A contrast agent (or contrast medium) is a substance used to increase the Display contrast of structures or fluids within the body in medical imaging.
Contrast agents are commonly used to improve the visibility of 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 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 low toxicity, but have a correspondingly higher cost attached to their use.
MRI contrast agents
Gadolinium is used in magnetic resonance imaging 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 examination, specifically
Echocardiography, 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
Capillary (blood vessels) of the
. 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
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
gives the liquid with these bubbles a high signal, which can be seen in the resulting image.
See also
