A lower gastrointestinal series, also called a barium enema, is a medical procedure used to examine and diagnose problems with the human colon (large intestine). X-ray pictures are taken while barium sulfate fills the colon via the rectum.

This test may be done in a hospital or clinic radiology department. The patient lies on the X-ray table and a preliminary X-ray is taken. The patient is then asked to lie on the side while a well lubricated enema tube is inserted into the rectum. As the enema enters the body, the patient might have the sensation that their stomach is being filled. The barium, a radiopaque (shows up on X-ray) contrast medium, is then allowed to flow into the colon. A small balloon at the tip of the enema tube may be inflated to help keep the barium inside. The flow of the barium is monitored by the health care provider on an X-ray fluoroscope screen (like a TV monitor). Air may be puffed into the colon to distend it and provide better images (often called a "double-contrast" exam). If air is used, the enema tube will be reinserted (if it had been removed; whether it is depends on who does the exam) and a small amount of air will be introduced into the colon, and more X-ray pictures are taken.

The patient is usually asked to move to different positions and the table is slightly tipped to get different views.

If there is a suspected bowel perforation, a water-soluble contrast is used instead of barium. The study is otherwise very similar, although the images are not quite as good. (The concern with existing perforation is that contrast will leak from the bowel to the peritoneal cavity, and water-soluble material, compared to barium is less obscuring at laparotomy.)

From http://en.wikipedia.org/wiki/Barium_enema