Virtual guide star helps focus light for medical imaging

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A researcher at Washington University in St. Louis has revealed a new technique that focuses diffuse light inside a dynamic scattering medium containing living tissue. 

The advance, by Lihong Wang, PhD and colleagues, is detailed today in Nature Communications.

In addition, the University team are said to have improved the speed of optical focusing deep inside tissue by two orders of magnitude, a development that represents an important step toward non-invasive optical imaging in deep tissue and photodynamic therapy.

In the new research, Wang and his team have built on a technique they developed in 2010 to improve the focusing speed of time-reversed ultrasonically encoded (TRUE) optical focusing for applications in living tissue.

To focus light, the engineers use a virtual internal guide star at the targeted location. By detecting the wavefront of light emitted from the guide star, they can determine an optimum phase pattern that allows scattered light moving along different paths to focus at the targeted location.

When light is shined into living biological tissue, breathing and blood flow changes the optical interference - or speckle pattern - which can cause previous methods to focus diffuse light inside scattering media to fail. Scientists have to act quickly to get a clear image.

The new TRUE technology combines two techniques: focused ultrasonic modulation and optical phase conjugation. Researchers use a type of mirror to record then time-reverse the ultrasound-modulated light emitted from the ultrasonic focus to achieve the best focus. Previously, technology limited the speed of TRUE focusing to no more than 1Hz.

To overcome this obstacle, the team is said to have used a fast-responding photorefractive crystal that is sensitive to light at the 790-nanometer wavelength, making it suitable to focus light deep into biological tissue.

The new TRUE technology is able to focus light inside a dynamic medium with a speckle correlation time as short as 5.6 milliseconds. The improved speed allowed Wang, the Gene K. Beare Professor of Biomedical Engineering at the University, to achieve the first optical focusing of diffuse light inside a scattering medium containing living biological tissue.

The team now plans to implement the system in a reflection configuration, where light is shined and detected on the same side of the tissue.