Magnetic resonance cholangiopancreatography (MRCP)

Magnetic resonance cholangiopancreatography (MRCP) is a non-invasive imaging technique to visualize the intra and extrahepatic biliary tree and pancreatic ductal system.

It can provide diagnostically-equivalent images to ERCP and and is a useful technique in high risk patients to avoid significant morbidity.

Indications

MRCP can be used to evaluate various conditions of the pancreaticobiliary ductal system, some of which are:

• identification of congenital anomalies of the cystic and hepatic ducts
• post-surgical biliary anatomy and complications
• pancreas divisum
• anomalous pancreaticobiliary junction
• choledocholithiasis
• biliary strictures
• chronic pancreatitis
• pancreatic cystic lesions
• biliary or pancreatic trauma

Physics

The technique exploits the fluid which is present in the biliary and pancreatic ducts as an intrinsic contrast medium by acquiring the images using heavily T2-weighted sequences. Since the fluid-filled structures in the abdomen have a long T2 relaxation time as compared to the surrounding soft tissue, these structures appear hyperintense against the surrounding non-fluid-containing tissues on a heavily T2-weighted sequence and can easily be distinguished.

Technique and protocols

No exogenous contrast medium is administered to the patient.

Fasting for 4 hours prior to the examination is required to reduce gastroduodenal secretions, reduce bowel peristalsis (and related motion artifact) and to promote distension of the gallbladder. MRCP is performed on a 1.5 T or superior MRI system, using a phased-array body coil.

All protocols obtain heavily T2-weighted sequences. Most commonly obtained sequences are:

• RARE: rapid acquisition and relaxation enhancement
• FRFSE: fast-recovery fast spin-echo coronal oblique 3D respiratory triggered
• HASTE: half-Fourier acquisition single shot turbo spin echo-axial 2D breath hold sequence which provide superior images and can be performed in single breath hold (<20 s) and a fat-suppressed sequence
• an additional sequence that can be acquired to evaluate the duct wall is a fat suppressed T1 GRE sequence
  • T1 sequences may also help differentiate biliary calculi from pneumobilia ⁷

For optimal visualization of ducts, acquired images are reformatted in different planes using multiplanar reconstruction (MPR) and maximum intensity projection (MIP).

The advantage of FRFSE, as a 3D technique, is the ability to perform multiplanar reconstructions. However, despite respiratory triggering, this sequence is often prone to motion artifact.

Technical modifications

With the evolution of MRCP, modified techniques came into existence. Commonly applied modified MRCP techniques are:

• secretin-stimulated MRCP
  • secretin (administered intravenously) causes exocrine secretion of the pancreas, dilating the pancreatic duct and improving its visualization; indications include ¹:
    • ​detection/characterization of pancreatic duct anomalies or strictures
    • characterizing communications between the pancreatic duct and pseudocysts/fistulas
    • characterizing pancreatic and sphincter of Oddi dysfunction 
• functional MRCP
  • intravenous administration of MR lipophilic paramagnetic contrast agents which are then excreted by the hepatobiliary system
• negative oral contrast to 'null' the duodenum
  • commercially available agents
  • natural products which are rich in manganese (e.g. pineapple or blueberry juice) shorten the T2 relaxation time

Practical points

Artifacts related to technique and reconstruction, and motion or susceptibility artifacts due to metal clips and gas, may give rise to poor spatial resolution and limited interpretation. Reviewing thin section and multiple planes may help overcome some of these issues ⁶.