A new procedure that is as simple as an ultrasound more than doubles the depth that light can be focused inside biological tissues and soon could enable doctors to perform incision-free surgery or to diagnose cancer by seeing tumors inside the body. While the previous limit for how deep light could be focused into tissue was only about 1 mm, researchers at the California Institute of Technology (Caltech) are now able to reach 2.5 mm. In principle, their method could focus light as much as a few inches into tissue. If the power of light is cranked up, doctors may someday do away with traditional scalpels. "It enables the possibilities of doing incisionless surgery," said Changhuei Yang, a professor of electrical engineering and bioengineering at Caltech and a senior author on the new study. "By generating a tight laser-focus spot deep in tissue, we can potentially use that as a laser scalpel that leaves the skin unharmed." The new technique builds on a previous method that Yang and his colleagues developed to see through a layer of biological tissue, which is opaque because it scatters light. (See: Tissue Made Transparent) In that study, the scientists shined light through a tissue sample and recorded the resulting scattered light on a holographic plate. The recording contained information about how the light beam scattered, zigzagging through the tissue. By playing the recording in reverse, they sent the light back through the other side of the tissue, retracing the beam’s path to the original source. The Caltech researchers’ new technique allows them to focus light deep inside biological tissue. In the experiment, the researchers shined green laser light into the tissue sample seen here in the center. (Image: Caltech/Benjamin Judkewitz and Ying Min Wang) In this way, the researchers could send light through a layer of tissue without the blurring effect of scattering. However, to make images of what is inside tissues, they would have to be able to focus a beam of light into the tissue. “For biologists, it’s most important to know what’s happening inside the tissue,” said Ying Min Wang, a graduate student in electrical engineering. To precisely focus light into tissue, the team expanded upon the recent work of Lihong Wang’s group at Washington University in St. Louis (WUSTL). (See: Photoacoustic imaging penetrates deeply in skin) The WUSTL group developed a technique to focus light using the high-frequency vibrations of ultrasound and two of ultrasound’s favorable properties. First, its high-frequency sound waves are not scattered by tissue; secondly, its ultrasonic vibrations interact with light in such a way that the light’s frequency is shifted ever so slightly. As a result of this acousto-optic effect, light that interacts with ultrasound changes into a slightly different color. Both teams focused ultrasound waves into a small region inside a tissue sample during their experiments. Next, they shined light into the sample, which scattered the light. Any light that passed through the region with the focused ultrasound changed color somewhat. The researchers identified and recorded the color-shifted light. Left: Light enters the tissue sample and is scattered (blue arrows). From above, ultrasound is focused into a small area inside the tissue. The ultrasound shifts the frequency of any light that passes through that area ever so slightly, changing its color. The color-shifted light (green) is then recorded. Right: The recorded light is sent back to retrace its steps to the small region where the ultrasound was focused — which means the light itself is focused on that area. (Image: Caltech/Ying Min Wang and Benjamin Judkewitz) Using Caltech’s playback technique, they sent the light back, inducing only the color-shifted portion to retrace the path to the small region where the ultrasound was focused. This means that the light itself is focused on that area, allowing an image to be created. By moving the ultrasound’s focus, the researchers can control where they want to focus the light. Only a very small amount of light could be focused in the WUSTL experiment, but Caltech’s new method allows scientists to fire a beam of light with as much power as they need for potential applications. The team demonstrated how the new method could be used with fluorescence imaging by embedding inside a tissue sample a patch of gel with a fluorescent pattern that spelled out “CIT.” The investigators scanned the sample with focused light beams, which hit and excited the fluorescent pattern, resulting in the glowing letters emanating from inside the tissue. They also used the technique to take images of tumors tagged with fluorescent dyes. "This demonstration that we can focus significant optical power deep within tissues opens up significant possibilities in optical imaging," Yang said. By tagging cells or molecules that are markers for disease with fluorescent dyes, doctors can use this technique to make noninvasive diagnoses. A 2.5-mm-thick tissue significantly scatters light, making it impossible to read the text below. (Image: Ying Min Wang) Doctors could also use the technique to treat cancer with photodynamic therapy, which currently can be used only at the surface of tissue because of the way light is easily scattered. The new method should make it possible to reach cancer cells deeper inside tissue. With future improvements on the optoelectronic hardware used to record and play back light, the engineers say, they may be able to reach 10 cm — the depth limit of ultrasound — within a few years. "It's a very new way to image into tissue, which could lead to a lot of promising applications," Caltech’s Wang said. The study appeared in the June 26 issue of Nature Communications. For more information, visit: www.caltech.edu