Bismuth-filled Carbon Nanotubes Improve CT Scans

Research has shown in recent years that carbon nanotubes can improve fluorescence imaging, replacing the dyes that are typically used. Now researchers at Rice University have demonstrated that carbon nanotubes combined with bismuth can improve the capabilities of computed tomography (CT) scans.

The Rice team has filled single-walled carbon nanotubes with bismuth compounds to make for a more effective contrast agent than is currently available for CT scans.

The research, which was published in the journal of Materials Chemistry B (“Bismuth@US-tubes as a potential contrast agent for X-ray imaging applications”), builds on previous work that examined the use of bismuth as a contrast agent in CT scans and on the Rice team’s own research into using carbon nanotubes as a contrast agent in magnetic resonance imaging (MRI).

According to Rice chemist Lon Wilson, this is the first time that bismuth has been combined with nanotubes for use as a contrast agent for imaging individual cells.

“At some point, we realized no one has ever tracked stem cells, or any other cells that we can find, by CT,” Wilson said in a press release. “CT is much faster, cheaper and more convenient, and the instrumentation is much more widespread (than MRI). So we thought if we put bismuth inside the nanotubes and the nanotubes inside stem cells, we might be able to track them in vivo in real time.”

The new contrast agent starts off as single-walled carbon nanotubes that have been chemically processed to make them into capsules with dimensions of between 20 and 80 nanometers long and about 1.4 nanometers in diameter. These nanocapsules are then are mixed with bismuth chloride to form what the researchers have dubbed Bi@US-tubes.

Once the Bi@US-tubes are introduced in vivo, they are small enough to diffuse into individual cells where they aggregate into a clump approximately 300 nanometers in diameter. “We think the surfactant used to suspend them in biological media is stripped off when they pass through the cell membrane,” Wilson said in the press release. “The nanotubes are lipophilic, so when they find each other in the cell they stick together.”

“The cells adjust over time to the incorporation of these chunks of carbon and then they go about their business,” Wilson adds in the release.

Bismuth is more effective at diffracting X-rays than the iodine in today's contrast agents, or just about any other material. When it is put in the nanocapsules, the resulting combination can produce high contrasts in very low concentrations.

The surfaces of the nanotubes can also be modified in ways to make them target different kinds of cells and make them more biocompatible.

The next step in the research will be to increase the amount of bismuth in the capsules so as to improve their contrasting capability.

Image: Eladio Rivera/Rice University

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