New ultrasound scan uses laser instead of acoustics to image human body

Researchers have developed an eye- and skin-safe laser system as an alternative to conventional ultrasound, an advance that lead to a new less-invasive imaging technique for seeing the inside of patients.

The scientists, including those from Massachusetts Institute of Technology (MIT) in the US, said conventional ultrasound instruments required contact with a patient's body, limiting its use in patients who don't tolerate the probe well, such as babies, burn victims, or those with sensitive skin.

In their current study, published in the journal Light: Science and Applications, the researchers described the first laser ultrasound images in humans.

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, the study noted, were produced using remote lasers focused on a volunteer from half a metre away, and were similar to conventional ultrasound.

"We're at the beginning of what we could do with laser ultrasound," said Brian W. Anthony, a senior author on the paper from MIT.

"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," Anthony explained.

As part of the study, the researchers sent light in the form of a pulsed laser at a particular wavelength, which penetrates the skin and is absorbed by blood vessels.

On getting heated by the laser, the blood vessels rapidly expanded and relaxed, cooling back to their original size -- only to be struck again by another light pulse.

The resulting mechanical vibrations generated sound waves which travelled back up, where they could be detected by signal converters placed on the skin, and translated into a an image.

However, the researchers said, the technique still requires a detector in direct contact with the body in order to pick up the sound waves.

They said until now the light can only travel a short distance into the skin before fading away, meaning the technique could be used to image blood vessels just beneath the skin, but not much deeper.

To overcome this hurdle, the team selected lasers which emitted light at a wavelength absorbed highly by water -- and is eye- and skin-safe with a large safety margin.

Since the human skin is essentially composed of water, the team reasoned that it would efficiently absorb this light, and heat up and expand in response.

As the skin oscillates back to its normal state, it may produce sound waves that propagate through the body, the researchers said.

They then used 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, the scientists noted, is a sensitive motion detector that measures vibrations on the skin surface caused by the sound waves bouncing off muscle, fat, and other tissues.

They said the laser's frequency changes the skin surface motion generated by the reflected sound waves in a measurable way.

Using these scans over the body, the 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. 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," Anthony said.