Generally, there are three types of contrast material or dye used in radiographic contrast study. These are:
1. Iodinated CM
2. Barium sulfate solution
3. Air
Air is the third type of CM used in contrast studies like the chest radiography, which is very common, Barium Enema and Upper GI series. For chest radiography, the patient is instructed to inhaled deeply and hold his breath while the radiologic technologist will do the exposure. The air collected within the lungs will then act as a contrast media and will appear black in the radiograph. One of its purposes is to clearly differentiate bones and vertebrae and ribs from the soft tissues of the lungs.
On the other hand, when air is employed in the double contrast technique or method of certain examinations like the Barium enema and the UGIS, the air is introduced to the patient by pumping air through his anus or the patient is positioned so that the air-fluid level inside the stomach can be seen. Just like in the chest radiography, air also appears black on the radiograph.