Silicon microparticles with gadonanotubes promise advance for MRI sensitivity

November 11, 2010

A way to trap contrast agents inside a silicon particle that, when injected into a patient’s bloodstream, would make them up to 50 times more effective¬† has been discovered by scientists at Rice University and other Texas Medical Center institutions and colleagues in Colorado, Italy and Switzerland. Contrast agents “light up” damaged tissue in the body in images produced by MRI instruments.

Silicon microparticles, or SiMPs served as a delivery device for contrast agents in the study. Nanoscale pores were created in the discs. Three types of contrast agents were drawn into the pores. Magnevist, a common contrast agent used worldwide, was one; the others were gadofullerenes and gadonanotubes, both pioneered by Wilson’s Rice lab. All three chemically sequester the toxic element gadolinium to make it safe for injection.

MRIs work by manipulating hydrogen atoms in water, which interact and align with the applied magnetic field from the instrument. The hydrogen atoms are then allowed to return to their original magnetic state, a process called relaxation. In the presence of the paramagnetic gadolinium ion, the atoms’ relaxation time is shortened, making these regions brighter against the background under MRI.

SiMPs are small — about a micrometer (a millionth of a meter) across — but when they trap both water molecules and bundles of nanotubes containing gadolinium, the protons appear much brighter in an MR image. Because SiMPs keep their form for up to 24 hours before dissolving into harmless silicic acid, the molecules can be imaged for a long time.

The trick is getting them to places in the body that doctors and technicians want to see. SiMPs are designed to escape the bloodstream, where they leak and aggregate at the sites of tumors and lesions, said said Lon Wilson, professor of chemistry at Rice and one of three senior co-authors of the research paper published online in Nature Nanotechnology. “Spherical particles, at least in mathematical models, flow down the center of the vasculature,” he said. “These particles are designed to hug the wall. When they encounter a leaky area like a cancer tumor, they can easily get out.”

The encapsulation within SiMPs enhanced the performance of all three contrast agents, but SiMPs with gadonanotubes (carbon nanotubes that contain bundles of gadolinium ions) showed the best results. “The performance was enhanced beyond what we had imagined,” he said.

SiMPs may also be functionalized with peptides that target cancer and other cells. SiMPs that contain contrast agents and medications could potentially be tracked as they home in on disease sites, where medications will be released as the silicon dissolves.

The work was supported by the Telemedicine and Advanced Technology Research Center-United States Army Medical Research Acquisition Activity through the Alliance for Nano Health and grants from the Department of Defense and National Institutes of Health, the Robert A. Welch Foundation, the Nanoscale Science and Engineering Initiative at Rice University, the Swiss National Science Foundation, European Cooperation in Science and Technology and TDA Research Inc.

Adapted from materials provided by Rice University