Endoscopic retrograde cholangiography (ERCP). This is a technique used to examine the bile ducts (thin lines), gall bladder (white sac, upper left), pancreas and liver. An endoscope (flexible camera, white tube, upper right to lower left) is inserted through the mouth, down the oesophagus, through the stomach to the second part of the duodenum and finally through the sphincter of Odi into the common bile duct. A contrast dye is injected into the tube and passes into the ducts. An X-ray is taken and the dye shows up any blockages or narrowing of the ducts. Further instruments can be used with the endoscope to remove obstructions, such as gallstones, widen the ducts or take a biopsy.