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contrast medium (contrast agent) (kon-trahst) n. a substance administered to enhance the visibility of structures (i.e. increase the contrast) during imaging. positive c. m. a contrast agent (e.g. barium sulphate) that increases the density of a structure in radiography. negative c. m. a contrast agent (e.g. gas) that decreases the density of a structure in radiography. magnetic resonance c. m. a contrast agent that contains either a positive contrast atom (usually gadolinium) to increase the signal or a negative contrast atom (such as iron) to decrease it. ultrasound c. m. a contrast medium consisting of tiny (1–10 μm diameter) bubbles of gas, which reflect back the sound waves strongly.