a type of x-ray test sometimes used to help look for colorectal cancer. Barium sulfate, a chalky liquid, is given through the rectum to enlarge and partly fill the colon (large intestine). When the colon is about half-full of barium, air is often pushed in to cause the colon to expand further. This allows good x-ray films to be taken. This procedure may also be called a double contrast barium enema, air-contrast barium enema, or a lower GI series. See also barium sulfate, colon, colorectal cancer.