Colonic polyp, double contrast coloured barium X-ray. The polyp is the grey sphere at upper left. This is a pedunculated polyp, one that extends on a stalk from the inner wall of the colon (large intestine). Polyps often cause no symptoms, but may sometimes cause a change of bowel habits and blood in the faeces. Although benign, some polyps can turn cancerous, and they are generally removed as a precautionary measure. This image was produced using a double contrast barium enema. Water containing radio-opaque barium was introduced into the rectum, which was then distended by air. SOVEREIGN, ISM / SCIENCE PHOTO LIBRARY.