radiopaque dye: dye that does not allow the passage of X rays or other radiation; used to outline certain organs during X-ray examination
dye: a usually soluble substance for staining or coloring e.g. fabrics or hair
clinical trial: a rigorously controlled test of a new drug or a new invasive medical device on human subjects; in the United States it is conducted under the direction of the FDA before being made available for general clinical use