1) Radio-opaque 
A radiopaque substance (for example metal) will be highlighted (appear white) on a plain X-ray. The use of iodine containing radiopaque contrast dyes allow enhancement of the anatomy demonstrable with conventional X-ray. ... (27 Sep 1997) ... 
(ra″de-o-pāk´) obstructing the passage of radiant energy, such as x-rays, the representative areas appearing light or white on the exposed film.
Type: Term Pronunciation: rā′dē-ō-pāk′ Definitions: 1. Exhibiting relative opacity to, or impenetrability by, x-rays or any other form of radiation. Synonyms: radiodense
Radiopaque: Anything that does not let X-rays or other types of radiation penetrate. Radiopaque objects block radiation. They are opaque to radiation. A metal object, for example, is typically radiopaque. If a child swallows a coin and it goes down into the stomach, it is easily visible in the stomach on an X-ray because it is radiopaque. Radiopaqu...
[adj] - not transparent to X-rays or other forms of radiation
radiopaque A substance (for example metal) will be highlighted (appear white) on a plain X-ray. The use of iodine containing radiopaque contrast dyes allow enhancement of the anatomy demonstrable with conventional X-ray.
radio-opaque adjective not transparent to X-rays or other forms of radiation; `barium sulfate is radiopaque`