Colon cancer. Coloured X-ray of colon cancer revealed by a barium enema. The cancer itself is not seen, but it narrows the lumen of the colon, producing the characteristic apple core" narrowing seen at centre (blue wisps). A solution of a barium salt was introduced into the rectum. It is opaque to X-rays, and appears black (lower left). In a normal colon, the barium would flow up the rectum into the sigmoid colon, but here it is blocked by the cancer, leaving the sigmoid colon empty (white, upper right). Colon cancer is relatively common, and is thought to be linked to a high fat, low fibre diet."