MRI contrast agent
Such MRI contrast agents shorten the relaxation times of nuclei within body tissues following oral or intravenous administration.
An intense radiofrequency pulse is applied that tips the magnetization generated by the hydrogen nuclei in the direction of the receiver coil where the spin polarization can be detected.
Thermally driven motion of the strongly paramagnetic metal ions in the contrast agent generate the oscillating magnetic fields that provide the relaxation mechanisms that enhance the rate of decay of the induced polarization.
[26] As a free solubilized aqueous ion, gadolinium(III) is highly toxic, but the chelated compounds are generally regarded as safe for individuals without kidney disease.
[30] Gadolinium has been found to remain in the brain, heart muscle, kidney, liver, and other organs after one or more injections of a linear or macrocyclic gadolinium-based contrast agents, even after a prolonged period of time.
[31][32] The amount differs with the presence of kidney injury at the moment of injection, the molecular geometry of the ligand, and the dose administered.
[38][39] Continuing evidence of the retention of gadolinium in brain and other tissues following exposure to gadolinium containing contrast media, led to a safety review by the Committee for Medicinal Products for Human Use (CHMP) which led the EMA to restrict or suspend authorization for the intravenous use of most brands of linear gadolinium-based media, in which Gd3+ has a lower binding affinity, in 2017.
The FDA also called for increased patient education and requiring gadolinium contrast vendors to conduct additional animal and clinical studies to assess the safety of these agents.
[43] The World Health Organization issued a restriction on use of several gadolinium contrast agents in November 2009 stating that "High-risk gadolinium-containing contrast agents (Optimark, Omniscan, Magnevist, Magnegita, and Gado-MRT ratiopharm) are contraindicated in patients with severe kidney problems, in patients who are scheduled for or have recently received a liver transplant, and in newborn babies up to four weeks of age.
"[44] In magnetic resonance imaging in pregnancy, gadolinium contrast agents in the first trimester is associated with a slightly increased risk of a childhood diagnosis of several forms of rheumatism, inflammatory disorders, or infiltrative skin conditions, according to a retrospective study including 397 infants prenatally exposed to gadolinium contrast.
If a contrast-enhanced MRI must be performed on a dialysis patient, it is recommended that certain high-risk contrast agents be avoided but not that a lower dose be considered.
[46] The American College of Radiology recommends that contrast-enhanced MRI examinations be performed as closely before dialysis as possible as a precautionary measure, although this has not been proven to reduce the likelihood of developing NSF.
These contrast agents consist of suspended colloids of iron oxide nanoparticles and when injected during imaging reduce the T2 signals of absorbing tissues.
[60] The complex specifically targeted human prostate cancer cells in vitro, and these results suggest that SIPPs may have a role in the future as tumor-specific contrast agents.
[69] This contrast agent works by reducing the number of hydrogen ions in a body cavity, thus causing it to appear dark in the images.
