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1 All authors: Department of Radiology, Asan Medical Center, University of Ulsan College of Medicine, 388-1, Poongnab-dong, Songpa-ku, Seoul 138-736, Korea.
Received September 4, 2001;
accepted after revision January 29, 2002.
Address correspondence to T. K. Kim.
Abstract
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MATERIALS AND METHODS. Of the 318 patients who underwent MRCP examinations at our institution during an 18-month period, we identified 49 patients who subsequently underwent surgery or cholangioscopic stone removal with proof of intrahepatic stones. Thirty-four of these patients also underwent ERCP; they made up our study population. All images were interpreted for the presence of bile duct stones: MRCP images were interpreted independently by two reviewers, and ERCP studies were interpreted by one reviewer who was unaware of the MRCP findings.
RESULTS. The sensitivity and specificity of MRCP for detecting intrahepatic stones were 97% and 93%, respectively, whereas those of ERCP were 59% and 97%, respectively. MRCP showed a significantly higher sensitivity than ERCP in the diagnosis of intrahepatic stones (p < 0.001). We found no significant difference between MRCP and ERCP in sensitivity or specificity for detecting calculi in the common duct or gallbladder.
CONCLUSION. MRCP is a more effective diagnostic method than ERCP for the evaluation of intrahepatic stones.
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The chief disadvantage of MRCP is that it is purely diagnostic, whereas ERCP is both diagnostic and therapeutic. However, although common bile duct stones are managed effectively by ERCP and sphincterotomy in many instances, the treatment of intrahepatic stones via the transpapillary route is difficult because of commonly accompanying ductal strictures, peripheral stone impaction, or ductal angulation [12]. Because ERCP is not considered to have a significant therapeutic role in the management of intrahepatic stones [12], MRCP may replace ERCP if it proves to be equal or superior to ERCP in the diagnosis of intrahepatic stones.
The purpose of our study was to compare the efficacy of MRCP with that of ERCP for the diagnosis of intrahepatic duct stones.
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Our study included 22 women (65%) and 12 men (35%) from 33 to 70 years old (mean age, 54 years). ERCP was performed within 16 days (mean, 3 days) of MRCP examination. MRCP preceded ERCP in eight patients, and ERCP preceded MRCP in the remaining 26 patients. The patients had various clinical symptoms of biliary obstruction with or without concomitant infection; these symptoms included abdominal discomfort or pain, abdominal tenderness, fever, chills, and jaundice.
Intrahepatic stones were confirmed by surgery (hepatic resection) in 21 patients and by stone removal using percutaneous transhepatic cholangioscopy in 13 patients. The decision to perform surgery or cholangioscopy was made from the findings of both MRCP and ERCP. For purposes of this study, the findings at surgery or cholangioscopic stone removal were accepted as the standard reference for evaluation of MRCP and ERCP. Therefore, we excluded patients who showed radiologic evidence of intrahepatic stones but underwent neither of the two treatment procedures.
Imaging Techniques
MRCP examinations were performed on a 1.5-T MR system (Magnetom Vision;
Siemens, Erlangen, Germany). A circular polarized phased array body coil with
four elements was used. No medication or contrast medium was administered
before imaging. Before MRCP was performed, T1-weighted axial and coronal
gradient-echo MR imaging (TR/TE, 149/4.8; flip angle, 70°; section
thickness, 8 mm; 18 sections in a 20-sec breath-hold) was performed to
localize the biliary system. These images were then used as guides to perform
MRCP. Two MRCP techniques were applied: single-slab rapid acquisition with
relaxation enhancement (RARE) and multislice half-Fourier acquisition
single-shot turbo spin-echo (HASTE). A radiologist was present during the
entire examination and determined the acquisition angles that optimally
delineated the biliary ducts. The slabs of a single-shot RARE sequence were
obtained at various angles to allow optimal visualization of the bile ducts;
the number of thick-slab acquisitions per patient ranged from three to 10
(mean, six acquisitions). Then multislice HASTE images were obtained in the
coronal and oblique planes. Typically, the oblique plane was at an angle of
20-35° to the coronal plane to simulate a right anterior oblique
projection on direct cholangiography. Each examination was performed during a
single breath-hold.
The imaging parameters for the single-shot RARE sequence were TR/effective TE, infinite/1200; echo spacing, 11.5 msec; echo-train length, 240; flip angle, 150°; slab thickness, 70-96 mm; field of view, 300 mm; number of signals acquired, 1; matrix, 240 x 256; and acquisition time, 6.32 sec. The imaging parameters for the multislice HASTE sequence were TR/effective TE, infinite/95; echo spacing, 11.9 msec; echo-train length, 128; flip angle, 150°; section thickness, 4 mm with no gap; number of slices, 13-15 (range of coverage, 52-60 mm); field of view, 300 mm; number of signals acquired, 1; matrix, 240 x 256; and acquisition time, 20-23 sec. Fat saturation was used to reduce strong fat signal during image acquisition. The total acquisition time for all MR imaging steps was less than 15 min. Postprocessing of the multislice HASTE images was not performed.
ERCP was performed with the patient under conscious sedation. An under-couch fluoroscopic unit was used for screening and taking the hardcopy conventional films. Under fluoroscopic guidance, 10-30 mL of water-soluble contrast material was injected. Then multiple images of the bile ducts were obtained under the supervision of a radiologist to optimally show the entire ductal anatomy and reveal any abnormality. An average of 15 images were acquired per examination to reveal the presence of stones, sludge, and air bubbles in various planes and patient positions. Because one of the purposes of the ERCP examination was to localize bile duct stones, the examiners attempted to fully opacify the intrahepatic ductal system.
Analysis
All MRCP and ERCP images were retrospectively reviewed to compare MRCP and
ERCP results. The MRCP images were reviewed independently by two radiologists
who were unaware of the ERCP and final results. Both reviewers were
experienced in abdominal MR imaging; neither had participated in the clinical
interpretation of the MRCP images. However, both reviewers were aware that
stones were present in the studies they were evaluating. All MRCP sequences in
each patient were reviewed, but no attempt was made to evaluate separately the
accuracy of the individual acquisitions. All direct ERCP images were reviewed
by one radiologist who was unaware of the MRCP findings. Both MRCP and ERCP
images were evaluated for the presence or absence of stones in each segment of
the bile ducts (i.e., right or left intrahepatic ducts, common duct) and the
gallbladder and for ancillary findings such as intrahepatic fluid collections
or masses. Biliary calculi were diagnosed on MRCP as round, oval, or
multiangular signal voids seen in the lumen. Surgical and pathologic findings
(n = 21) or cholangioscopic findings identified during the procedure
of stone removal (n = 13) were used as the standard of reference for
determining the diagnostic performance of MRCP and ERCP.
We determined the statistical differences between findings on MRCP and on
ERCP indicating the presence of stones in each segment of the bile ducts and
gallbladder by applying the McNemar test with a significance level of
p as less than 0.05. The sensitivity and specificity of MRCP and ERCP
for the diagnosis of stones in the intrahepatic ducts, common duct, and
gallbladder were calculated. Finally, agreement between the MRCP reviewers was
evaluated using kappa values. Interobserver agreement was interpreted as very
good (
> 0.80), good ( = 0.80-0.61), moderate ( =
0.60-0.41), fair ( = 0.41-0.21), or poor ( 
0.20)
[13].
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All MRCP examinations were adequate for analysis. The sensitivity and specificity of MRCP for detecting right intrahepatic duct stones were 92% and 95%, respectively; two independent reviewers had the same diagnostic performance (Table 1). The sensitivity and specificity of ERCP for detecting right intrahepatic duct stones were 38% and 100%, respectively. For the two reviewers, the sensitivity of MRCP for detecting left intrahepatic duct stones was 96% and 100%, and the specificity was 88%. The sensitivity and specificity of ERCP for detecting left intrahepatic duct stones were 69% and 88%, respectively. The low sensitivity of ERCP for detecting intrahepatic duct stones appeared to be caused by incomplete filling of the contrast material in the stone-containing intrahepatic ducts because of severe ductal stenosis (Figs. 1A,1B,1C and 2A,2B,2C). MRCP showed significantly higher sensitivity than ERCP for the diagnosis of intrahepatic stones (p < 0.001). We found no significant difference between MRCP and ERCP in the specificity for the diagnosis of intrahepatic stones (p > 0.05). Although individual sequences were not directly compared in this study, the multislice HASTE images appeared to be superior to the single-slab RARE images for confidence in diagnosing intrahepatic stones (Figs. 1A,1B,1C and 2A,2B,2C).
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For detecting calculi in the common duct or gallbladder, ERCP showed a slightly better diagnostic performance than MRCP; however, no statistically significant difference was seen between MRCP and ERCP in the sensitivity or specificity (p > 0.05). One of the MRCP reviewers interpreted biliary air as a common duct stone (Fig. 3A,3B,3C) in three patients, and the other reviewer made that interpretation in two patients (Table 1).
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In the diagnosis of right and left intrahepatic duct stones, interobserver agreement between the two MRCP reviewers was very good, with kappa values of 0.92 and 0.85, respectively. In common duct stones and gallbladder stones, the observed agreement was good and very good, respectively, and the kappa values were 0.67 and 0.88, respectively.
An intrahepatic fluid collection suggesting an abscess was diagnosed on MRCP by both reviewers, but it was not detected on ERCP. Aspiration performed under sonographic guidance confirmed that this lesion was an abscess. In two patients (10%) who underwent surgery, intrahepatic stones were associated with cholangiocarcinoma; however, neither the MRCP reviewers nor the ERCP reviewer had suggested the possibility of cholangiocarcinoma.
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Compared with common bile duct stones, which can be managed effectively by therapeutic procedures during ERCP, endoscopic removal of intrahepatic stones is not easy [12]. Therefore, surgical intervention is indicated when the stones and strictures are localized to a single atrophic liver segment or lobe. Radiologic intervention also plays a significant role in the treatment of intrahepatic stones, particularly in patients who present with symptoms and signs of cholangitis. After effective percutaneous drainage has been established, subsequent cholangioscopic stone removal using a small-caliber endoscope [12] or fluoroscopy-guided stone removal using preshaped catheters and stone baskets [16] is commonly used. Accurate localization of intrahepatic stones is important for determining the treatment of choice for this disease.
Intrahepatic stones can be depicted on unenhanced CT, but the detection rate is not satisfactory (63-81%) because pigmented stones are sometimes not sufficiently hyperattenuating to be visible on CT [14]. With the improved spatial resolution provided by recently implemented MR techniques, MRCP has become an important technique in the evaluation of biliary calculi. Because of the narrow spacing of the radiofrequency refocusing pulses, single-shot RARE and multislice HASTE MRCP eliminate artifacts arising from respiratory motion and negligible susceptibility effects from bowel gas and metallic foreign bodies [17, 18]. The capability of MRCP to visualize peripheral intrahepatic bile ducts makes this technique well suited to the evaluation of intrahepatic stones. The combination of thick-slab RARE and multislice HASTE imaging takes no more than 15 min and results in a reliable test for evaluating the biliary tree.
Our findings showed that MRCP was superior to ERCP for the diagnosis of
intrahepatic stones. This conclusion was based on the fact that ERCP could not
opacify peripheral intrahepatic bile ducts because of the common occurrence of
associated severe ductal stricture. Visualization of the intrahepatic ducts
peripheral to the ductal stricture is often possible with ERCP but
necessitates forced injection of contrast material, which may increase the
risk of cholangitis and may actually worsen the course of the disease. In
addition, the right intrahepatic bile ducts are normally more difficult to
fill on ERCP performed with the patient prone, because they are the
nondependent ducts. Air block may also be a factor in patients with
intraductal air (Fig. 1A).
These observations may be additional explanations for the lower sensitivity of
ERCP in right intrahepatic stones. Moreover, ERCP has the disadvantage of
being highly operator-dependent and of having procedure-related complications,
including pancreatitis, sepsis, perforation, and hemorrhage. The principal
advantage of MRCP for the diagnosis of intrahepatic stones is its ability to
depict the entire biliary tract and to allow identification of dominant
strictures without instrumentation
[4]. In addition to the
excellent diagnostic performance of MRCP, the very good ( > 0.80)
interobserver agreement indicates that MRCP interpretation is objective and
reliable.
Another potential advantage of MRCP is the simultaneous imaging of the hepatic parenchyma, which allows better differentiation between benign stenosis and the malignant processes that can be associated with intrahepatic stone disease. A number of investigations suggest that chronic infection and bile stasis induced by intrahepatic stones can lead to the development of epithelial adenomatous hyperplasia and cholangiocarcinoma [12, 14, 19]. However, the diagnosis of cholangiocarcinoma associated with intrahepatic stones is difficult to make because inflammatory lesions often show similar radiologic findings. Cholangiocarcinomas associated with intrahepatic stones have a poor prognosis because of the usually delayed diagnosis [19]. In our study, two patients (10%) were proven to have cholangiocarcinoma, but neither of the MRCP reviewers made this diagnosis. This failure to detect cholangiocarcinoma might be partly attributable to the fact that the imaging sequences performed in this study were not adequate for a comprehensive evaluation of the hepatic parenchyma. MRCP can also detect other abnormalities associated with intrahepatic stones, such as perihepatic biloma or the collection of free fluid in the perihepatic spaces [15].
Biliary air may also be confused with biliary calculi because both appear as filling defects in the high-signal-intensity bile on MRCP images. Many of our patients had a history of recent biliary intervention or biliary—enteric anastomosis and therefore had pneumobilia. However, differentiation is usually possible because biliary air is located in nondependent portions of the bile duct, whereas calculi tend to be located in dependent positions. Because stones have the typical appearance of biliary air, axial T2-weighted MR images and lateral projection of single-shot RARE MRCP images are helpful in differentiating biliary air from stone; the filling defects sometimes produce an air—fluid level on axial or sagittal images [1, 4, 20, 21] (Fig. 3A,3B,3C).
Our study has limitations. Because all patients were referred for MRCP examination rather than for both MRCP and ERCP, a selection bias was introduced. Specifically, this method of selection may result in examining a large number of patients who present technical difficulties or who have undergone incomplete ERCP examinations. However, because most patients in our hospital who were suspected of having intrahepatic stones underwent MRCP to determine treatment method, the selection bias might be small. In addition, most patients in our study had moderately or markedly dilated intrahepatic bile ducts. Therefore, our results may not apply to patients with mildly dilated or non-dilated ducts.
In summary, MRCP is a rapid, accurate, noninvasive means of showing the presence of intrahepatic stones. Findings in our study show the clinical applications of MRCP in intrahepatic stone disease. Because ERCP is neither an effective treatment method nor an accurate diagnostic modality, MRCP is a more effective diagnostic method for the evaluation of intrahepatic stones.
Acknowledgments
We thank Bonnie Hami, Department of Radiology, University Hospitals of
Cleveland, for her editorial assistance in the preparation of this
manuscript.
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