A series of x-ray pictures of the esophagus, stomach, and duodenum (the first part of the small intestine). The x-ray pictures are taken after the patient drinks a liquid containing barium sulfate (a form of the silver-white metallic element barium). The barium sulfate coats and outlines the inner walls of the upper gastrointestinal tract so that they can be seen on the x-ray pictures. Also called upper GI series.