The upper gastrointestinal tract contour can be seen with the use of barium testing. On standard X-ray images, the gut (gastrointestinal tract) does not appear particularly clearly. On the other hand, when you consume a white beverage that includes the chemical barium sulphate, the shape of the upper digestive tract—including the oesophagus, stomach, and small intestines—is plainly visible on X-ray images. This is so that X-rays can't penetrate barium.