Radiopaque

adj. impermeable to radiation, not allowing radiation to pass through

Radiopacity refers to the relative inability of electromagnetism to pass through a particular material, particularly X-rays. Dense materials that prevent the passage of electromagnetic radiation are called 'radiopaque'. The term refers to the relative opaque white appearance in radiographic imaging, when passing x-rays through dense matter .
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Adjective
1. not transparent to X-rays or other forms of radiation; "barium sulfate is radiopaque"
(synonym) radio-opaque
(antonym) radiolucent

adj. radiopaque, impermeable to radiation, not allowing radiation to pass through

X-rays cannot penetrate things that are radiopaque (opaque to X-ray). Radiopaque structures appear white on exposed X-ray film.