Cholangiopancreatography using magnetic resonance.
The history of biliary tract imaging began with Graham and Cole, who performed the first cholecystogram in 1924. This procedure yielded information about the function and anatomical shape of the gallbladder. A disadvantage of this method was that it gave only an approximation of the gallbladder function. In addition, the procedure often was unsuccessful due to gallbladder malabsorption and poor storage of the contrast in the gallbladder. For these reasons, researchers looked for other methods of visualizing the biliary tract.
Ludwig and Struthers published their findings in 1949, suggesting ultrasound as a method of biliary imaging. They proposed that the procedure be used to detect gallstones and foreign bodies in soft tissues. Through advancing technologies, ultrasound remains a useful method for demonstrating the biliary tract.
In 1952 Carter and Saypol introduced transabdominal cholangiography, a transabdominal method for visualizing the biliary tract. Known today as percutaneous transhepatic cholangiography (PTC), this procedure successfully demonstrates the biliary ducts, but requires passing a needle through the abdominal wall into the liver.
Another method for visualizing the biliary tract is intravenous cholangiography (IVC), introduced in 1953. This procedure used sodium or meglumine iodipamide as the contrast media. The contrast was administered intravenously via drip infusion. Even though this method was reported to be accurate, it often yielded poor visualization of the gallbladder due to obstruction of the image by bowel gas.
In 1955 Taplin et al performed the first clinical test demonstrating biliary function using radioactive material. This approach requires the injection of a radioisotope followed by scintillation, a technique in which a special camera is used to detect the presence of the isotope within the biliary tract. This exam is called a HIDA, DISIDA or MIBlVA scan, depending on the pharmaceutical used. These scans are indicated for cholecystitis and gallbladder uptake and emptying studies.
In 1965 Rabinov and Simon introduced a method of visualizing the biliary tract through direct oral cannulation with a flexible fiber optic endoscope. Called endoscopic retrograde cholangiopancreatography (ERCP), this method has imaging and therapeutic capabilities and today is considered the standard for biliary imaging.
In 1976 Alfidi et al suggested computed tomography as a diagnostic tool for evaluating the liver and biliary tract. The advantage of CT is its ability to produce images of high spatial and contrast resolution in the axial plane.
In 1983 Hricak et al used magnetic resonance imaging to demonstrate the gallbladder and surrounding tissues. They proposed that MRI would be the first imaging modality to evaluate gallbladder function as well as anatomy and that it could become a safe, simple and accurate clinical test.
Technical Aspects of Magnetic Resonance Cholangiopancreatography
Magnetic resonance cholangiopancreatography (MRCP) is a recent MRI technique that allows projectional and cross-sectional imaging of the biliary tract without the use of contrast media. Successful MRCP requires careful consideration of coil selection, respiration control, pulse sequence, slice mode acquisition and postprocessing.
The first option that must be considered is the coil. The three types of coils used for MRCP exams are the body coil, surface coil and torso multicoil array. The majority of MRCPs to date have relied on the body coil for signal retrieval. This coil offers some benefits. First, it is part of the standard equipment of MRI systems, eliminating the need for additional, costly coils. The body coil also produces moderate signal-to-noise ratios (SNR), which yield images of acceptable spatial resolution.
The shoulder surface coil also can be used for MRCPs. Its closer proximity to the body improves image quality. This coil allows a small field of view (FOV), which decreases the SNR but allows for higher spatial resolution images. However, a surface coil requires proper positioning and is limited to thinner patients.
To improve the SNR, a torso multicoil array can be used. The torso multicoil array offers advantages over the body coil. It allows for a more efficient method of signal retrieval. Because the torso multicoil surrounds the patient, a larger percentage of the signal from the tissue is obtained. The torso multicoil yields higher SNR and contrast-to-noise ratio (CNR) than the body coil. It is larger than a surface coil and covers a greater area, accommodating anatomical variances without the need for time-consuming repositioning.
The second technical factor that should be considered is respiration control. To date, there are two methods of respiration control used for MRCP exams: breath hold and nonbreath hold. Breath hold was first reported by Wallner et al in 1991. Patients were required to hold their breath for 12-second intervals. This breath-hold sequence was repeated throughout the exam, which lasted between 10 and 15 minutes. The drawback to this approach is that it required patient cooperation.
To increase the probability of success, Reinhold et al administered oxygen to patients who had trouble holding their breath. However, this modification was not possible for patients who have chronic obstructive pulmonary disease. Another disadvantage of the breath-hold technique was the length of breath hold required. In an experiment conducted by Guibaud et al, patients were required to hold their breath for approximately 1 minute. Most patients are unable to do this, thereby limiting the applicability of the exam.
To resolve these problems, nonbreath-holding imaging was first attempted by Macaulay et al in 1995. There are two forms of the nonbreath-hold technique. The first one is limited to facilities where respiratory compensation devices are available, such as respiratory triggering for Philips equipment and respiratory bellows for General Electric equipment. Respiratory compensation devices decrease respiration artifacts by acquiring the data at the end of each respiration phase. A disadvantage of using a respiratory compensation device is the extended scan time required.
The second variation of nonbreath-hold technique is called shallow breathing. This requires the use of fast spin-echo pulse sequences available on most MRI units. The main advantages of shallow breathing are that it can be used to image patients who are unable to stop their breathing and it can be performed in facilities where respiration compensation devices are not available. This has made MRCP more widely used.
The next step is to choose an appropriate pulse sequence. Today MRCP is most commonly imaged with a fast spin echo (FSE) pulse sequence. Previously, gradient echo (GE) and spin echo (SE) were used to produce images of diagnostic quality. Regardless of the pulse sequence, it must yield images that are heavily T2 weighted. This T2 weighting produces high-intensity signals in stationary fluids such as pancreatic secretions and bile and lower-intensity signals in solid organs.
Earlier attempts at MRCP used SE pulse sequences. Spin-echo sequences were used because higher field strengths and faster gradients were not available. The images that resulted from this attempt were not heavily T2 weighted and were of limited diagnostic value. In fact, the images obtained in the axial plane were comparable to those obtained with CT. In 1991 Wallner et al proposed GE pulse sequence as the best method for imaging the biliary tract. They used a steady-state free precession (SSFP) gradient pulse echo to achieve the heavily T2-weighted effect. In 1992 Morimoto et al suggested that acquiring the data in a 3-D set would optimize the SSFP and improve the contrast between structures.
The main limitation of GE pulse sequences is the inability to demonstrate nondilated bile ducts. This is due in part to the thick sections and large field of view necessary for 3-D GE. Another disadvantage to SSFP is its sensitivity to artifacts. For example, surgical clips and intestinal gas affect images of GE pulse sequences.
The most frequently used pulse sequence for MRCP today is fast spin echo (FSE). FSE was introduced in 1993 by Outwater and Gordon as a pulse sequence that demonstrates the biliary tree in obstructed and nondilated individuals. FSE also offers the advantage of reducing the time required to perform MRCP. FSE has the ability to incorporate extended echo train length (ETL), which increases the number of echoes produced per repetition time.
By using longer ETLs (16-32), FSE reduces the possibility of artifacts and allows for the option of breath-hold sequences. This permits imaging of patients who have surgical clips or are unable to hold their breath. Soto et al noted in 1995 the importance of FSE's ability to accurately demonstrate pancreatic duct dilatation, strictures, stones and cystic diseases. In addition, FSE sequences produce higher SNR and CNR from stationary fluids than SE or GE. This allows FSE pulse sequences to use thin slice selections, which increases spatial resolution.
Slice Mode Acquisition
After coil, respiration control and pulse sequence have been chosen, the radiologic technologist must consider an appropriate slice mode acquisition. The two types of slice mode acquisition are 2-D and 3-D. Two-dimensional slice mode is the only option with SE pulse sequence. However, GE and FSE pulse sequences allow for either mode.
In reference to 2-D slice mode, Guibaud et al reported difficulty in demonstrating small ducts after reconstruction as a result of low SNR. Barish et al also noted that the 2-D slice mode was limited in demonstrating normal intrahepatic ducts. 3-D slice acquisition solves the problems associated with 2-D slice mode. By choosing 3-D, SNR is increased, allowing for increased spatial resolution after postprocessing. Using a 3-D FSE sequence, Soto et al produced better quality images because each pixel had an equal voxel size. The choice between 2-D and 3-D slice selection is important when postprocessing image analysis is considered.
MRCP source images have been postprocessed into 3-D images since 1991. The source images are obtained in the coronal and axial planes, then the data is processed using the maximum intensity projection (MIP) algorithm. By using the MIP algorithm, the biliary tract can be viewed in 3-D format. This gives the viewer the ability to rotate the images, viewing them from any angle. However, there are disadvantages of the MIP reconstructed images. As noted by Soto et al, MIP images can hide intraductal abnormalities due to the bright bile. Outwater and Gordon also reported that MIP frequently degrades the data. Ascites and other upper abdominal fluid collections also can limit the effectiveness of the MIP algorithm. Due to these disadvantages, Barish et al recommend that both the source and postprocessed images be viewed when making a diagnosis.
MRCP is capable of imaging the biliary tract along with surrounding tissue without the aid of contrast. By utilizing heavily T2-weighted pulse sequences, MRCP can produce high signal intensity images in fluid-filled structures. In a normal exam, intrahepatic ducts (IHD) and extrahepatic ducts (EHD) should be visualized within normal limits, along with the biliary tract. (See Fig. 1.) However, if an obstruction is present, IHDs, EHDs and the biliary tract will have a dilated appearance. (See Fig. 2.)
[Figures 1-2 ILLUSTRATION OMITTED]
Pathology of the biliary tract can have many different appearances. Choledocholithiasis, for instance, appears as low-density round objects surrounded by high-density bile. (See Fig. 3.) Chronic pancreatitis can present as a normal-appearing biliary tract with a dilated pancreatic duct. Gallbladder sludge has a medium-intensity appearance in contrast to high-intensity bile. Biliary cystadenocarcinoma has the same intensity as bile but, given its cystic appearance and location, diagnosis is possible. Choledocholcyst has a similar appearance to biliary cystadenocarcinoma except it is located within the common bile duct (See Fig. 4.)
[Figures 3-4 ILLUSTRATION OMITTED]
MRCP is a useful diagnostic procedure for biliary imaging. While it offers advantages, MRCP is limited in comparison to other modalities. As reported by Soto et al, MRCP is restricted to imaging the biliary tract whereas ERCP has both imaging and therapeutic capabilities. Since MRCP depends on fluid-filled structures, it often misses small lesions distal to an obstruction.
On the other hand, MRCP offers some benefits. In comparison to ERCP and PTC, MRCP demonstrates ducts in their normal state. ERCP and PTC often cause ducts to dilate as a result of contrast injection. This may lead to inaccurate interpretation of ductal size. Macaulay et al consider MRCP to be an accurate method for identifying biliary obstruction and measuring ductal size. MRCP also has been shown to be more sensitive in diagnosing choledocholithiasis compared to ultrasound and CT.
Another advantage of MRCP is that there are no morbidity or mortality rates with this exam, since MRCP is noninvasive and does not require the use of contrast media. ERCP has a morbidity rate of 1% to 3.7% [22,23] and a mortality rate of 0.2% to 1%.[24,22]
The success rate of MRCP also is comparable to that of ERCP. ERCP has been reported to be unsuccessful in approximately 3% to 10% of cases. MRCP, on the other hand, has been reported to be 90% to 95% effective in demonstrating strictures, biliary and pancreatic dilatation[25-27,14,16] and 72% to 95% effective in demonstrating choledocholithiasis.[27,14,16]
Through advances in technology, MRI is emerging as a promising modality for biliary imaging. By selecting the correct technical factors, radiologic technologists can produce heavily T2-weighted images of the biliary tract. MRCP has advantages over other modalities in that it can image in any plane and does not require the use of contrast media. Diagnostically, MRCP has proven to be an accurate and effective method of visualizing the biliary tract.
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Jason Douglas, B.S., R.T.(R), is a 1997 graduate of the Indiana University School of Allied Health Sciences Medical Imaging Technology Program at Indiana-Purdue University at Indianapolis. Mr. Douglas thanks all those at the school who provided assistance and advice and Stephen Stockberger, M.D., Indiana University Department of Radiology, for his help in obtaining original source images.
Reprint requests may be sent to the American Society of Radiologic Technologists, Communications Department, 15000 Central Ave. SE, Albuquerque, NM 8 7123-3917.
[C] 1998 by the American Society of Radiologic Technologists.
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|Date:||May 1, 1998|
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