The first technique to produce cholangiographic images by MRI
was reported in 1991. However, it was not until the past three
years that investigators have begun to extensively evaluate and
refine the technique. MR cholangiopancreatography (MRCP) seems to
be a very attractive alternative to imaging of the bile and
pancreatic ducts, as it allows cholangiographic images to be
obtained in a noninvasive manner.
Endoscope retrograde cholangiopancreatography (ERCP) has long
considered the gold standard of cholangiographic imaging;
however, it is not perfect. In the detection of
choledocholithiasis, sensitivities are approximately 90%.2 ERCP
also requires sedation, has potential complications, and is
expensive. Complication rates for ERCP may be as high as 7%, and
because of this, noninvasive MRCP is gaining attention. MRCP can be
performed without sedation or contrast on any commercially
available MR unit, at various field strengths.3 Given its potential
usefulness, the question of whether MRCP is ready for widespread
implementation remains. In this article, we will review the history
of MRCP, discuss the various performance techniques which have been
utilized, explain tips on how to perform and interpret MRCP,
discuss findings of various disorders, discuss potential pitfalls,
and review the role of MRCP.
Patient preparation-Patient preparation for an MRCP examination
is minimal. Some investigators have chosen to keep patients NPO for
at least four hours to help reduce peristalsis and gastroduodenal
fluid. Others have promoted the use of glucagon or buscopan to help
reduce peristalsis, and recently, two studies4,5 have utilized a
negative oral contrast agent to reduce the signal intensity from
the bowel. Further investigation will be necessary to fully
determine the role these agents should play.
Pulse sequences-At present, there is no clear consensus on the
proper MRCP protocol. Initial techniques utilized gradient echoes
with steady state-free precession (SSFP) or CE-FAST, which is very
sensitive to slow flow and will only produce signal in tissues with
very long T2 relaxation times. Therefore, only stationary fluid
such as bile appears bright using this technique. These sequences
have several limitations, however, including low signal-to-noise
ratios, increased susceptibility to artifacts, high signal
intensity of fat, difficulty in visualization of non-dilated ducts,
sensitivity to motion (requiring long breath-holds), and a
relatively limited area that can be scanned.1,6
The development of rapid acquisition with relaxation enhancement
(RARE) techniques in 1986 was an important step in refining MRCP.
Utilizing hybrid RARE techniques such as fast spin echo (FSE) or
turbo spin echo (TSE), heavily T2-weighted images could be obtained
rapidly. These sequences have several advantages over the gradient
echo techniques, including improved signal-to-noise and
contrast-to-noise ratios, better overall image quality, and less
motion and susceptibility artifact.7,8
Recently, faster gradients have been developed which can now
allow imaging of the entire biliary system in a single breath-hold.
Those sequences are referred to as half-Fourier acquisition
single-shot turbo spine echo (HASTE) or single-shot RARE. A few
studies have evaluated this technique, demonstrating improved
resolution and image quality over other breath-hold
techniques.8,9,10 In a comparison study of three breath-hold
techniques (gradient echo, FSE, and half-averaged single-shot
RARE), the single-shot RARE sequence provided the best image
quality and duct conspicuity. Contrast-to-noise ratios also were
highest with the RARE sequence; however, FSE demonstrated higher
signal-to-noise ratio. In this study, the FSE sequence was not
Acquisition geometry-Most series have utilized 2D acquisition
modes; however, some have shown excellent results with 3D
acquisitions.11,12 Advantages of the 3D acquisition include
improved resolution, though this sequence is long and, as such, is
more susceptible to motion artifact than the 2D sequences.
Usual planes of imaging include axial, coronal, and oblique
coronal, all oriented in an RAO projection to the bile duct
bifurcation. When using 3D techniques or projection images, the
coronal or oblique coronal plane should be utilized.
Images can be obtained as sequential slices, which can be
reconstructed into 3D models or a single projectional image (figure
10). Slices less than or equal to 3 mm thickness allow for adequate
reformations but must be weighed with the signal available on the
scanner. If available, the use of torso phased-array coils will
allow improved resolution.
Motion correction-Several options and breathing techniques have
been examined as to their abilities to reduce motion artifact. Some
researchers have been able to obtain long breath-holds from the
patients, while others have had to segment the acquisition into
several breath-holds. Increasing the number of excitations has been
shown to allow shallow respiration, and respiratory triggering also
has been attempted in order to suppress artifact. It was determined
that resolution can be sacrificed to reduce imaging time and allow
breath-holding in some instances, while increasing the resolution
by increasing the number of excitations was indicated in order to
reduce respiratory artifacts in other cases.2,9,10,12-17
At our institution, we routinely review the individual source
images acquired on a workstation. These images are critical in the
identification of abnormalities. Most investigators have shown the
source images demonstrate more abnormalities than the 3D
reconstructions or single slice projection images.
Three dimensional reconstruction can be useful in displaying the
relationship of structures and abnormalities. However, its current
challenge is to gain
clinical acceptance by demonstrating cholangiographic images
similar to those obtained by ERCP. Maximum intensity projection
(MIP) algorithms are used to create 3D reconstructions. By rotating
the image, the proper obliquities can be obtained to best depict
anatomy and pathology. However, when using the MIP alorithm,
valuable information such as visualization of smaller ducts may be
lost during reconstruction, and stones may be obscured by the
surrounding high intensity bile (figure 2). Axial source images
which are projected into the coronal plane show significant
degradation and misregistration. Therefore, source images obtained
in the coronal plane provide the best coronal reconstructions.
Normal anatomy-Several studies have assessed the rate of
visualization of normal ducts at MRCP. Non-breath-held TSE has
demonstrated peripheral intrahepatic ducts in all dilated cases and
82% of non-dilated ducts.13 The main intrahepatic ducts were
visualized in 97 to 100%, confluence of intrahepatic ducts in 86%,
cystic duct in 59%, cystic duct insertion in 34%, and confluence of
the CBD and pancreatic duct in 38% of the cases studied; the
pancreatic duct was able to be visualized in the head in 66% and in
the body in 31% of cases.13 The main pancreatic duct has been
demonstrated using 2D FSE sequences in the head in 95% and
body/tail in 96% of cases.2 In contrast, earlier studies utilizing
a simple body coil showed a portion or all of the duct in only 65%
of cases. Using respiratory gated 3D TSE, diagnostic pancreatograms
were obtained in 67% of patients and these demonstrate a
specificity of 69% in the confirmation of a normal pancreatic duct;
this can be increased to 81% by utilizing source images.11 Using
HASTE sequences and a phased-array coil, normal cystic ducts and
branches of the pancreatic duct were demonstrated in 88% and 75% of
It seems that most MR techniques are capable of visualization of
the extrahepatic bile ducts, central intrahepatic ducts, and
portions of the pancreatic duct. Visualization of the cystic duct,
ampullary region, and the pancreatic duct and its branches still
remains problematic, however.
In several reports, measurements of the ducts with MRCP have
underestimated the true size, as obtained at ERCP.14 This likely is
secondary to lack of distention at MRCP and decreased signal at the
periphery of the bile duct.
Anatomic variants-It is important to identify variations of
normal anatomy in the biliary system, as the anomalous ducts can be
injured during surgical procedures such as laparoscopic
Using non-breath-hold 2D FSE and a phased-array coil, MRCP
proved useful for the detection of an anomalous course of the
cystic duct. Sensitivities were 83%, 85%, 91%, and 86% for low
insertion, medial insertion, parallel course to the hepatic ducts,
and overall diagnosis of variant anatomy, respectively, with 100%
The main anomaly of the pancreatic duct that is of clinical
importance is pancreatic divisum (figure 3). Studies have reported
up to 100% sensitivity (n=25) by using a combination of a body coil
and torso multicoil for visualization.17 The majority of anomalous
pancreatic ducts are best identified on axial source images. Other
methods have had less success; on 3D TSE without routine use of
axial images, only 67% (n=6) of anomalies were detected.11 Further
studies utilizing newer techniques and a larger population will be
necessary to truly assess the accuracy of MRCP.
Choledocholithiasis-Most gallstones will appear as a dark signal
intensity structure on MRCP surrounded by high signal intensity
bile. However, some stone compositions may have central areas of
high signal. Also, soft stones and debris may not appear as low
signal and can approach the signal intensity of bile. Experienced
observers are now reporting sensitivities of greater than 90%
(table 1) in the detection of cholelithiasis;2,12-14,16,17,19,20
however, larger studies using single-shot techniques are still
needed to accurately assess the sensitivity.
There are several pitfalls which can lower the specificity in
the diagnosis of choledocholithiasis. Presence of hemorrhage,
protein, and debris all can alter the signal intensity of bile,
mimicking the presence of a stone (figure 4). Strictures also may
give the appearance of a stone. Examination of the duct in various
locations may help in the differentiation. Small stones, especially
in non-dilated ducts, also can lead to false-negative
interpretation. Because of the narrowing of the CBD distally at the
ampulla, the appearance of small stones impacted distally may be
subtle (figure 5). If the duct is dilated, air may rise to the
non-dependent portion, making distinction possible (figure 6).
Therefore, obtaining proper history such as prior sphincterotomy or
surgery is important. As with other abnormalities, evaluation of
the source images is critical, as small stones may be obscured by
the surrounding bile on MIP reconstructions.
MRCP is fairly accurate in diagnosing obstruction secondary to
strictures and also as to the site (figure 7). Initial studies
using SSFP demonstrated good sensitivity in detecting obstruction;
however, visualization of the stricture and distal duct was
limited.1,6,8 TSE techniques have demonstrated sensitivities of 86
to 100% in the diagnosis of obstruction and 93% in identification
of the correct site.13,14,17 Up to 90% of strictures can be
identified using 3D TSE.20 In their study, Macaulay et al showed no
false-negative results, suggesting that, in the appearance of a
normal duct, ERCP may be unnecessary.13 Distal obstructions can be
difficult to diagnose and differentiation of benign from malignant
strictures may not be possible. However, Guibaud et al demonstrated
a sensitivity of 86% and specificity of 95% in diagnosing malignant
A potential problem in the MRCP evaluation of a stricture is
that the duct distal to the narrowing may be difficult to see
because of the collapsed nature and size of the duct. This can be
problematic when evaluating length of strictures.13 The use of
phased-array coils and single-shot techniques, which improve
resolution, may allow better visualization of the distal
As with choledocholithiasis, several pitfalls exist. Anything
which lowers the signal intensity of the bile, such as stone,
debris, or hemorrhage, can give an appearance of narrowing.
Ampullary masses and sphincter of Oddi dysfunction can lead to
dilatation and may be difficult to diagnose on MRCP.
As indicated above, the pancreatic duct is more difficult to
visualize than the bile ducts because of its smaller size, though
at least a portion of the duct can be seen in a high percentage of
individuals. This lack of visualization does not indicate a
stricture and should be interpreted cautiously, especially in the
absence of proximal dilatation. Three-dimensional TSE demonstrated
an 87 to 100% sensitivity in detecting pancreatic duct dilatation.
Also, HASTE sequences have been shown to identify 100% of dilated
pancreatic ducts. However, only 50% of dilated branches off the
main pancreatic duct may be visualized. The level of obstruction of
the main duct is usually well demonstrated.9
Role of MRCP
In the preceding section, we discussed the accuracy of MRCP in
the detection of various disorders of the biliary system and
pancreas. However, exactly when to perform MRCP still needs to be
defined. Potential clinical applications include the following:
Unsuccessful ERCP-MRCP has been shown to be useful in patients
who underwent unsuccessful ERCP.21 Failure of ERCP may be due to
many factors, including the inability to cannulate, postoperative
biliary-enteric surgery, and significant duct obstruction. In these
settings, MRCP seems a reasonable alternative compared to
Preoperative evaluation-MRCP may provide useful information in
patients undergoing laparoscopic cholecystectomy, demonstrating
anomalous anatomy and choledocholithiasis.
Detection of choledocholithiasis-In patients with high suspicion
of choledocholithiasis, ERCP should be performed regardless of the
findings on MRCP. However, MRCP may be useful in the evaluation of
patients with a low suspicion of choledocholithiasis or in patients
considered to be at high risk for developing a complication during
a diagnostic ERCP. Patients presenting with acute pancreatitis also
may benefit from undergoing MRCP to exclude gallstone pancreatitis,
as there is an increased risk of complications in performing ERCP
in these individuals.
Strictures-In the evaluation of biliary and pancreatic
strictures, ERCP still provides information unobtainable with MRCP.
ERCP allows distention of the stricture, which may help
differentiate benign from malignant causes, allow brushings to be
obtained, and allow therapeutic procedures such as dilatation and
stenting to be performed. However, one potential advantage of MRCP
over ERCP is the visualization of masses associated with
strictures. In tight strictures, ERCP may not be able to visualize
the ducts proximal to the stricture. MRCP allows excellent
visualization of the proximal dilated ducts and length of the
stricture, and can guide surgical or percutaneous procedures.
Chronic pancreatitis-MRCP has shown the ability to visualize
dilated ducts and to diagnosis chronic pancreatitis and its
possible etiologies, such as pancreatic divisum. MRCP also is able
to visualize intraductal stones. However, it is limited in its
ability to demonstrate the side branches, in which dilatation can
be an early change of chronic pancreatitis (figure 8).
Pancreatic function-MRCP has been reported to have the ability
to evaluate for papillary stenosis and exocrine insufficiency.5
However, further studies will be necessary to corroborate these
Although the above scenarios seem to be reasonable, MRCP
currently is being used mainly in a problem solving role. The goal
of MRCP should be to replace diagnostic ERCP. At this point, the
resolution of MRCP is still inferior to ERCP. Further technical
developments such as HASTE imaging should allow MRCP to narrow the
difference in resolution, while the shorter scan times will lower
the cost. Further outcome studies will be necessary to confirm the
equality of MRCP and ERCP. Refinement and optimization of the
technique will need to be performed before widespread
implementation of MRCP in order to assure acceptable accuracy
rates. Despite this uncertainty, our clinical referrals are
increasing. We believe that, while techniques are improving, MRCP
is not yet ready to completely replace diagnostic ERCP. AR
Thanks to Kris McPeck, Heidi Wiedenfeld, and Dr. Ken Vitellas
for their assistance in preparation of this manuscript.
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Dr. Fidler is Assistant Professor of Radiology and Section Chief
of CT/MRI at the University of Nebraska Medical Center in Omaha,
NE. Dr. Spritzer is an Associate Professor of Radiology at Duke
University Medical Center in Durham, NC.