Modern ultrasonography is well established in diagnostic and interventional medical imaging and is the most commonly used imaging technique for soft tissue.
Ultrasound has advantages compared to other imaging methods, including being nonionizing, relatively low cost, and portable. Current embodiments of ultrasound technology range from cart-based bedside systems to portable hand-held devices.
Conventional ultrasound imaging requires the placement of piezoelectric transducers in contact with the patient to transmit and then detect reflected and scattered acoustic waves at the body surface.
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 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 only travel 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, MIT researcher Brian Anthony and 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.
“We’re at the beginning of what we could do with laser ultrasound,” Dr. Anthony said.
“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 team selected 1,550-nm 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 scientists 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.
They tested this idea with a laser setup, using one pulsed laser set at 1,550 nm 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.
Dr. Anthony and co-authors used the new setup to image metal objects embedded in a gelatin mold roughly resembling skin’s water content.
They imaged the same gelatin using a commercial ultrasound probe and found both images were encouragingly similar.
They moved on to image excised animal tissue — in this case, pig skin — where they found laser ultrasound could distinguish subtler features, such as the boundary between muscle, fat, and bone.
Finally, they carried out the first laser ultrasound experiments in humans.
After scanning the forearms of several healthy volunteers, they produced the first fully noncontact laser ultrasound images of a human.
The fat, muscle, and tissue boundaries are 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.
“I can imagine a scenario where you’re able to do this in the home,” Dr. 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. You could imagine deploying this in the ambient environment to get an understanding of your internal state.”
The team’s work was published in the journal Light: Science and Applications.