Stealth micelles for improved MRI scans

06 September 2010

Novel gadolinium complexes could improve magnetic resonance imaging (MRI) of blood vessels, say Belgian scientists.

Gadolinium complexes have a long history of use as contrast agents in MRI. Modern MRI machines operate at high magnetic fields giving images with high resolution and sensitivity. At these high fields Gd complexes have low relaxivity, meaning that a large amount of Gd is needed to produce high quality images. However, the Gd complexes are potentially toxic and scientists have been searching for ways to reduce the amount of Gd required for sensitive scans.

Now, a team led by Christophe Detrembleur, at the University of Liège, have designed poly(3-caprolactone)-b-poly(ethylene oxide) copolymers that are biocompatibile and are able to remain undetected by the immune system. The diblock polymer forms micelles in water with reactive groups on the outside. Reacting these micelles with Gd complexes forms a stealth contrast agent. The poly(ethylene oxide) chains in the micelle prevent complex being recognised by the immune system, says Detrembleur, allowing the contrast agent to circulate in the blood for longer.

Veins and arteries in the brain could be easier to image using the new complex

Attaching the complexes to the micelles slows down the rotational motion of the Gd complex in solution, explains Detrembleur, which improves their relaxivity rates to give better contrast efficiency and allows lower dose of gadolinium to be used.

The micelle-Gd agents show three times increased reflexivity in high magnetic fields compared to the gadolinium complex alone. This, along with the micelles long circulation time, makes them ideal for use as MRI blood pool contrast agents. Kris Thurecht, an expert in biomedical imaging using polymers from the University of Queensland, Australia, explains, 'most conventional imaging agents become much less effective at higher field. In order to overcome these issues, new contrast agents, such as these, with enhanced relaxivity and longer circulation times in vivo are an important step towards more information-rich MR images."

The team are continuing to make agents which will give even better images, 'we have demonstrated a powerful way for imparting stealth character to macromolecular contrast agents. Now, we have to improve contrast efficiency,' says Detrembleur.

Catherine Bacon

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