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Friday, April 11, 2008

Magnetic Resonance Cholangiopancreatography

Magnetic Resonance Cholangiopancreatography represents a relatively new development in MR technology that allows for rapid evaluation of the biliary tract, pancreatic duct and gallbladder without contrast material administration, instrumentation or radiation. To date, over 2000 MRCPs have been performed at the Medical College of Virginia Hospitals. Special imaging sequences that are heavily-T2-weighted are utilized to depict the biliary tract, pancreatic duct and gallbladder as high signal intensity or bright structures owing to the fluid within them. Studies performed at the Medical College of Virginia Hospitals as well as at other institutions have shown that the accuracy of MRCP is comparable to that known as ERCP (endoscopic retrograde cholangiopancreatography, the traditional but invasive means of imaging the pancreaticobiliary system) in the evaluation of choledocholithiasis, malignant obstruction, anatomic variants and chronic pancreatitis. In most instances, MRCP can be completed in 10 minutes and is easily performed as an outpatient examination.

Since its introduction in 1991, the role of MRCP in evaluating pancreaticobiliary disease has continued to evolve. MRCP is assuming a larger role as a rapid, accurate and non-invasive alternative to diagnostic ERCP. During the past several years, radiologists and nonradiologists alike have shown a keen interest in MRCP and its clinical applications. Technical refinements such as fast MR sequences that allow for imaging of the entire biliary tract and pancreatic duct in a single breathhold have resulted in marked improvement in the quality and diagnostic yield of MRCPs. As the quality of MRCPs has improved, the clinical applications of this technique have expanded such that MRCP is now replacing diagnostic ERCP in many instances.

Current techniques allow for depiction of obstructed or dilated bile and pancreatic ducts in essentially all patients. Normal caliber extrahepatic bile ducts and central intrahepatic ducts are routinely depicted in as many as 100% of patients. Although the normal caliber pancreatic duct may be more difficult to visualize than the bile duct, the normal pancreatic duct can be visualized in 80-95% of cases. Dilated ducts proximal to an obstruction are well visualized, usually better than with ERCP where there can be difficulty in opacifying ducts proximal to a high-grade obstruction.

MRCP avoids the complications of ERCP such as pancreatitis (3-5%), sepsis, perforation and hemorrhage. The main disadvantage of MRCP is that it is purely diagnostic and does not provide access for therapeutic intervention.

Clinical Applications

Screening Examination In Patients With Low or Intermediate Probability Of Choledocholithiasis:

MRCP is useful as a noninvasive means of determining the presence or absence of common bile duct stones as well as their number, size and location. With the use of state-of-the-art MRCP techniques, MRCP has a sensitivity of 95-100% in the detection common bile duct stones. MRCP is particularly useful in the evaluation of patients with suspected gallstone pancreatitis and in patients with non-specific abdominal pain and normal liver-associated enzymes. In these settings, an MRCP that shows no evidence of a common bile duct may result in avoidance of an unnecessary diagnostic ERCP.

Failed or Incomplete ERCP:

MRCP provides a means of demonstrating the biliary tract and pancreatic duct after a failed or incomplete ERCP. Although ERCP is still regarded as the standard of reference for evaluating the bile duct and pancreatic duct, ERCP is technically challenging and is associated with a failed cannulation rate of 10-20%. Anatomic alterations such as Billroth II anatomy, periampullary diverticula, duodenal stenosis and periampullary masses may contribute to failed ERCP attempts. MRCP also allows for evaluation of ducts in patients who may not be candidates for ERCP due to cervical spinal fractures, head and neck tumors, sleep apnea, or other diseases or injuries that preclude placement of the endoscope or positioning the patient for ERCP.

Variant Ductal Anatomy:

MRCP is useful in demonstrating variant ductal anatomy and congenital anomalies of the biliary tract and pancreatic duct such as pancreas divisum, choledochal cyst, annular pancreas, abnormal pancreaticobiliary junctions and aberrant bile ducts. There may also be a role for MRCP in the evaluation of patients prior to laparoscopic cholecystectomy to identify and define variant biliary anatomy that might complicate the surgical procedure. MRCP may be useful in sorting out complex anatomic variants of the pancreaticobiliary tract.

Post-operative Anatomy:

In our experience, MRCP has been shown to be helpful in demonstrating the bile ducts in patients with surgically-altered biliary anatomy such as that associated with biliary-enteric anastomoses and liver transplantations. MRCP is particularly useful in the setting of surgical alterations of the gastrointestinal tract since the performance of ERCP may be difficult or impossible in this setting.

Primary Sclerosing Cholangitis (PSC):

MRCP provides a noninvasive means of diagnosing PSC and determining disease extent. In the detection of PSC, MRCP has been shown to have a sensitivity of 88%, a specificity of 97%, and positive and negative predictive values of 94%. MRCP depicts the mural irregularities, strictures and biliary calculi that characterize PSC. In contrast to ERCP, MRCP delineates the duct proximal to a complete obstruction and avoids the risk of ERCP-induced sepsis related to the injection of contrast material into an obstructed system.

Complications of Chronic Pancreatitis:

MRCP demonstrates ductal dilatation, strictures, intraductal calculi, fistulas and pseudocysts that occur as a result of chronic pancreatitis. One of the major roles of MRCP in this setting lies in defining the ductal anatomy and extent of ductal disease prior to surgical drainage procedures.

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