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, 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 gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
.
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
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
Th ...
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 of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...
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
echocardiogram
An echocardiography, echocardiogram, cardiac echo or simply an echo, is an ultrasound of the heart.
It is a type of medical imaging of the heart, using standard ultrasound or Doppler ultrasound.
Echocardiography has become routinely used in ...
s, for the detection of a
cardiac shunt A cardiac shunt is a pattern of blood flow in the heart that deviates from the normal circuit of the circulatory system. It may be described as right-left, left-right or bidirectional, or as systemic-to-pulmonary or pulmonary-to-systemic. The dire ...
. These microbubbles are composed of agitated
saline solution
Saline (also known as saline solution) is a mixture of sodium chloride (salt) and water. It has a number of uses in medicine including cleaning wounds, removal and storage of contact lenses, and help with dry eyes. By injection into a vein it ...
, 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
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...
or
perfluorocarbon
Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commerc ...
s 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 back to the probe. This process of
backscatter
In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, ...
ing 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