contrast medium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A substance, such as barium or air, used in radiography to increase the contrast of an image. A positive contrast medium absorbs x-rays more strongly than the tissue or structure being examined; a negative contrast medium, less strongly.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun medicine Any substance, such as barium sulfate, used in radiography to increase the visibility of internal structures

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a substance that is opaque to x-rays; when administered it allows a radiologist to examine the organ or tissue it fills