Chemical substances, containing chemically bound iodine, used to highlight tissues and organs that are not distinguished from their surroundings on plain X-ray pictures. Usually injected with a needle into an arm vein, or directly into an artery via a catheter inserted into an artery in the groin and moved to the target organ/tissue. Other types of contrast media are based on barium, and are used to highlight the stomach and/or gut, and can either be swallowed or given as an enema. Contrast media are also used in MRI, but most of these are based on a metal called gadolinium. CM are also called contrast agents.