Substances used in radiography that allow visualization of certain tissues.

Any internally administered substance that has a different opacity from soft tissue on radiography or computed tomography; includes barium, used to make opaque parts of the gastrointestinal tract; water-soluble iodinated compounds, used to make opaque blood vessels or the genitourinary tract; may refer to air occurring naturally or introduced into the body; also, paramagnetic substances used in magnetic resonance imaging.