Fluorescent Nanotubes Provides Better Imaging of Mouse Innards

For an animal the size of a mouse, a few centimeters makes a great difference.

Developing drugs to fight or cure human disease frequently involves a phase of testing with mice, so being able to see clearly into a living mouse's insides has real benefits. However, with the fluorescent dyes now used to image the interior of laboratory mice, the view becomes so cloudy several millimeters under the skin that researchers do not have success extracting usable data.

"We have already used similar carbon nanotubes to deliver drugs to treat cancer in laboratory testing in mice, but you would like to know where your delivery went, right?" said Dr. Hongjie Dai, a professor of chemistry at Stanford University (Stanford, CA, USA; www.stanford.edu) . "With the fluorescent nanotubes, we can do drug delivery and imaging simultaneously--in real time--to evaluate the accuracy of a drug in hitting its target."

Researchers injected the single-walled carbon nanotubes into a mouse and watched as the tubes are delivered to internal organs by the bloodstream. The nanotubes fluoresce brightly in response to the light of a laser directed at the mouse, while a camera attuned to the nanotubes' near infrared wavelengths records the images. By attaching the nanotubes to an agent, researchers can see how the drug is progressing through the mouse's body.

Dr. Dai is the one of the authors of an article describing the research published online in May 2011 in the journal Proceedings of the [US] National Academy of Sciences (PNAS). The basis to the nanotubes' usefulness is that they shine in a different region of the near infrared spectrum than most dyes. Biologic tissues--whether mouse or human--naturally fluoresce at wavelengths below 900 nm, which is in the same range as the available biocompatible organic fluorescent dyes. That results in undesirable background fluorescence, which fogs the images when dyes are used. However, the nanotubes used by Dr. Dai's group fluoresce at wavelengths between 1,000 nm and 1,400 nm. At those wavelengths, there is barely any natural tissue fluorescence, so background "noise" is minimal.

The nanotubes usefulness is additionally enhanced because tissue scatters less light in the longer wavelength region of the near-infrared, reducing image smearing as light moves or travels through the body, another advantage over fluorophores emitting below 900 nm. "The nanotubes fluoresce naturally, but they emit in a very oddball region," Dr. Dai said. "There are not many things--living or inert--that emit in this region, which is why it has not been explored very much for biological imaging."

By selecting single-walled carbon nanotubes (SWNTS) with different chiralities diameters and other properties, Dr. Dai and his team can customize the wavelength at which the nanotubes fluoresce. The nanotubes are imaged immediately upon injection into the bloodstream of mice. Dr. Dai and graduate students Sarah Sherlock and Kevin Welsher, who are also coauthors of the PNAS article, observed the fluorescent nanotubes passing through the lungs and kidneys within seconds after injection. The spleen and liver lit up a few seconds later. The group also did some postproduction work on digital video footage of the circulating nanotubes to further enhance the image quality using a process called principal component analysis.

"In the raw imaging, the spleen, pancreas, and kidney might appear as one generalized signal," Ms. Sherlock said. "But this process picks up the subtleties in signal variation and resolves what at first appears to be one signal into the distinct organs."

"You can really see things that are deep inside or blocked by other organs such as the pancreas," Dr. Dai noted.

There are some other imaging methods that can produce deep tissue images, such as magnetic resonance imaging (MRI) and computer tomography (CT) scans. However, fluorescence imaging is widely used in research, and it requires simpler methods.

According to Dr. Dai, that the fluorescent nanotubes are not capable of reaching the depth of CT or MRI scans, but nanotubes are a step forward in widening the potential uses of fluorescence as an imaging system beyond the surface and near-surface applications it has been restricted to up until now.

Since nanotube fluorescence was discovered about 10 years ago, researchers have been trying to make the fluorescence brighter, according to Dr. Dai. However, he has been a little surprised at just how well they now work in animals. "I did not imagine they could really be used in animals to get deep images like these," he said. "When you look at images like this, you get a sense that the body almost has some transparency to it."

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