Which medium is used as negative contrast in imaging?

• A. Iodine solution
• B. Air
• C. Barium sulfate
• D. Gadolinium

Answer: (B)

Negative contrast media are substances that appear darker on imaging because they are radiolucent. Air fits this role perfectly due to its very low density and low X-ray attenuation. When air is present in a hollow organ or space, it creates a dark region on the image, helping outline the walls and separate surrounding tissues, which improves visualization of the lumen and any potential pathology. This is especially useful in gastrointestinal imaging to distend and clearly delineate the bowel. In contrast, iodine solutions and barium sulfate are positive contrast media; they appear bright on X-ray by increasing attenuation. Gadolinium is used primarily as an MRI contrast agent, not for radiographic imaging.