The upper gastrointestinal tracts 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 tractincluding the oesophagus stomach and small intestinesis plainly visible on X-ray images. This is so that X-rays cant penetrate barium.