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Noncontact Laser Ultrasound Safely Images Human Tissue

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MIT engineers have devised an alternative to conventional ultrasound imaging that doesn’t require contact with the body and can be used on patients who may not tolerate a probe on their body, such as infants, burn patients, or patients with sensitive skin. In tests scanning the forearms of volunteers, the researchers were able to observe common tissue features such as muscle, fat, and bone down to about 5 cm below the skin. The images, comparable to conventional ultrasound, were produced using remote lasers focused from half a meter away.

Since sound waves travel farther into the body than light, the researchers began by investigating how to convert a laser beam’s light into sound waves at the surface of the skin, in order to image deeper in the body. The team selected 1550-nm lasers, a wavelength that is readily absorbed by water and is eye- and skin-safe with large safety margins. Since skin is composed largely of water, the team reasoned that it should efficiently absorb this wavelength, and that it would heat up and expand in response. As it oscillated back to its normal state, the skin could be expected to produce sound waves that propagated through the body.

The researchers tested this idea by using one pulsed laser set at 1550 nm to generate sound waves, and a second continuous laser, tuned to the same wavelength, to remotely detect reflected sound waves. The second laser, a motion detector, measured vibrations on the skin surface caused by the sound waves bouncing off muscle, fat, and other tissues. Skin surface motion, generated by the reflected sound waves, caused a measurable change in the laser’s frequency. By mechanically scanning the lasers over the body, the researchers were able to acquire data at different points and generate an image of the region.

Noncontact laser ultrasound, MIT.
A new ultrasound technique uses lasers to produce images beneath the skin, without making contact with the skin like conventional ultrasound probes do. The new laser ultrasound technique was used to produce an image of a human forearm (left) , which was also imaged using conventional ultrasound (right). Courtesy of X. Zhang et al.

The researchers first used the new setup to image metal objects embedded in a gelatin mold resembling skin’s water content. They imaged the same gelatin using a commercial ultrasound probe and found both images to be similar. They then imaged excised animal tissue — in this case, pig skin — and found that laser ultrasound could distinguish subtle features, such as the boundaries between muscle, fat, and bone.

Finally, the team carried out the first laser ultrasound experiments in humans, using a protocol that was approved by the MIT Committee on the Use of Humans as Experimental Subjects. After scanning the forearms of several healthy volunteers, the researchers produced what they believe to be the first fully noncontact laser ultrasound images of a human. The fat, muscle, and tissue boundaries were clearly visible and comparable to images generated using commercial, contact-based ultrasound probes.

The researchers plan to improve their technique, and they are looking for ways to boost the system’s performance to resolve fine features in the tissue. They are also looking to hone the detection laser’s capabilities. Further down the road, they hope to miniaturize the laser setup, so that laser ultrasound might one day be deployed as a portable device.

“I can imagine a scenario where you’re able to do this in the home,” researcher Brian W. Anthony said. “When I get up in the morning, I can get an image of my thyroid or arteries, and can have in-home physiological imaging inside of my body.”

Anthony believes that scientists are just beginning to explore the possibilities of noncontact laser ultrasound. “Imagine we get to a point where we can do everything ultrasound can do now, but at a distance,” he said. “This gives you a whole new way of seeing organs inside the body and determining properties of deep tissue, without making contact with the patient.”

The research was published in Light: Science & Technology (www.doi.org/10.1038/s41377-019-0229-8).  

BioPhotonics
Mar/Apr 2020
Research & TechnologyeducationAmericasMITMassachusetts Institute of TechnologyimaginglasersopticsSensors & Detectorsmotion detectorscontinuous wave lasersBiophotonicsmedicalphotoacousticsnoncontact laser ultrasoundBioScan

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