Radiocontrast
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Radiocontrast agents (or simply contrast agents) are compounds used to improve the visibility of internal bodily structures in an X-ray image.
Contrast agents (usually based on gadolinium) are also used in MRI (Magnetic Resonance Imaging). Although MRI is usually considered a branch of radiology, it is not based on X-rays. Gadolinum-based MRI contrast agents work not by being radioopaque, but rather by altering the magnetic properties of nearby hydrogen nucleii.
Types and uses
There are two basic types of contrast agents used in X-ray examinations.
One type of contrast agent is based on barium sulphate, an insoluble white powder. This is mixed with water and some additional ingredients to make the contrast agent. As the barium sulphate doesn't dissolve, this type of contrast agent is an opaque white mixture. It is only used in the digestive tract; it is usually swallowed or administered as an enema. After the examination, it leaves the body with the faeces.
The other type of contrast agent is based on iodine. There are different molecules, such as iohexol, iodixanol, ioversol and many others. The basic idea is to chemically bind the iodine in an large organic molecule making it water soluble and as harmless as possible to the body. These contrast agents are sold as clear colorless water solutions, the concentration is usually expressed as mg I/ml. Modern iodinated contrast agents can be used almost anywhere in the body. Most often they are used intravenously, but for various purposes thay can also be used intraarterially, intrathecally and intraabdominally.
An older type of contrast agent, Thorotrast was based on thorium dioxide, but this was abandoned since it turned out to be carcinogenic.
Side-effects
Modern iodinated contrast agents are safe drugs; adverse reactions exist but they are uncommon.
For patients with previously impaired renal function, however, large doses of iodinated contrast agents may cause contrast-induced nephropathy. The risk is augmented if the patient also has other predisposing conditions; notably diabetes mellitus, dehydration and myeloma.
The osmolality of the contrast agent is believed to be of great importance in contrast-induced nephropathy. Ideally, the contrast agent should be isoosmolar to blood. Modern iodinated contrast agents are non-ionic, the older ionic types caused more adverse effects and are not used much anymore.
To minimize the risk for contrast-induced nephropathy, various actions can be taken if the patient has predisposing conditions. Low-osmolar or iso-osmolar contrast media slould be chosen. The dose of contrast media should be as low as possible, while still being able to perform the necessary examination. Dehydration should be corrected before the examination, this is usually done by means of intravenous fluids.
Some recent studies suggest that N-acetylcysteine protects the kidney from the toxic effects of the contrast agent (Gleeson & Bulugahapitiya 2004). This effect is, in any case, not overwhelming.
Other pharmacological agents, such as theophylline and furosemide have been tried, but they have not been shown to have beneficial effects against contrast-induced nephropathy.
References
- Gleeson TG, Bulugahapitiya S. Contrast-induced nephropathy. AJR Am J Roentgenol 2004;183:1673-89. PMID 15547209.ja:造影剤