Ultrasound examinations are relatively easy: The procedure is noninvasive, and it doesn’t expose patients to radiation. However, it does require the examiner to place a probe in contact with a patient’s body, and there are situations—such as with infants or burn victims—where that may not be ideal. Furthermore, ultrasound probe contact induces significant image variability, which is a major challenge in modern ultrasound imaging.
Now, engineers at the Massachusetts Institute of Technology (MIT) have developed an alternative to conventional ultrasound with an eye- and skin-safe laser system that doesn’t require contact with the patient’s body. One laser, trained on a person’s skin, generates sound waves within the body. A second laser remotely detects the sound waves, which researchers then translate into an image similar to conventional ultrasound.
In a paper1 published in December 2019, the research team reports generating the first laser ultrasound images in humans. The researchers scanned the forearms of several volunteers and observed common tissue features such as muscle, fat, and bone, down to about 6 centimeters below the skin. These images, comparable to conventional ultrasound, were produced using remote lasers focused on a volunteer from half a meter away.
“We’re at the beginning of what we could do with laser ultrasound,” says Brian W. Anthony, a principal research scientist in MIT’s Department of Mechanical Engineering and Institute for Medical Engineering and Science (IMES), a senior author on the paper. “Imagine we get to a point where we can do everything ultrasound can do now, but at a distance. 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 new technique amplifies on advances in photoacoustics. Instead of directly sending sound waves into the body, the idea is to send in light, in the form of a pulsed laser tuned at a particular wavelength, that penetrates the skin and is absorbed by blood vessels.
The blood vessels rapidly expand and relax— instantly heated by a laser pulse then rapidly cooled by the body back to their original size—only to be struck again by another light pulse. The resulting mechanical vibrations generate sound waves that travel back up, where they can be detected by transducers placed on the skin and translated into a photoacoustic image.
While photoacoustics uses lasers to remotely probe internal structures, the technique still requires a detector in direct contact with the body in order to pick up the sound waves. What’s more, light can travel only a short distance into the skin before fading away. As a result, other researchers have used photoacoustics to image blood vessels just beneath the skin, but not much deeper.
Since sound waves travel further into the body than light, the MIT researchers looked for a way to convert a laser beam’s light into sound waves at the surface of the skin, in order to image deeper in the body.
Based on their research, the team selected 1,550-nanometer lasers, a wavelength which is highly absorbed by water (and is eye- and skin-safe with a large safety margin). As skin is essentially composed of water, the team reasoned that it should efficiently absorb this light and heat up and expand in response. As it oscillates back to its normal state, the skin itself should produce sound waves that propagate through the body.
The researchers tested this idea with a laser setup, using one pulsed laser set at 1,550 nanometers to generate sound waves, and a second continuous laser, tuned to the same wavelength, to remotely detect reflected sound waves. This second laser is a sensitive motion detector that measures 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, causes a change in the laser’s frequency, which can be measured. By mechanically scanning the lasers over the body, scientists can acquire data at different locations and generate an image of the region.
“It’s like we’re constantly yelling into the Grand Canyon while walking along the wall and listening at different locations,” Anthony says. “That then gives you enough data to figure out the geometry of all the things inside that the waves bounced against—and the yelling is done with a flashlight.”
The MIT 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. 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,” Anthony says. “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. You could imagine deploying this in the ambient environment to get an understanding of your internal state.”
- Zhang X, Fincke JR, Wynn CM, et al. Full noncontact laser ultrasound: first human data. Light Sci Appl. 8, 119 (2019) doi:10.1038/s41377-019-0229-8
Reconstructed laser ultrasound image of a volunteer’s forearm with multiple visible tissue layers. Image courtesy Zhang, et al., Light Sci Appl., Licensed by Creative Commons.