Contrast media for röntgen rays (X-rays)

In conventional radiology and computerized tomography, contrast media may be classified as positive or negative media.

Negative contrast media (air, carbon dioxide and other gases) attenuate X-rays less than the soft tissues of the body, because a gas (the negative contrast medium) contains per unit volume a much lower number of radiation attenuating atoms than the patient's soft tissues.

Positive contrast media and the body's soft tissues contain a similar number of atoms per unit volume. Some atoms in the contrast medium (e.g. iodine or barium) have a much higher atomic number than those of the soft tissues (hydrogen, carbon, nitrogen, oxygen). A higher atomic number is generally associated with an increased ability to attenuate X-rays. At the photon energies which are used in diagnostic radiology, the radiation is attenuated by photoelectric absorption and Compton scatter. Such attenuation is proportional to the third or fourth power of the atomic number. In positive contrast media, iodine or barium atoms attenuate radiation 50-1000 times more than an atom of the human soft tissue (such as carbon-, nitrogen- and oxygen).

The positive contrast media can either be water soluble, which in clinical practice today means water solutions of organic compounds with iodine covalently bound to an aromatic structure, or water insoluble contrast media, which in daily practise means an aqueous suspension of in soluble crystals of barium sulphate.

Torsten Almén and Peter Aspelin