An organic iodine salt that is radio-opaque and therefore used as a CONTRAST MEDIUM to outline various organs in the body in X-ray films (see X-RAYS). It is given intravenously. Its main use is in PYELOGRAPHY – that is, in rendering the kidneys radio-opaque – but it is also used to outline the blood vessels (ANGIOGRAPHY) and the gall-bladder and bile ducts (CHOLANGIOGRAPHY).