New Compound Improves MRI Contrast Agents

A nano diamond agent is 15 times more sensitive.

Magnetic resonance imaging (MRI) has become an indispensable medical diagnostic tool because of its ability to produce detailed, 3D pictures of tissue in the body. Radiologists often inject patients with contrast agents to make certain tissues, such as tumors, stand out more on the final image. Now, researchers have synthesized an MRI contrast agent that is 15 times more sensitive than the compounds currently used. This could allow less contrast agent to be used, thus reducing the potential for harmful side effects.

Nano pyramid: Gadolinium ions (green) are chemically linked to the surface of a nano diamond to form an MRI contrast agent 15 times more sensitive than current ones. Credit: American Chemical Society

The researchers created the new compound by chemically linking gadolinium ions to nano diamonds--tiny clusters of carbon atoms just a few nanometers in diameter. Gadolinium, a rare-earth metal, is used in MRI contrast agents because of its strong paramagnetic properties (magnetism in response to an applied magnetic field). But alone, gadolinium is toxic, so it has to be bonded to other, biocompatible molecules to be used clinically. Many groups have been trying to improve the properties of gadolinium-based contrast agents by attaching the metal to a variety of materials, ranging from large organic molecules to nanoparticles.

"We've done this with many classes of nanoparticles and have never seen this extraordinary increase in sensitivity," says Thomas J. Meade, the Eileen M. Foell professor of chemistry and director of the Center for Advanced Molecular Imaging at Northwestern University. He and his colleagues published their findings online in Nano Letters last month.

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Meade collaborated with Dean Ho, assistant professor of biomedical and mechanical engineering at Northwestern, and his group, which has been studying nano diamonds as vehicles for drug delivery. Unlike some carbon nanomaterials, Ho says nanodiamonds are well-tolerated by cells and do not change gene expression in adverse ways. The researchers coupled the nano diamonds to gadolinium and tested the properties of the resulting complex to assess how good of an MRI contrast agent it might be.

MRI works by surrounding a patient with a powerful magnetic field, which aligns the nuclei of hydrogen atoms in the body. Radio wave pulses systematically probe small sections of tissues, knocking those atoms out of alignment. When they relax back into their previous state, the atoms emit a radio frequency signal that can be detected and translated into an image.