What is the difference between PTC and PTBD?

The main differences between PTC (percutaneous transhepatic cholangiography) and PTBD (percutaneous transhepatic biliary drainage) are:

- Purpose - PTC is a diagnostic procedure to image the biliary system. PTBD is a therapeutic procedure to drain bile and relieve obstruction.

- Contrast use - PTC involves injection of contrast material into the bile ducts to opacify them and image on X-ray. PTBD does not require contrast.

- Catheter placement - In PTC a catheter is placed temporarily into the bile duct just to inject contrast. In PTBD a catheter is left in place for ongoing drainage.

- Drainage - PTC does not involve drainage, just contrast injection and imaging. PTBD involves the insertion of a drainage catheter to divert bile externally.

- Indications - PTC is used to identify blockages or stones causing biliary obstruction. PTBD is used to provide biliary drainage if a blockage cannot be resolved and is causing life-threatening symptoms.

- Outcome - PTC results in x-ray images of the biliary system. PTBD results in external bile drainage to relieve the obstruction. 

So in summary, PTC is diagnostic while PTBD is interventional and therapeutic. PTC visualizes while PTBD drains. PTBD usually follows a diagnostic PTC if drainage is required.