MR Cholangiopancreatography

MR cholangiopancreatography (MRCP) is a non-invasive MR technique that shows the biliary tree and pancreatic duct using heavily T2-weighted sequences. Stationary fluid in the ducts appears very bright, while solid tissue and flowing blood appear dark. No contrast is needed for the basic study.

The patient fasts for 4 to 6 hours to empty the stomach and gallbladder and to reduce bowel fluid. A negative oral contrast (pineapple or blueberry juice, or ferumoxsil) can be given to suppress fluid in the stomach and duodenum.

Thick slab single shot images (30 to 80 mm) are acquired in a breath hold of 2 to 5 seconds, in coronal and oblique planes. These give a rapid projection of the biliary tree. A 3D respiratory triggered thin slice sequence (1 mm) gives a high resolution volume, which is reformatted with maximum intensity projection.

Axial and coronal T2-weighted images, T1-weighted images and diffusion weighted imaging of the liver and pancreas are added to show the parenchyma. The study takes 20 to 30 minutes.

Secretin (0.2 micrograms each kg intravenously) can be given to stimulate pancreatic secretion and improve visualisation of the pancreatic duct, ductal anomalies and exocrine function.

Gallstones are present in about 10 to 15% of adults, and 10 to 15% of those have common bile duct stones. MRCP has a sensitivity of 90 to 95% and a specificity of 95 to 98% for common bile duct stones, comparable to endoscopic ultrasound and avoiding the 3 to 5% risk of pancreatitis and other complications of diagnostic endoscopic retrograde cholangiopancreatography.

It shows biliary strictures with a sensitivity of over 90%, and diagnoses malignant obstruction with an accuracy of about 85 to 90%.

Indications.

• Suspected common bile duct stones (jaundice, abnormal liver tests, gallstone pancreatitis) with an intermediate probability, to avoid unnecessary endoscopy.
• Biliary obstruction of unknown cause (stricture, tumour, cholangiocarcinoma, pancreatic head cancer).
• Primary sclerosing cholangitis and other cholangiopathies.
• Pancreatic duct disease: chronic pancreatitis, strictures, stones, pancreas divisum, anomalous pancreaticobiliary junction.
• Cystic pancreatic lesions: intraductal papillary mucinous neoplasm and its communication with the duct.
• Biliary complications after surgery (bile leak, stricture, retained stone), and after liver transplantation.
• Congenital anomalies (choledochal cysts, biliary atresia in infants, aberrant ducts).
• Recurrent acute pancreatitis of unknown cause.
• Failed or incomplete endoscopic retrograde cholangiopancreatography, or when the ducts cannot be reached (surgical anatomy, obstruction).
• Pregnancy with suspected biliary obstruction (non-contrast MR).
• Mirizzi syndrome and cystic duct anatomy before surgery.

Not for patients already known to need therapeutic endoscopic retrograde cholangiopancreatography (for example, cholangitis with a visible stone and high probability), where intervention should not be delayed.

Contraindications and precautions.

• The usual MRI safety screening. Biliary stents are safe, but metal clips from cholecystectomy cause artefact.
• Claustrophobia. Patients who cannot hold their breath: use respiratory triggered sequences.
• Pregnancy: MRCP without gadolinium is safe and preferred over CT or endoscopy.
• Secretin: avoid in acute pancreatitis (can worsen it) and in hypersensitivity.
• Gadolinium (when added): renal precautions apply.
• Oral negative contrast: avoid in patients with swallowing difficulties.
• Patients with severe obstruction, sepsis or cholangitis should not wait for MRCP if drainage is urgent.

Preparation and technique.

The patient fasts for 4 to 6 hours. If an oral agent is used, 200 to 400 mL of juice or contrast is taken 15 to 30 minutes before the study.

The patient lies supine, with the coil over the upper abdomen. Scout images are followed by coronal thick slab breath hold MRCP, and then 3D respiratory triggered thin slice MRCP. Coronal and axial T2 images with and without fat suppression, and T1 in- and opposed phase images, are acquired.

For secretin enhanced MRCP, thick slab images are acquired every 30 to 60 seconds for 10 minutes after injection, to measure fluid entering the duodenum (normally over 10 mL in 10 minutes, and filling the duodenum to the second part).

Post-contrast imaging is added for suspected tumour or inflammation, with hepatobiliary agents (gadoxetate) to show bile leaks or biliary obstruction function.

Normal appearances.

The intrahepatic ducts converge into the right and left hepatic ducts, which form the common hepatic duct. The cystic duct joins it to form the common bile duct. The common bile duct measures up to 6 mm in adults (up to 7 to 8 mm after cholecystectomy, and slightly more in older people). It tapers smoothly at the ampulla.

The gallbladder is a pear shaped bright structure with a thin wall. The cystic duct is usually spiral.

The main pancreatic duct measures up to 3 mm in the head, 2 mm in the body and 1 mm in the tail. It is smooth with fine side branches, and joins the common bile duct at the ampulla.

The duodenum and stomach may contain bright fluid unless suppressed. Variants include a low cystic duct insertion, an aberrant right posterior duct, and pancreas divisum.

Interpretation.

• Common bile duct stones: a rounded or faceted signal void (dark filling defect) surrounded by bright bile, with a meniscus of bile over it. Stones as small as 3 mm are seen. Multiple stones, sludge, air (a bubble in a non-dependent location), and a stone impacted at the ampulla are recognised.
• Bile duct dilatation: common bile duct over 6 to 7 mm, with intrahepatic duct dilatation. Look for the level and cause of obstruction.
• Benign stricture: smooth, short, tapered narrowing (post-surgical, inflammatory, chronic pancreatitis).
• Malignant stricture: irregular, abrupt cut off, longer length, wall thickening and enhancement, shouldering, and a mass. The "double duct sign" (dilated common bile duct and pancreatic duct) suggests pancreatic head tumour or ampullary tumour.
• Cholangiocarcinoma: hilar tumour with dilated intrahepatic ducts and separated confluence. Extension to the right and left ducts is described for surgery.
• Primary sclerosing cholangitis: multifocal strictures and dilatation (beaded appearance), pruned peripheral ducts, and wall enhancement. Dominant strictures need exclusion of cholangiocarcinoma.
• Choledochal cyst: cystic or fusiform dilatation of the common bile duct, with anomalous pancreaticobiliary junction (common channel over 15 mm).
• Biliary atresia: absent or atrophic extrahepatic ducts and gallbladder in infants (triangular cord sign on ultrasound).
• Chronic pancreatitis: dilated, irregular main pancreatic duct with beading and side branch ectasia, strictures, stones (signal voids), pseudocysts, and atrophy.
• Pancreas divisum: a dominant dorsal duct that drains through the minor papilla and does not join the common bile duct.
• Intraductal papillary mucinous neoplasm: main duct (dilated over 5 to 10 mm), branch duct (cluster of small cysts communicating with the duct, "bunch of grapes"), and mixed type. Worrisome features are a cyst over 3 cm, an enhancing mural nodule, main duct dilatation of 5 to 9 mm, thickened wall, and abrupt duct calibre change. High risk features are a main duct over 10 mm, an enhancing solid component, and obstructive jaundice.
• Other pancreatic cysts: serous cystadenoma (honeycomb, central scar, no duct communication), mucinous cystic neoplasm (thick wall, no communication, in women), and pseudocyst (after pancreatitis).
• Bile leak: free fluid in the subhepatic space with a biliary connection. Hepatobiliary agent excretion outside the duct confirms it.
• Post-surgical anatomy: surgical clips (artefact), anastomotic strictures, hepaticojejunostomy, and transplant ducts (donor and recipient size mismatch).
• Cholecystitis and gallbladder disease: stones, wall thickening, pericholecystic fluid, and Mirizzi syndrome (stone in the cystic duct compressing the hepatic duct).
• Parasites and other causes: hydatid cyst communicating with the ducts, and Ascaris or liver fluke (linear filling defects in ducts).
• Recurrent pancreatitis causes: microlithiasis (sludge), sphincter of Oddi dysfunction, anatomical variants, and tumours.

Critical values and findings needing immediate action.

• Common bile duct stone with cholangitis (fever, jaundice, pain): urgent biliary decompression with endoscopic retrograde cholangiopancreatography, antibiotics and resuscitation.
• Gallstone pancreatitis with persistent obstruction: urgent endoscopic retrograde cholangiopancreatography within 24 to 72 hours.
• Malignant biliary obstruction: multidisciplinary review, staging and stenting or resection.
• Bile leak after surgery: drainage and endoscopic stent.
• Perforation or biliary peritonitis: surgical review.
• High risk pancreatic cyst features: endoscopic ultrasound and surgical review.
• Cholangiocarcinoma: staging and specialist hepatobiliary referral.

Limitations and artefacts.

• Spatial resolution is lower than for direct cholangiography. Small stones under 3 mm, small papillary tumours, and subtle strictures may be missed, and side branch details of the pancreatic duct are less clear.
• Flow artefacts: pulsation of the hepatic artery, duodenal fluid and fast bile flow can mimic stones or strictures. Pseudo stricture of the common hepatic duct occurs where the right hepatic artery crosses.
• Air bubbles (pneumobilia) mimic stones. They lie in the non-dependent part of the duct, while stones sit in the dependent part.
• Susceptibility artefact from surgical clips and metal stents causes signal voids and false strictures. Stents obscure the duct lumen.
• Fluid in the duodenum and stomach hides the ducts, so the preparation must be followed.
• Motion artefact, ascites and a large number of intraductal stones reduce clarity.
• Stones mimic tumours and vice versa, and sludge can resemble a mass. Diffusion weighted imaging and post-contrast sequences help.
• MRCP is a diagnostic test with no therapy, so a positive study means a second invasive procedure in many patients.
• Secretin responses vary, and a normal MRCP does not exclude sphincter of Oddi dysfunction.
• Dilated ducts in older people and after cholecystectomy are normal in some cases.

Special groups.

• Pregnant patients: non-contrast MRCP is the preferred test for suspected common bile duct stones.
• Children: choledochal cysts, biliary atresia, and pancreatic anomalies. Sedation may be needed.
• Patients with surgically altered anatomy (Roux-en-Y, hepaticojejunostomy): endoscopy is difficult, so MRCP is first.
• Patients with primary sclerosing cholangitis: surveillance for cholangiocarcinoma, with MRCP and CA 19-9.
• Patients after liver transplant: strictures and leaks.
• Elderly or frail patients: avoids the risks of endoscopic retrograde cholangiopancreatography.
• Regions: gallstone disease, biliary ascariasis, liver flukes (opisthorchiasis, clonorchiasis), hydatid disease, recurrent pyogenic cholangitis (hepatolithiasis), and biliary tuberculosis occur. MRCP is available in few centres. Ultrasound is the first test, and endoscopic or surgical exploration is the main approach.

Follow up tests.

• Endoscopic ultrasound with fine needle aspiration for cysts and ampullary lesions.
• Endoscopic retrograde cholangiopancreatography for stone removal, stenting, brushings and biopsy.
• Percutaneous transhepatic cholangiography and drainage.
• Contrast CT or MRI of the liver and pancreas for staging.
• Blood tests: liver function (bilirubin, alkaline phosphatase, gamma glutamyl transferase, transaminases), lipase or amylase, full blood count, inflammatory markers, blood cultures, CA 19-9, IgG4.
• Cholescintigraphy for bile leak and function.
• Faecal elastase for exocrine function.
• Stool and serology for parasites.
• Cholecystectomy for gallstones.
• Referral to gastroenterology, hepatobiliary surgery, interventional radiology or oncology.