Seeing Through Skin with Visible Light

A patterned spot of laser light appears on the slide filled with yogurt. Moussa N’Gom and his team measured the brightness of the light getting through for hundreds of patterns, which their algorithm built into a mathematical representation of the yogurt’s scattering pattern. (Source: J. Xu, U. Michigan)

With yogurt and crushed glass, Uni­versity of Michigan resear­chers have taken a step toward using visible light to image inside the body. Their method for focu­sing light through these materials is much faster and simpler than today’s dominant approach. Dense structures like bone show up clearly in x-rays, but softer tissues like organs and tumors are difficult to make out. That’s because x-rays are strongly deflected by bones, while they cut straight through soft tissue. Visible light, on the other hand, is deflected by soft tissue. Until recently, this has made seeing through skin with visible light a non­starter: while light can get through, it’s scattered every which way. At the same time, visible light would be safer for diagnostic imaging than higher-energy x-rays.

“Light comes in, it hits a molecule, hits another, hits another, does something really crazy, and exits this way,” said Moussa N’Gom, assistant research scientist in electrical engi­neering and computer science. By under­standing exactly how a patch of skin scatters the light, researchers hope to care­fully pattern light beams so that they focus inside the body. In their experiments, the researchers spelled “MICHIGAN” with a beam of light shone through yogurt and crushed glass. They chose those materials because they scatter light strongly and serve as good models for skin. Their demon­stration, remi­niscent of writing a name with a flashlight, shows that they can take a single, quick scan of the material and focus through it at many points.

The field of imaging objects through materials, from layers of paint to eggshells and even mouse skulls, has made great strides in the last decade. The typical holographic method untangles the scattering pattern by looking at how the light waves interfere with each other. This gives infor­mation about how different rays were delayed on their way through the material. This method is very precise, said N’Gom, but it is slow. To speed things up, researchers typically figure out just enough of the scat­tering pattern to focus on a parti­cular point. To focus on a different point, the material has to be scanned again. This would slow the process of measuring the size or texture of a tumor, for example. “Our method is significantly faster and more convenient because we use a single set of measure­ments to generate all these points, and we don’t have to rescan,” N’Gom said.

As is typical for focusing-through-materials experi­ments, the researchers used a spatial light modulator to produce patterns of light. If you shone a laser through frosted glass, it would enter at a point on one side, at a parti­cular angle, and then leave the other side through many points, in different directions. By combining a screen with an array of mirrors, a spatial light modu­lator can do the reverse, sending light to a surface at many points, at many angles, so that these rays converge on a point on the other side of the material. They set up the spatial light modulator to shine in hundreds of different patterns, 461 in all. But rather than ana­lyzing the paths of individual light rays emerging from the other side, N’Gom’s team measured the bright­ness.

They developed an algo­rithm to trawl through the incoming light patterns and outgoing bright­ness measure­ments, using the infor­mation to build up a mathe­matical repre­sentation of the material’s scattering pattern, called the trans­mission matrix. “Previous techniques, instead, used complex holo­graphic setups to extract the necessary infor­mation,” said Raj Rao Nada­kuditi, associate professor of electrical engineering and computer science. “We were able to achieve the same through simple bright­ness measure­ments and as a result operate much faster.”

Using the trans­mission matrix, N’Gom’s team could figure out exactly how to set the spatial light modu­lator to get a bright spot at any point on the other side of the ground glass or yogurt. In the yogurt, there was a time limit on how long the map was good, just a few minutes. It was enough time for N’Gom and his colleagues to spell “MICHIGAN” in 157 shots. In skin, the time constraints are much tighter. They would need a new map about every milli­second. Even so, with state-of-the-art electronics, N’Gom thinks their algo­rithm could run that fast.

Another challenge in seeing through skin is that they wouldn’t be able to position a detector beneath it to measure the bright­ness of the light. For this, N’Gom said that researchers are using ultra­sound to detect heating in the target tissue – a measure of how much light is getting through. Finally, with the light focused inside, an imaging device would still need to focus the light coming back out of the skin. For this, they could essen­tially run the pattern of light back through the trans­mission matrix to deduce where the reflec­tion was coming from. Considering recent progress and ongoing studies in focusing light through trans­lucent materials, N’Gom anti­cipates that we may see the first visible light images taken through skin within the next five years. (Source: U. Michigan)

Reference: M. N’Gom et al.: Controlling Light Transmission Through Highly Scattering Media Using Semi-Definite Programming as a Phase Retrieval Computation Method, Sci. Rep. 7, 2518 (2017); DOI: 10.1038/s41598-017-02716-x

Link: Dept. of Electrical & Computer Engineering, University of Michigan, Ann Arbor, USA

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