Researchers at Massachusetts Institute of Technology (MIT) have produced the first laser ultrasound images of humans, according to an MIT News report. The advancement is important because the technique they used may help remotely image and assess the health of infants, burn victims, and accident survivors in hard-to-reach places.

Conventional ultrasound requires contact with a patient’s body, and while it is generally coonsidered noninvasive, it may not be ideal in situations where the probe would have to touch burn victims, babies, or other patients with sensitive skin. The report also noted that ultrasound probe contact induces significant image variability, which is a major challenge in modern ultrasound imaging.

MIT engineers have come up with an alternative method that doesn’t require contact with the body to see inside a patient. The new laser ultrasound technique leverages an eye- and skin-safe laser system to remotely image the inside of a human body. 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 MIT team said they 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,” said Brian W. Anthony, a principal research scientist in MIT’s Department of Mechanical Engineering and Institute for Medical Engineering and Science (IMES), and 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.

The concept of using laser-based methods in ultrasound excitation is not new. Researchers have explored the idea in a field known as photoacoustics. Only, instead of sending sound waves into the body, other researchers have tried sending light, in the form of a pulsed laser, at a particular wavelength that penetrates the skin and is absorbed by blood vessels. The problem is that this technique still requires a detector in direct contact with the body and light can only travel a short distance into the skin before fading away.

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 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 said. “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.”

Click here to read more about the research, which was supported in part by the MIT Lincoln Laboratory Biomedical Line Program for the United States Air Force and by the U.S. Army Medical Research and Material Command's Military Operational Medicine Research Program.