Caltech researchers focus light deeper inside biological tissue

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Engineers at the California Institute of Technology (Caltech) have developed a technique to focus light efficiently inside biological tissue to a depth of 2.5mm. The technique could lead to a less invasive way of diagnosing diseases and even perform surgery without cutting through the skin.

The previous limit for focusing light in living tissue was only about one millimetre. The team has been able to more than double this and with future improvements might be able to reach 10cm, the engineers say.

The technique involves focusing light using the high-frequency vibrations of ultrasound, which interact with light in such a way that they slightly shift the light's frequency. Ultrasound waves are focused into a small region inside a tissue sample and, due to the acousto-optic effect, any scattered light passing through the region will shift in wavelength, which can be recorded.

The researchers then send the light back, having only the wavelength-shifted bits retrace their path to the small region where the ultrasound 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 ultrasound focus.

The team demonstrated how the method could be used with fluorescence imaging. The researchers embedded a patch of gel with a fluorescent pattern that spelled out ‘CIT’ inside a tissue sample, which was scanned and imaged. The team also demonstrated their technique by taking images of tumours tagged with fluorescent dyes.

‘This demonstration that we can focus significant optical power deep within tissues opens up significant possibilities in optical imaging,’ said Changhuei Yang, a professor of electrical engineering and bioengineering at Caltech and a senior author on the study.

By tagging cells or molecules that are markers for disease with fluorescent dyes, doctors can use this technique to make diagnoses non-invasively, much as if they were doing an ultrasound procedure.

Doctors might also use this process to treat cancer with photodynamic therapy and, by increasing the power of light, there might be the possibility of incision-less surgery. ‘By generating a tight laser-focus spot deep in tissue, we can potentially use that as a laser scalpel that leaves the skin unharmed,’ said Yang.

With future improvements on the optoelectronic hardware used to record and play back light, the engineers say, they may be able to reach 10cm – the depth limit of ultrasound – within a few years.

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