The tube is passed through the stomach and into the upper part of the small bowel (duodenum). A further tube within this scope called a catheter is slide further down until the bile duct and bilary tree are identified. A contrast medium (dye) is injected and X-ray flims taken to obtain very detailed images of the bile and pancreatic duct system, to identify diseases or obstructions which can then be corrected by passing "stents" to open the blocked ducts.