MRCP Features: PSC vs PBC
PSC shows characteristic multifocal strictures and dilatations ("beading") of the bile ducts on MRCP, while PBC typically has normal bile ducts on cholangiography—making MRCP essential for diagnosing PSC but not necessary for PBC diagnosis.
Key Distinguishing Features
PSC on MRCP 1, 2:
- Multifocal strictures with intervening dilatations or ectasias affecting intra- and/or extrahepatic bile ducts (classic "beading" appearance)
- Ductal thickening and pruning of peripheral branches
- Pseudodiverticula formation
- Short segmental strictures distributed throughout the biliary tree
- Bile ducts up to third order should be visible on high-quality MRCP
- Sensitivity 86% and specificity 94% for PSC diagnosis 1
PBC on MRCP 3:
- Normal bile ducts on cholangiography—this is the critical differentiator
- MRCP is not required for diagnosis of PBC (diagnosis relies on cholestatic biochemistry and positive AMA antibodies)
- When cirrhosis develops, may see:
- Periportal halo sign (relatively specific for PBC)
- General cirrhotic changes (nodular liver contour, splenomegaly, varices)
- No bile duct stricturing or beading
Diagnostic Algorithm
For cholestatic liver disease workup 1:
- Elevated ALP/GGT → Check ultrasound to exclude obstruction
- If ducts normal → Check AMA antibodies:
- AMA positive → PBC (MRCP not needed)
- AMA negative → Proceed to MRCP:
- Strictures/beading present → PSC
- Normal ducts → Consider liver biopsy for small duct PSC or other diagnoses
Important Clinical Caveats
- May miss very early intrahepatic PSC affecting only peripheral ductules (small duct PSC—requires liver biopsy)
- False positives can occur in cirrhosis of any etiology due to duct distortion
- Less sensitive than ERCP for early changes, but MRCP remains first-line due to non-invasive nature
When to use ERCP instead 2:
- Tissue acquisition needed (cytology brushings for suspected cholangiocarcinoma)
- Therapeutic intervention required (stricture dilation)
- Not for routine diagnosis
Additional MRI information beyond MRCP 1:
- Bile duct wall enhancement and thickness
- Hepatic parenchymal changes
- Portal hypertension complications
- Can add MR elastography for fibrosis staging
The Bottom Line
The fundamental difference is that PSC is a disease of the bile ducts visible on MRCP, while PBC is a disease of small intrahepatic bile ductules not visible on cholangiography. If you see bile duct abnormalities on MRCP in cholestatic disease (after excluding secondary causes), think PSC. If the ducts look normal and AMA is positive, think PBC 1, 2.