A new method enables light to be focused efficiently inside biological tissue, potentially allowing doctors to perform surgery without having to cut through the skin.

Researchers at the California Institute of Technology (Caltech; Pasadena, USA), expanding on a study conducted at Washington University, St. Louis (WUSTL; MO, USA; wustl.edu) developed a method to focus light using the high-frequency vibrations of ultrasound (US), taking advantage of two properties of US. First, the high-frequency sound waves are not scattered by tissue, which is why it is efficient, for example, for imaging fetuses in utero. Second, US vibrations interact with light to create an acousto-optic effect by a slight frequency shift, so that any light that has interacted with US changes into a slightly different color.

In both the WUSTL and Caltech experiments, the researchers focused US waves into a small region inside a tissue sample. They then shined light into the sample, which, in turn, scattered. Due to the acousto-optic effect, any of the scattered light that passed through the region with the focused US changed to a slightly different color. The researchers can pick out this color-shifted light and record it; using a playback technique, they can then send the light back, retracing the color-shifted sections to the small region where the US was focused, which means that the light itself is focused on that area, allowing an image to be created. The researchers can control where they want to focus the light simply by moving the US focus.

Doctors can also use this process to treat cancer with photodynamic therapy (PDT), using a drug that contains light sensitive, cancer-killing compounds injected into a patient. Cancer cells absorb those compounds preferentially, so that the compounds kill the cells when light shines on them. PDT can now only used at tissue surfaces, because of the way light is easily scattered, but the new technique should allow doctors to more than double the current limit for how far light can be focused into tissue. With future improvements on the optoelectronic hardware used to record and play back light, the researchers may be able to reach 10 centimeters, the depth limit of US, within a few years. The study was published in the June 26, 2012, issue of Nature Communications.

“It enables the possibilities of doing incision-less surgery,” said senior author professor of electrical engineering and bioengineering Changhuei Yang, PhD. “By generating a tight laser-focus spot deep in tissue, we can potentially use that as a laser scalpel that leaves the skin unharmed.”

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