Acontrast agent (orcontrast medium) is asubstance used to increase thecontrast of structures or fluids within the body inmedical imaging.[1] Contrast agents absorb or alter externalelectromagnetism orultrasound, which is different fromradiopharmaceuticals, which emitradiation themselves. InX-ray imaging, contrast agents enhance theradiodensity in a target tissue or structure. Inmagnetic resonance imaging (MRI), contrast agents shorten (or in some instances increase) the relaxation times ofnuclei within body tissues in order to alter the contrast in the image.
Contrast agents are commonly used to improve the visibility ofblood vessels and thegastrointestinal tract.
The types of contrast agent are classified according to their intended imaging modalities.
Forradiography, which is based onX-rays,iodine andbarium are the most common types of contrast agent. Various sorts of iodinated contrast agents exist, with variations occurring between theosmolarity,viscosity and absolute iodine content. Non-ionicdimers are favored for their low osmolarity and low toxicity, but have a correspondingly higher cost attached to their use.[2]
Gadolinium is used inmagnetic resonance imaging as anMRI contrast agent or gadolinium-based contrast agent (GBCA).[3] In the 3+ oxidation state, the metal has seven unpaired electrons. This causes water around the contrast agent to relax quickly, enhancing the quality of the MRI scan.
Microbubbles are used as contrast agents forsonographic examination, specificallyechocardiograms, for the detection of acardiac shunt. These microbubbles are composed of agitatedsaline solution, most of which are too large to pass through thecapillaries (blood vessels) of thelungs. Therefore, the only ones that reach the left side of the heart pass through an abnormal connection between the two sides of the heart, known as aright-to-left shunt. In addition, pharmaceutically prepared microbubbles are composed of tiny amounts ofnitrogen orperfluorocarbons strengthened and supported by aprotein,lipid, orpolymer shell.[4] These are small enough to pass through the capillaries and are used to increase the contrast in the left ventricle, improving the visualization of its walls. The drop in density on the interface between the gas in the bubble and the surrounding liquid strongly scatters and reflects theultrasound back to the probe. This process ofbackscattering gives the liquid with these bubbles a high signal, which can be seen in the resulting image.
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