A contrast agent (or contrast medium) is a
substance
Substance may refer to:
* Matter, anything that has mass and takes up space
Chemistry
* Chemical substance, a material with a definite chemical composition
* Drug substance
** Substance abuse, drug-related healthcare and social policy diagnosis ...
used to increase the
contrast of structures or fluids within the body in
medical imaging
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues ( physiology). Medical imaging seeks to rev ...
. Contrast agents absorb or alter external
electromagnetism or
ultrasound
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
, which is different from
radiopharmaceuticals, which emit radiation themselves. In x-ray imaging, contrast agents enhance the
radiodensity in a target tissue or structure. In magnetic resonance imaging, contrast agents shorten (or in some instances increase) the relaxation times of nuclei within body tissues in order to alter the contrast in the image.
Contrast agents are commonly used to improve the visibility of
blood vessels and the
gastrointestinal tract.
The types of contrast agent are classified according to their intended imaging modalities.
Radiocontrast media
For
radiography, which is based on
X-rays,
iodine
Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
and
barium are the most common types of contrast agent. Various sorts of iodinated contrast agents exist, with variations occurring between the
osmolarity, viscosity and absolute iodine content. Non-ionic
dimers are favored for their low osmolarity and toxicity, but have a correspondingly higher price attached to their use.
MRI contrast agents
Gadolinium is used in
magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio ...
as an
MRI contrast agent or gadolinium-based contrast agent (GBCA). 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.
Ultrasound contrast agents
Microbubbles are used as contrast agents for
sonographic
Medical ultrasound includes diagnostic techniques (mainly imaging techniques) using ultrasound, as well as therapeutic applications of ultrasound. In diagnosis, it is used to create an image of internal body structures such as tendons, muscl ...
examination, specifically
echocardiograms, for the detection of a
cardiac shunt. These microbubbles are composed of agitated
saline solution, most of which are too large to pass through the lung capillaries. 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 a
right-to-left shunt. In addition, pharmaceutically prepared microbubbles are composed of tiny amounts of
nitrogen or
perfluorocarbons strengthened and supported by a protein, lipid, or polymer shell.
[}] 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 the
ultrasound
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
back to the probe. This process of
backscattering gives the liquid with these bubbles a high signal, which can be seen in the resulting image.
See also
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References
{{DEFAULTSORT:Contrast Medium