary-enteric anastomosis. Am J Roentgenol 1997;169:807-11. Kim MJ, Mitchell DG, Ito K, Outwater EK. Biliary dilatation: differentiation of benign from malignant causes—value of adding conventional MR imaging to MR cholangiopancreatography. Radiology 2000;214:173-81.
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IN IMAGING OF THE
HEPATOBILIARY TREE
Magnetic resonance cholangiopancreatography Fergus V. Coakley, MD, Aliya Qayyum San Francisco, California
Magnetic resonance cholangiopancreatography (MRCP) refers to selective or partially selective MR imaging of the pancreatic and biliary ducts. Complete or partial selectivity for ductal imaging is an important component of this definition, because this selectivity distinguishes MRCP from other cross-sectional modalities such as CT and ultrasound, which can also be used to assess pancreatic and biliary ductal disease. MRCP was first described in the 1980s. Since then MRCP has undergone considerable technical evolution and maturation and has reached the stage at which MRCP is a viable alternative to diagnostic endoscopic retrograde cholangiopancreatography (ERCP) in most settings (Fig. 1). This article discusses some of the relative advantages and disadvantages of MRCP, important aspects of technique, and major clinical applications. Emphasis is placed on the scientific data supporting the contention that MRCP is competitive with ERCP. It should be noted that many clinical studies of MRCP use ERCP as the standard of reference. This carries the implicit and unstated assumption that ERCP results are absolute and objective. It is true that in practice ERCP is often considered the definitive test for structural disease of the pancreatic and biliary ducts. However, as with any other imaging test, ERCP is subject to error and variability, as illustrated by studies in which clinical or surgical results are used as the standard of reference. For example, in a study of 124 patients with suspected pancreatic cancer in which the standard of reference was biopsy or one year of clinical followup, the sensitivity and specificity of MRCP for the diagnosis of pancreatic cancer were 84% and 97%, respectively, compared with a sensitivity and specificity of 70% and 94% for ERCP.1 Similarly, in a study of 72 patients undergoing cholecystectomy, preoperative ERCP had a sensitivity and specificity of 91% and 100%, respectively, for the diagnosis of From the Department of Radiology, Abnormal Imaging, University of California–San Francisco, California. Reprint requests: Fergus V. Coakley, MD, Radiology Department, USCF Medical Center, San Francisco, CA 94122. Copyright © 2002 by the American Society for Gastrointestinal Endoscopy 0016-5107/2002/$35.00 + 0 37/0/124751 doi:10.1067/mge.2002.124751 S2
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B Figure 1. Comparison of an ERCP (A) and a contemporaneous MRCP (B) in a patient with an unremarkable biliary duct system. As commonly performed, MRCP uses heavily T2-weighted sequences to depict the fluid within the ducts as high signal intensity. State of the art MRCP now provides very similar images to ERCP.
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common duct stones, using intraoperative cholangiography and bile duct exploration as a standard of reference.2 ADVANTAGES AND DISADVANTAGES OF MRCP MRCP is less expensive than ERCP and does not require deep sedation. However, the major advantage of MRCP over direct (or conventional) cholangiography such as ERCP or percutaneous transhepatic cholangiography is the noninvasive nature of the procedure. This is not a trivial consideration, because the major complication rate of convention cholangiography is approximately 3%. Such complications include sepsis, bleeding, bile leak, and death.3,4 The major disadvantage of MRCP compared with conventional cholangiography is a somewhat lower spatial resolution, such that MRCP continues to be partially limited in the assessment in fine detail (Fig. 2), such as subtle small duct changes of sclerosing cholangitis and side branch changes of chronic pancreatitis. The suboptimal depiction of small duct disease is a result not only of current technical limitations of spatial resolution but also of the lack of duct distention during MRCP compared with the direct introduction of contrast during conventional cholangiography. Another important advantage of MRCP is that ductal depiction can be entirely selective, so only the ductal content is depicted, or semi-selective, so the ductal content is depicted with the surrounding tissue. This latter technique can be especially useful in neoplastic disease of the pancreatic or biliary ducts.5 In contrast, conventional cholangiography directly depicts only the duct lumen, and periductal pathology is only indirectly inferred from these images.6 Furthermore, when MRCP is performed as part of a full abdominal MRI study, the examination can provide a one-stop evaluation of the nature and site of ductal disease and the extent and stage of any underlying tumor, including detection of associated adenopathy or liver metastases.7 Finally, MRCP also has the advantage of displaying all fluid-filled structures within the field of view, not simply those structures that have been made opacified by contrast injection. This can be particularly useful for the depiction of excluded duct segments or cystic tumors that do not communicate with the pancreaticobiliary tree (Fig. 3). A potential criticism of MRCP is that endobiliary therapy cannot be performed as part of the procedure. This may not always be a disadvantage because MRCP results can be utilized for interdisciplinary planning of definitive therapy without the potentially negative effects of endobiliary intervention. For example, two separate studies of periampullary tumors and proximal cholangiocarcinoma performed VOLUME 55, NO. 7 (SUPPL), 2002
A
B Figure 2. ERCP and MRCP in a 45-year-old patient with primary sclerosing cholangitis and a previous right hepatectomy for cholangiocarcinoma. A, Multifocal ductal irregularity and stricturing are better appreciated on ERCP. B, MRCP remains somewhat limited in the assessment of small duct disease, because of a somewhat lower special resolution and the lack of duct distention by exogenous contrast during MRCP.
at Memorial Sloan-Kettering Cancer Center have shown that preoperative endobiliary stent placement is associated with increased bacterial contamination of the biliary system, wound and intra-abdominal complications, and postoperative infections.8 In addition, many patients undergoing ERCP have only a diagnostic study and no therapeutic interventions are performed. Such patients would be ideal candidates for MRCP, provided they could be identified with reasonable accuracy before imaging evaluation. TECHNIQUE As routinely performed, MRCP utilizes high-resolution breathhold T2-weighted sequences to obtain GASTROINTESTINAL ENDOSCOPY
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A
A
B
B Figure 3. ERCP and MRCP in a 75-year-old man with vague upper abdominal symptoms. A, ERCP shows dilatation of the gallbladder and biliary system to the level of the ampulla of Vater where there is a gradual tapered narrowing. The cause of this narrowing is not clear. B, MRCP shows a large noncommunicating cystic tumor is the cause of obstruction. Pathologic analysis showed a mucinous cystadenoma with foci of carcinoma in situ. One of the advantages of MRCP is the ability to depict noncommunicating cystic pathology and excluded duct segments.
diagnostic quality images of the pancreatic and biliary ducts. Fluid is bright on T2-weighted sequences, and, therefore, such sequences can be used to depict the fluid within the ductal lumen. That is, MRCP utilizes the intrinsic brightness of fluid on T2weighted sequences; this contrast mechanism is S4
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Figure 4. Gradient echo T1-weighted coronal image and SSRARE T2-weighted coronal image in a patient who has had a cholecystectomy. A, A large signal void is seen in the gallbladder fossa on the gradient echo sequence as a result of magnetic susceptibility artifact induced by the cholecystectomy clips. B, The SS-RARE sequence is much less susceptible to artifact, and no signal void is evident on this sequence. Low susceptibility to artifact is one of the advantages of the SS-RARE sequence, which is currently the most widely used sequence for MRCP.
entirely endogenous and no exogenous contrast is required. As described it would appear to be a simple and straightforward procedure, but the requirement for high quality T2-weighted imaging that is also rapid enough to be performed during breathholding has been one of the major challenges in abdominal MRI. When first described, MRCP was usually performed with gradient echo-based sequences, because at the time these represented the fastest available T2-weighted sequences with reasonable spatial resolution. Later, fast spin-echo sequences were developed, which gradually replaced gradient echo-based MRCP. More recently, singleshot rapid acquisition with refocused echoes (SSRARE) has emerged as the optimal sequence for VOLUME 55, NO. 7 (SUPPL), 2002
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B
C
D
Figure 5. Comparison of MRCP techniques in a 64-year-old patient with choledocholithiasis. A and B, Thin section heavily fluid weighted images can be combined using maximum intensity projection post processing to create a 3-dimensional cholangiographic image (B). C, A cholangiogram-like image can also be obtained by using a single thick (e.g., 4 to 6 cm) slab which includes all of the pancreaticobiliary system in one image. While this is similar to the MIP image, it is not three-dimensional and cannot be rotated. D, Another approach uses a less heavily T2-weighted sequence to preserve periductal tissue detail. This mixed fluid and tissue weighting has the advantage of demonstrating periductal anatomy but cannot be used to generate cholangiographic type images.
MRCP and has the dual advantages of rapid image acquisition with relatively high spatial resolution.10 SS-RARE is an ultra-fast T2-weighted sequence, which allows subsecond slice acquisition. This largely overcomes the problem of motion artifact because physiologic motion is “frozen” and imaging of the pancreatic and biliary ducts can be performed in a single breathhold. Breathhold imaging is crucially important, since image degradation by physiologic motion, particularly breathing, has been the greatest barrier to uniformly successful MRCP. Section misregistration or motion artifacts severely limit assessment of small structures. SS-RARE is less sensitive to magnetic susceptibility artifacts than gradient-echo sequences (Fig. 4). SS-RARE is available as a software upgrade from major manufacturers, at an approximate cost of $20,000. Hardware VOLUME 55, NO. 7 (SUPPL), 2002
requirements include a medium to high field strength (≥0.5T) and high performance gradients.11 Common commercial versions of SS-RARE include single-shot fast spin-echo (SSFSE; General Electric, Florence, South Carolina) and half-Fourier acquisition single-shot turbo spin echo (HASTE; Siemens, Munich, Germany). In addition to using the optimal sequence, MRCP should be performed with surface phased-array multicoils because the resulting improvement in signal-to-noise ratio increases spatial resolution.12 Typically, MRCP is performed using parameters in which fluid is the main signal source. In SS-RARE this is achieved by using a long echo time (TE) value (e.g. 150-600 msec) with fat suppression or by using an extremely long TE value (e.g. 600-1200 msec) without fat suppression.13 Cholangiogram-like GASTROINTESTINAL ENDOSCOPY
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Table 1. UCSF magnetic resonance cholangiopancreatography protocol
Plane Axial
Figure 6. Maximum intensity projection image of the biliary system using a T1-weighted sequence after intravenous injection of manganese. The clinical utility of such T1-based excretory MRCP remains under investigation, but has the potential to add a functional element to conventional T2-based MRCP.
images can be produced, either by acquiring signal from a single thick slice or by maximum intensity projection (MIP) postprocessing of multiple thin slices (Fig. 5). The conceptual basis for such “fluidonly” techniques is an implicit assumption that MRCP images must closely resemble conventional cholangiograms. However, such cholangiogram-like images may convey less diagnostic information than the source images. In a study of 108 patients with a variety of biliary and pancreatic disease, bile duct stenoses, dilatation, and stones were all better seen on source thin sections than on either MIP reconstructions or single thick slice MRCP.14 For example, the sensitivity for 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 (e.g. 100 msec) without fat suppression. These parameters result in images where fluid is bright, but periductal structures are also well depicted and are not deliberately excluded from the final image (Fig. 5). This allows assessment of extra luminal detail, which may be of critical importance, such as in the assessment of neoplastic duct obstruction. Another advantage of this technique is that the duodenal lumen is clearly distinguishable, which facilitates identification of the ampulla of Vater, irrespective of whether the duodenum contains air (dark) or fluid (bright). Therefore, administration of oral fluid, which has been recommended to highlight the duodenum on “fluid-only” sequences, is unnecessary.15 A disadvantage of this technique is that MIP postprocessing is not possible. However, these two approaches to MRCP are not mutually exclusive because of the very short acquisition times with SS-RARE. Both “fluid-only” and S6
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Slice thickness and gap (mm) TE (ms)
Matrix
4/0
80-100
256/160
Coronal (+/– 4/0 oblique) Coronal 4/0
80-100
256/160-192
Coronal slab
40-60
Comment Thin section mixed fluid and tissue sequence. Useful for evaluation of periductal tissues.
600-1200 256/160-192 Thin section “fluidonly” sequence; can be used for MIP 600-1200 320/320 Single thick slice (slab) “fluid-only”
UCSF, University of California–San Francisco; TE, echo time; MIP, maximum intensity projection.
mixed intensity imaging can be performed on every patient with only a minimal overall increase in total imaging time. Based on these considerations, a practical approach to MRCP would be to perform axial and coronal SS-RARE imaging of the pancreatic and biliary ducts with additional planes, including oblique planes as indicated. With careful selection of imaging volume, acquisition time is usually short enough to perform each series in a single breathhold. In patients with limited breath-holding capacity, images can be acquired as two or more contiguous or overlapping stacks rather than interleaved stacks to reduce slice misregistration. In uncooperative patients, images acquired during quiet respiration are often surprisingly cooperative. Contiguous 3- to 5mm 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 256 192-256 acquisition matrix is usually adequate. Higher resolution matrix (e.g. 512 or 1024) can be used but may result in grainy images, at least with current technology. Mixed fluid and tissue images can be acquired with a TE of 100 msec without fat suppression. “Fluid-only” images can be acquired with a longer TE (150-300 msec) with fat suppression, or extremely long TE (e.g. 1200 msec) without fat suppression. The current protocol for MRCP at our institution is shown in Table 1. The description of MRCP in the previous paragraphs is based on T2-weighted contrast, which is the usual basis for MRCP in practice. A recent development has been the emergence of manganese as a biliary contrast agent (Fig. 6). Manganese is bright on T1-weighted images and was initially developed as an agent for hepatic parenchymal enhancement. However, manganese is predominantly excreted by VOLUME 55, NO. 7 (SUPPL), 2002
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A A
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B
C Figure 7. ERCP performed in a 72-year-old man with recurrent right upper quadrant pain and a remote history of cholecystectomy. A, Multiple stones are seen in the common duct, and were initially presumed to be the cause of symptoms. B and C, An MRCP performed two months later confirms the presence of multiple stones in the common duct but also demonstrates a constricting pancreatic carcinoma surrounding the distal common duct. These sequences were obtained with a mixed fluid and tissue weighting, and illustrate the importance of assessing the periductal anatomy in addition to assessment of the luminal content. VOLUME 55, NO. 7 (SUPPL), 2002
Figure 8. ERCP (A) and MRCP (B) in a patient with a cholangiocarcinoma of the middle common duct. Biliary dilatation is less pronounced on the MRCP image because MRI was performed after endobiliary stenting. Nonetheless, the stricture is well seen.
the biliary route, and on delayed images the biliary tree is depicted as high signal intensity on T1-weighted images.16 The utility of such excretory T1-based MRCP remains under investigation, but does add an element of functional assessment that may become a useful clinical application. MAJOR CLINICAL APPLICATIONS It would be impractical to attempt an encyclopedic review of all of the reported applications of GASTROINTESTINAL ENDOSCOPY
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Table 2. Results of recent studies investigating the accuracy of MRCP in choledocholithiasis.19-21 Total patients
Number with stones
Sensitivity (%)
191 50 278
34 32 76
91 88 92
MRCP, and this review concentrates on the major clinical applications of MRCP. These major applications include the evaluation of common causes of duct obstruction (stones, tumors, and strictures), primary ductal disease (including chronic pancreatitis, sclerosing cholangitis, and mucinous ductal ectasia), important anatomic variants of ductal anatomy (including pancreas divisum and surgically important biliary variants), and postcholecystectomy biliary disorders. Choledocholithiasis An early study of MRCP in choledocholithiasis demonstrated a sensitivity and specificity of 81% and 98%.17 With additional experience and improvements in technique, the same group subsequently reported a sensitivity and specificity of 90% and 100%.18 These latter figures are similar to the reported accuracy of ERCP and illustrate the study improvement in MRCP of choledocholithiasis (Fig. 5). Other more recent studies19-21 have again confirmed that MRCP is of comparable accuracy to ERCP in this condition (Table 2). Potential causes of signal voids in or adjacent to the common duct, which may result in a false positive diagnosis of choledocholithiasis, include air bubbles, surgical clips, and the appearance of the right hepatic artery as it traverses the upper portion of the common duct.22,23 Neoplastic obstruction Tumors obstructing the pancreatic and biliary ducts present a major diagnostic and therapeutic challenge. These tumors may arise directly from the ducts, or involve the ducts by extension from primary or metastatic tumors of the liver, gallbladder, pancreas, or adjacent lymph nodes. Identification of the level and cause of obstruction is critical to treatment planning. Earlier reports of MRCP in the assessment of neoplastic pancreatic and biliary ductal obstruction were not encouraging. For example, level and cause of obstruction were depicted by a gradientecho MRCP technique in only 7 (78%) and 5 (56%) of 9 patients with malignant obstructive jaundice.24 More recent studies have shown better results, most likely reflecting improvements in MRCP technology. S8
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Specificity (%)
Positive predictive value (%)
Negative predictive value (%)
98 94 97
91 97 92
98 81 97
Table 3. Accuracy of MRCP in chronic pancreatitis26 Main duct dilatation Main duct narrowing Side branch dilatation Filling defects
Sensitivity (%)
Specificity (%)
54-75 69 38-62 43-83
94-98 96-97 89 98
In a study of breathhold SS-RARE MRCP without fat suppression and an intermediate T2 value of 100 msec in 32 patients with pathologically confirmed neoplastic duct obstruction, the level of obstruction was correctly identified in 27 (84%) and 28 (88%) of the 32 cases by two independent observers, respectively, and the site of underlying tumor was correctly identified in 27 (84%) and 29 (91%) cases.5 Of particular note, in this study the readers looked only at the MRCP images and did not review the additional images performed as part of the standard abdominal MRI examination. That is, these results reflect the ability of the MRCP technique in isolation to provide the same information that would traditionally have been expected from conventional cholangiography (Fig. 7). Strictures MRCP remains limited in the ability to detect subtle strictures of smaller ducts because of a combination of limited spatial resolution and lack of distention by exogenous contrast. However, MRCP is of high accuracy in the evaluation of higher grade strictures and strictures in larger ducts (Fig. 8). For example, in a recent study of 34 postoperative patients, 6 with strictures as proven by direct cholangiography, MRCP showed a sensitivity and specificity of 100% and 86% to 87%, respectively.25 Chronic pancreatitis One of the more comprehensive studies of MRCP in chronic pancreatitis was published several years ago. In this study, 2 independent reviewers evaluated the MRCP images in 39 patients with chronic pancreatitis for a variety of ductal findings, using ERCP as the standard of reference. The sensitivity VOLUME 55, NO. 7 (SUPPL), 2002
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Figure 9. MRCP image in a 57-year-old patient with marked changes second to chronic pancreatitis. The grossly dilated main pancreatic duct and multiple dilated side branches are well demonstrated.
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and specificity of MRCP for these ductal changes are shown in Table 3.26 As might be expected, the results indicate a greater accuracy for MRCP in the diagnosis of main duct changes rather than more subtle side branch findings (Fig. 9). Primary sclerosing cholangitis As with the side branch changes of chronic pancreatitis, the confident MRCP diagnosis of subtle small duct changes in primary sclerosing cholangitis is somewhat problematic (Fig. 2). However, a recent study suggests that MRCP is becoming increasingly accurate.27 Two independent readers examined the MRCP images of 136 patients, 34 of whom had the diagnosis of primary sclerosing cholangitis established by ERCP. MRCP showed a sensitivity of 85% to 88% and a specificity of 92% to 97% with the diagnosis of primary sclerosing cholangitis and had a very reasonable kappa value for interobserver agreement of 0.62.
B
Mucinous ductal ectasia
Figure 10. ERCP and MRCP in a 52-year-old patient with mucinous ductal ectasia (intraductal papillary mucinous tumor). A and B, Focal cystic dilatation in the pancreatic head that communicates with the pancreatic duct system is demonstrated by both modalities.
Mucinous ductal ectasia refers to a condition in which the pancreatic ducts are distended by excessive amounts of mucin as a result of the presence of small intraductal papillary mucinous tumors. In addition to focal, multi-focal, or diffuse pancreatic ductal dilatation, ERCP demonstrates the characteristic finding of large amounts of jelly-like mucin leaking from the ampulla of Vater. This is a relatively recently described condition, and in the past many cases may have been misinterpreted as chronic pancreatitis. The preferred pathologic term is intraductal papillary mucinous tumor. However, the tumor nodules themselves are rarely visualized by imaging, and, therefore, the term mucinous ductal ectasia is more descriptive of the endoscopic and
radiologic appearances. The condition is usually found in patients over the age of 60 years, with a male predominance. The condition clinically mimics acute or chronic pancreatitis. This diagnosis should be suggested when MRCP demonstrates focal, multi-focal, or diffuse dilatation of the pancreatic duct (Fig. 10). The radiological distinction from chronic pancreatitis can be difficult, and ERCP may be required to confirm the diagnosis.28,29 Both side branch and main duct forms of mucinous ductal ectasia have been described. The condition is a predilection for the head of the pancreas. The presence of cystic dilated tubular structures, which may or may not have been seen to communicate with the main duct, is suggestive of the diagno-
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duct, a short cystic duct, and aberrant drainage of the right posterior segmental duct to the common duct confluence or left hepatic duct. The ability of MRCP to depict these variants has been studied in a series of 171 patients, using direct cholangiography as a standard of reference.31 MRCP showed a sensitivity and specificity of 86% and 100%, respectively, for the diagnosis of variance cystic duct anatomy and a sensitivity and specificity of 71% and 100%, respectively, for the depiction of an aberrant right posterior segmental duct (Fig. 11). Postcholecystectomy biliary disorders
Figure 11. MRCP image in a 28-year-old patient being assessed as a potential right hepatic lobe living donor. An aberrant drainage of the right posterior segmental duct into the commencement of the common duct is evident. This is the trifurcation pattern.
sis. The natural history of the condition has not been well established, although malignant transformation is recognized and surgery has been suggestive as the definitive treatment in patients who are good operative candidates. Anatomic variants Pancreas divisum is the major congenital anatomic variant that should be considered when assessing MRCP images of the pancreatic duct. The reported autopsy frequency of pancreas divisum varies from 4% to 14%. Pancreas divisum is characterized by drainage of the main pancreatic duct to the minor papilla, rather than by the conventional anatomic path of drainage to the ampulla of Vater. 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. A study of 108 patients, of whom 6 were found to have pancreas divisum by ERCP, showed MRCP to have a sensitivity and specificity of 100%.30 Although limited by the small number of patients with pancreas divisum, the study indicates that MRCP is of high accuracy in this diagnosis. Several important congenital variants of biliary anatomy have been described. These are of clinical relevance because they predispose to bile duct injury during laparoscopic cholecystectomy, or because they may represent relative contrary indications to right hepatectomy in living related liver donors. These variants include a low cystic ductal insertion, a medial cystic ductal insertion, a long paralleled course of the cystic duct with the common S10
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Biliary complications of cholecystectomy include bile duct injury, retained bile duct stones, biliary leak, and biliary fistula.32 Such postcholecystectomy disorders are rare, but the rapid acceptance of laparoscopic cholecystectomy by surgeons and patients has resulted in an absolute increase in the number of biliary complications encountered. The traditional algorithm for imaging of postcholecystectomy biliary complications has been ultrasound or CT, followed by ERCP or percutaneous transhepatic cholangiography. A recent study investigated the use of breath-hold SS-RARE MRCP in 17 consecutive patients referred for specialist surgical evaluation of suspected postcholecystectomy biliary disorders. 33 Final diagnoses established by surgery or other imaging were biliary occlusion, peribiliary lesions, nonspecific biliary dilatation, bile duct stones, and biliocolic fistula. Two independent readers correctly categorized these diagnoses in 15 (88%) and 13 (76%) cases, respectively, with excellent interobserver agreement (kappa = 0.82). An important limitation of the study, which illustrates a general problem with MRCP, is that biliary strictures and transections were not distinguished at MRCP, but grouped together as occlusion. The lack of distention and limitation in fine spatial resolution at MRCP make it difficult to visualize nondilated bile ducts below a stricture; abrupt termination of a dilated duct may be the result of stricture or transection. CONCLUSION MRCP is a technique which remains in evolution, but has reached sufficient maturity to be clinically useful and of comparable accuracy to ERCP. Common current indications for MRCP include unsuccessful or contraindicated ERCP.34,35 Patient preference for noninvasive imaging may also be a consideration. Other possible indications for MRCP include patients with a low index of suspicion for pancreatic or biliary disease, patients in whom therapeutic ERCP is considered unlikely, and patients with susVOLUME 55, NO. 7 (SUPPL), 2002
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pected neoplastic, pancreatic, or biliary obstruction. An unresolved issue is whether MRCP should be performed alone, with a limited charge, or as part of a full abdominal study. There is a case that a full study should always be performed, because tumors or other significant pathology may be missed on MRCP images alone. However, if the referring clinician has 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. One recent study has addressed this issue.36 Two independent readers reviewed the MRI studies of 62 patients with biliary dilation of known cause, looking first at the MRCP images alone and then at the MRCP images in combination with the standard T1, T2, and postgadolinium images. The combined approach was found to improve diagnostic accuracy in the differentiation of benign from malignant causes of obstruction for both readers (areas under the receiver operator characteristic curve of 0.95 and 0.86 for combined images versus 0.81 and 0.81 for MRCP alone, respectively). Acknowledged current limitations of MRCP include the limited resolution for smaller ducts disease and the lack of dynamic information. Both limitations may gradually recede because of continuing technical advances. For example, secretin stimulation of pancreatic exocrine secretion has been used to demonstrate reduced duodenal filling and persistent ductal distention as a result of papillary stenosis in patients with suspected pancreatic disease15; manganese may be used as a biliary contrast agent, providing a functional component to MRCP (much like cholescintigraphy). These and other advances should help overcome any residual concerns about the utility of MRCP. In the coming years, it seems likely that the role of therapeutic ERCP will remain unchallenged, but diagnostic ERCP will gradually be replaced by MRCP. REFERENCES 1. Adamek HE, Albert J, Breer H, Weitz M, Schilling D, Riemann JF. Pancreatic cancer detection with magnetic resonance cholangiopancreatography and endoscopic retrograde cholangiopancreatography: a prospective controlled study. Lancet 2000;356:190-3. 2. Frey CF, Burbige EJ, Meinke WB, Pullos TG, Wong HN, Hickman DM, et al. Endoscopic retrograde cholangiopancreatography. Am J Surg 1982;144:109-13. 3. Bilbao MK, Dotter CT, Lee TG, Katon RM. Complications of retrograde cholangiography (ERCP): a study of 10,000 cases. Gastroenterology 1976;70:314-20. 4. Harbin WP, Meuller P, Ferrucci JT. Transhepatic cholangiography: complications and use pattern of the fine-needle technique. Radiology 1980;135:15-22. 5. Schwartz LH, Coakley FV, Sun Y, Blumgart LH, Fong Y, Panicek DM. Neoplastic pancreaticobiliary duct obstruction: evaluation with breath-hold MR cholangiopancreatography. Am J Roentgenol 1998;170:1491-5. VOLUME 55, NO. 7 (SUPPL), 2002
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