Whilst under anaesthetic, the doctor threads an endoscope down your oesophagus, through your stomach and into the small intestine. The doctor then looks through the endoscope to find the point where the pancreatic duct and the common bile duct empty their secretions into the small interestine.  A small plastic tube called a catheter is then passed down through the endoscope and use this to inject special dye into the pancreatic and bile ducts. The special dye shows up on X-rays. The X-rays can show gallstones or blockages in the bile duct and demonstrate narrowing or blockage of the pancreatic duct.

Gallstones
If the X-rays show a gallstone in the common bile duct, the doctor can remove the stone or leave it to pass through the intestine.

Blockages and Narrowing
The X-rays may show a narrowing (called a stricture) or blockage of a duct that is preventing the free flow of bile. In these cases the doctor may be able to insert a device called a stent. A stent is a narrow plastic tube that is inserted into the duct to hold it open to work normally.