The advances in diagnostic imaging and therapy have not only been made by improvements in equipment and techniques but also in the “dye” used. A “dye” or contrast medium is a liquid substance that is used during a radiological examination for the purpose of delineating internal structures or an organ that is being studied. This would otherwise not be possible. Contrast medium may be given by injection (through the veins or through the arteries), by mouth, or from below, through the rectum. Generally, there are 3 types of contrast media:  

• Iodinated contrast medium is given by injection. It may also be given by mouth
• Barium sulphate is given by mouth or through the rectum
• Gadolinium is given by injection

Contrast media work by one of the following ways. When a contrast medium is given by injection, it will make vessels and organs with a good blood supply appear opaque or white when imaged. Therefore on angiography, the vessels will appear dense or opaque when x-ray films are taken. On computed tomography, the vessels and the organs with a good blood supply will appear opaque. The contrast medium will then be excreted by the kidneys and therefore on intravenous urography, the urinary system will appear opaque on the x-ray films. In this way, the various organs or structures can be seen and studied. When contrast medium is given by mouth for a barium meal, the upper gut will appear opaque or white on the x-ray films. Similarly, when given through the rectum for a barium enema, the lower gut will appear as opaque or white when x-ray films are taken. In this way, the large intestine can be seen and studied. Gadolinium is the contrast medium given by injection during magnetic resonance imaging. It works by making the vessels and the organs with a good blood supply appear white on the MRI images.

Modern intravenous contrast media are highly tolerable and are generally rapidly cleared with no adverse side effects. Contrast medium containing iodine may cause a feeling of warmth when injected. Sometimes, but uncommonly, it may cause nausea. That is why it is not advisable to eat before any radiological examination requiring contrast media. Rarely, iodine based contrast media may cause an allergic reaction. Contrast media containing barium sulphate is harmless. Barium will be passed out in the motion within a day or two. Contrast medium containing gadolinium rarely causes any side effects. However, it has not been proven to be safe in pregnant women.

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Last Updated:
Thursday, 21 August, 2003