Contrast agents are chemical substances introduced to the anatomical or functional region being imaged, to increase the differences between different tissues or between normal and abnormal tissue, by altering the relaxation times.
MRI contrast agents are classified by the different changes in relaxation times after their injection.
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Negative contrast agents (appearing predominantly dark on
MRI) are small particulate aggregates often termed
superparamagnetic iron oxide (
SPIO). These agents produce predominantly
spin spin relaxation effects (local field inhomogeneities), which results in shorter
T1 and
T2 relaxation times.
SPIO's and ultrasmall superparamagnetic iron oxides (
USPIO) usually consist of a crystalline iron oxide core containing thousands of iron atoms and a shell of polymer, dextran, polyethyleneglycol, and produce very high
T2 relaxivities. USPIOs smaller than 300 nm cause a substantial
T1 relaxation.
T2 weighted effects are predominant.
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A special group of negative
contrast agents (appearing dark on
MRI) are perfluorocarbons (
perfluorochemicals), because their presence excludes the hydrogen atoms responsible for the signal in
MR imaging.
The design objectives for the next generation of MR
contrast agents will likely focus on prolonging intravascular retention, improving tissue targeting, and accessing new
contrast mechanisms. Macromolecular
paramagnetic contrast agents are being tested worldwide. Preclinical data shows that these agents demonstrate great promise for improving the quality of MR
angiography, and in quantificating capillary
permeability and myocardial perfusion.
Ultrasmall
superparamagnetic iron oxide (
USPIO) particles have been evaluated in multicenter clinical
trials for lymph node
MR imaging and MR
angiography, with the clinical impact under discussion. In addition, a wide variety of
vector and carrier molecules, including antibodies, peptides, proteins, polysaccharides,
liposomes, and cells have been developed to deliver magnetic labels to specific sites. Technical advances in
MR imaging will further increase the efficacy and necessity of tissue-specific
MRI contrast agents.
See also
Adverse Reaction and
Nephrogenic Systemic Fibrosis.
See also the related poll result: '
The development of contrast agents in MRI is'
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Further Reading:
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