Radiolucent

Anything that permits the penetration and passage of X-rays or other forms of radiation.

Radiolucent is as opposed to radiopaque (which refers to anything that blocks the penetration of X-rays).

Plastic is usually radiolucent.

If a child swallows a radiolucent plastic paperclip, that paperclip will not be visible on X-ray so it may not be possible to tell whether it is in the stomach or down in the intestines (where it represents no problem) or lodged in the airway to the lungs (where it can be a big problem).

To make radiolucent structures visible on X-ray, radiopaque dyes may be used.

For instance, in an intravenous pyelogram (an IVP), a radiopaque dye is injected into the bloodstream and passes into the urine, outlining the otherwise radiolucent renal pelvis and ureters -- the renal pelvis where the urine is collected and the ureters that serve as conduits for the urine from the kidneys to the bladder.