01 Jan 2020
Technique may help remotely image and assess health of infants, burn victims, and accident survivors in hard-to-reach places.
When trained on a patient’s skin, one laser remotely generates sound waves that bounce through the body. A second laser remotely detects the reflected waves, which researchers then translate into an image similar to conventional ultrasound.
In a paper published by Nature in the journal Light: Science and Applications, the team reported 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 60mm below the skin surface. 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,” commented Brian Anthony, a principal research scientist in MIT’s Department of Mechanical Engineering and Institute for Medical Engineering and Science, 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.”
Early concepts for noncontact laser ultrasound for medical imaging originated from a Lincoln Laboratory program established by Rob Haupt of the Active Optical Systems Group and Chuck Wynn of the Advanced Capabilities and Technologies Group, who are co-authors on the new paper along with Matthew Johnson. From there, the research grew via collaboration with Anthony and his students, Xiang (Shawn) Zhang, who is now an MIT postdoc and is the paper’s first author, and recent doctoral graduate Jonathan Fincke, who is also a co-author.
The project combined the Lincoln Laboratory researchers’ expertise in laser and optical systems with the Anthony group's experience with advanced ultrasound systems and medical image reconstruction.
In recent years, researchers have explored laser-based methods in ultrasound excitation in a field known as 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.
Since sound waves travel further into the body than light, Zhang, Anthony, and their colleagues 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 1550nm lasers, a wavelength which is highly absorbed by water. As skin is essentially composed of water, the team reasoned that it should efficiently absorb this light, and heat up and expand in response.
The researchers tested this idea with a laser setup, using one pulsed laser set at 1550nm 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.
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,” 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.”