Multifunctional Nanoparticle Enables New Type of Biological Imaging

Spotting a single cancerous cell that has broken free from a tumor and is traveling through the bloodstream to colonize a new organ might seem like finding a needle in a haystack. But a new imaging technique from the University of Washington is a first step toward making this possible. A research team headed by Xiaohu Gao has developed a multifunctional nanoparticle that eliminates the background noise, enabling a more precise form of medical imaging — essentially erasing the haystack, so the needle shines through. A successful demonstration with photoacoustic imaging was reported in the journal Nature Communications.

Nanoparticles are promising contrast agents for ultrasensitive medical imaging. But in all techniques that do not use radioactive tracers, the surrounding tissues tend to overwhelm weak signals, preventing researchers from detecting just one or a few cells. "Although the tissues are not nearly as effective at generating a signal as the contrast agent, the quantity of the tissue is much greater than the quantity of the contrast agent and so the background signal is very high," said Dr. Gao.

The newly presented nanoparticle solves this problem by combining two properties to create an image that is different from what any existing technique could have produced. The new particle combines magnetic properties and photoacoustic imaging to erase the background noise. Researchers used a pulsing magnetic field to shake the nanoparticles by their magnetic cores. Then they took a photoacoustic image and used image processing techniques to remove everything except the vibrating pixels.

Dr. Gao compares the new technique to "Tourist Remover" photo editing software that allows a photographer to delete other people by combining several photos of the same scene and keeping only the parts of the image that aren't moving. "We are using a very similar strategy," Dr. Gao said. "Instead of keeping the stationary parts, we only keep the moving part." As a result, experiments with synthetic tissue showed the technique can almost completely suppress a strong background signal. Future work will try to duplicate the results in lab animals, Dr. Gao said.

The 30-nanometer particle consists of an iron-oxide magnetic core with a thin gold shell that surrounds but does not touch the center. The gold shell is used to absorb infrared light, and could also be used for optical imaging, delivering heat therapy, or attaching a biomolecule that would grab on to specific cells.

This work is detailed in a paper titled, "Multifunctional nanoparticles as coupled contrast agents." An abstract of this paper is available at the journal's Web site.

Source: National Cancer Institute; August 20, 2010

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