Factors affecting the appearance  
 

INTRALUMINAL CONTRAST AGENT: BARIUM

The image on the left shows barium which has been swallowed and is traveling down the esophagus into the stomach. Barium is very dense, absorbing much of the incident x-ray beam and so appears white on the film. Without the barium, the stomach and esophagus wouldn't be visible on an x-ray. Barium coats the mucosa and fills the lumen. Note the lungs appear dark and the ribs white, just like on an X-ray.

The image on the right is from a barium enema. Through a rectal catheter, barium is instilled into the rectum and flows retrograde into the colon. Its movement is observed fluoroscopically to ensure that the entire colon is coated and evaluated. Air is also instilled into the colon to distend it so that all of the walls are seen. This is called a "double contrast barium enema". In this image, the patient has been rolled so that they are lying on their right side. (Note the air-barium level, with the barium located dependently). The flow of barium is observed fluoroscopically as the patient is moved. Barium coats the mucosa of the colon to make it visible on x-rays.