contrast medium

contrast medium n a substance (as a solution of iodine or suspension of barium sulfate) comparatively opaque to X-rays that is introduced into the body (as by injection or swallowing) to contrast an internal part (as the gastrointestinal tract, kidneys, or blood vessels) with its surrounding tissue in radiographic visualization called also contrast agent, contrast material

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a substance administered to enhance the visibility of structures (i.e. increase the contrast) during imaging. In radiography a positive contrast agent (e.g. barium sulphate or a water-soluble iodine-containing compound) increases the density of a structure. Gas is a negative contrast agent. Positive and negative contrast media can be used together (e.g. barium sulphate and gas in a double-contrast barium enema). Magnetic resonance (MR) contrast agents contain either a positive contrast atom (usually gadolinium) to increase the signal or a negative contrast atom (such as iron) to decrease it. Ultrasound contrast medium consists of tiny (1-10 µm diameter) bubbles of gas, which reflect back the sound waves strongly. They can also be made to resonate or rupture to increase the signal to the ultrasound probe.

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a substance that is introduced into or around a structure or tissues and has a significant difference from the structure or tissues in the extent of absorption of x-rays, thus allowing radiographic visualization. See radiopaque m. and radiolucent m. Called also contrast agent.