A barium enema is a diagnostic procedure used to examine the large intestine, or colon. During the procedure, a contrast material containing barium is inserted into the rectum and X-rays are taken to create images of the colon. The barium helps to highlight the colon
From Wikipedia
Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.