Confine and contrast: Nanoporous nests offer relaxing home for contrast agent

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  • Published: Jan 5, 2011
  • Author: David Bradley
  • Channels: MRI Spectroscopy
thumbnail image: Confine and contrast: Nanoporous nests offer relaxing home for contrast agent

Enhanced contrast

Magnetic resonance imaging contrast agents are currently designed by modifying their structural and physiochemical properties to improve relaxivity and to enhance image contrast. A new approach based on porous, disk-shaped "nests" for nanotubes could offer a way to improve contrast by increasing relaxivity through the confinement of the contrast agent within nanoporous silicon.

Researchers at Rice University, Texas Medical Center institutions, Colorado, Italy and Switzerland have found that they can trap gadolinium-based contrast agents within a silicon particle, which could make them fifty times more effective than the naked contrast agent injected into a patient undergoing an MRI scan.

"Making MRIs better is no small matter," explains Rice's Lon Wilson. He points out that almost 30 million MRI scans are performed each year in the US alone and contrast agents were required for almost half of them. "MRI is one of the most powerful medical tools for imaging, if not the most powerful," he adds. "It's not invasive, it's not ionizing harmful radiation and the resolution is the best you can get in medical imaging. The sensitivity, however, is poor. So anything you can do to improve performance and increase sensitivity is a big deal - and that's what this does."

Microscopic hockey pucks

The team has used nanoscale chunks of porous silicon to create microscopic particles that are shaped somewhat like a hockey puck. They infiltrated the silicon nano pucks with three types of contrast agents: Magnevist, a common contrast agent used worldwide and two novel classes developed at Rice University, gado-fullerenes and gado-nanotubes. All three safely sequester the toxic element gadolinium to make it viable for injection prior to a scan. The silicon microparticles retain their form for up to two days allowing for long imaging, before they dissolve into harmless silicic acid, which is then excreted.

The key to successfully using these silicon hockey pucks in MRI is targeting the site in the body that is of interest from the clinical perspective. Wilson explains that the microparticles have been designed to leak from capillaries and to aggregate at the sites of tumours and other lesions. "Spherical particles, at least in mathematical models, flow down the centre of the vasculature," he explains. "These particles are designed to hug the wall. When they encounter a leaky area like a cancer tumour, they can easily get out." Their design means that they should not readily leak from intact vasculature into non-diseased areas, however.

The team found that the impact of all three contrast agent classes was improved using this technology. The encapsulation gado-nanotubes (carbon nanotubes that contain bundles of gadolinium ions) worked the best. "The performance was enhanced beyond what we had imagined," Wilson adds. The team adds that the particles could easily be functionalized with peptides that target cancer and other cells or be loaded with a therapeutic agent as well as the contrast agent to home in on a disease site and treat it.

Gado all over

The team has also demonstrated efficacy of the related gado-nanotube technology, invented by Wilson in 2005, in tracking the progress of stems cells through the body. The technique, reported elsewhere, holds the promise of tracking tagged cells as they travel towards their target. The researchers also hope that the magnetic properties of tagged stem cells might one day allow doctors to manipulate them directly and to direct the cells to specific locations for therapeutic purposes.

 

 


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 
Credit: Lesa Tran/Rice University Magnetic resonance imaging contrast agents are currently designed by modifying their structural and physiochemical properties to improve relaxivity and to enhance image contrast. A new approach based on porous, disk-shaped

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