All-Organic MRI Contrast Agent Tested In Mice

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http://cen.acs.org/articles/90/web/2012/09/organic-MRI-Contrast-Agent-Tested.html

To produce sharper images of organs through magnetic resonance imaging, doctors and researchers often inject patients with so-called contrast agents. The most commonly used agents contain the metal gadolinium. Researchers are looking for all-organic alternatives, in part, because the gadolinium compounds can cause serious adverse reactions in patients with kidney disease. Now scientists have designed a new practical organic contrast agent based on nitroxide radicals and demonstrated its use in living mice (J. Am. Chem. Soc., DOI: 10.1021/ja3079829).
Like gadolinium, nitroxide radicals are paramagnetic. Because of this similarity, researchers have long thought the radicals could serve as organic contrast agents. But research in this area has run into problems, says Murali C. Krishna, of the National Cancer Institute. Conditions inside the body quickly reduce nitroxides to hydroxyl amines, which are not paramagnetic. Also, compared to gadolinium, the radicals have low relaxivities. Relaxivity is a measure of the effect that a contrast agent has on nuclei in nearby molecules, typically water. These magnetic effects enhance the MRI signal.

Organic Contrast
A polypropylenimine dendrimer decorated with hindered nitroxide radicals (red dots) and polyethylene glycol chains (blue squiggles) could prove to be an alternative to gadolinium-based MRI contrast agents.
Credit: J. Am. Chem. Soc.