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International Day of Light Pre-celebration Edition: An Ultrasound Without the Goo?

Ultrasound is a powerful tool for looking inside the body. The scans see through layers of tissue to reveal pumping hearts, developing fetuses, troublesome blood clots, and injured muscles. They are relatively low-cost, portable, and have few side effects. Patients aren’t exposed to ionizing radiation or confined in a small space. They are, however, slathered in goo.
Most of the time, having a body part temporarily coated in a cold, sticky substance and then pressed on by a technician is a small price to pay for an accurate diagnosis. But in some situations, like when it’s necessary to image a wound, this contact can be painful. That’s one of the downsides of ultrasound technology. Another is that some results can be influenced by the amount of pressure applied by the technician–it’s a very “hands-on” technique, especially compared to other types of medical imaging.

In new research published in the journal Light: Science & Applications, a team of researchers from the Massachusetts Institute of Technology and MIT Lincoln Laboratory has pioneered the use of laser ultrasound (LUS) technology in humans. Not only does LUS skip the goo, the technology doesn’t require any direct patient contact, eliminating any technician-dependent ambiguities that can pop up in conventional ultrasound results. In addition, LUS could support 3D ultrasound imaging that would especially valuable in places where access to more expensive, less-portable CT, PET, and MRI scanning equipment is limited.

Conventional ultrasound is an imaging tool based on sound waves, not light. Using a handheld probe, a technician sends sound waves into the body at the location of interest. You don’t hear the sound because the frequency is higher than human ears can hear–hence the name ultrasound.

Ultrasound waves travel through tissue pretty well, but when they hit a boundary between different materials (such as tissue and bone, or tissue and amniotic fluid), some of the sound bounces back. A detector in the probe picks up these echoes. Based on their intensity and timing, a computer program can identify different features. Sonograms, the resulting images, display differences in echo intensity as differences in contrast.

Why goo? The gel mimics the acoustic properties of tissue and smooths out the signal’s transition from probe to tissue. Without it, the signal would encounter an air-tissue boundary that would block the sound waves from entering the body. The gel is also a lubricant, making it easier to move the probe around, but its main purpose is to keep air from getting between the probe and skin.

LUS works similarly, except you don’t need gel and the probe is replaced by two lasers. If that sets off alarm bells, rest easy. The laser in question is safe for eyes and skin, with a wavelength of about 1500 nm (invisible). A key to this technology is that when you shine a laser pulse on the skin, it excites a sound wave that travels through the body just like those generated by a conventional ultrasound probe.

The hands-off echo detection is accomplished with a laser doppler vibrometer, a tool commonly used in materials and structural research. A laser scans a surface, maybe a bridge, aircraft part, or, in this case, skin. If that surface is vibrating, the amplitude and frequency of the vibrations are captured in the laser light reflections, which are picked up by a detector. Sound waves are essentially traveling vibrations. Even though you don’t feel your skin vibrating during a scan, it is, and these vibrations can be measured by a vibrometer. Once sound waves and generated and echoes detected, all that’s left is processing the data into a sonogram.

In this new research, the team imaged the forearms of four volunteers with LUS and conventional ultrasound. During LUS, a pulsed laser was placed ~1 m from the patient’s forearm. This laser scanned the area of interest, generating sound waves. A second laser, part of the laser doppler vibrometer, scanned the same area and picked up vibrations from reflected sound waves. After processing, the results were encouraging, tissue features and bone surfaces were clearly visible.

(a) Simplified schematic of the laser ultrasound system (LUS). (b) Photograph of a volunteer’s forearm with the spot imaged illuminated in green. (c) LUS image of volunteer's forearm. (d) Conventional ultrasound image of the same region. Image credit: Xiang Zhang, Jonathan R. Fincke, Charles M. Wynn, Matt R. Johnson, Robert W. Haupt, Brian W. Anthony.

When the researchers compared LUS results to conventional ultrasound results, the same features showed up at the same depth and in the same location. The conventional results were better quality, but considering the conventional technique and its equipment have undergone decades of research and development, the initial LUS results were encouraging. According to the researchers, the LUS images are comparable to the quality of early medical ultrasound images.

“While further work remains prior to commercialization and clinical use, the core enabling technologies of LUS are available,” the team writes in the article. They expect that improvements in the key elements, the sound wave-generating laser and the detector, will lead to even better results. Down the line, that could lead to clinical ultrasound technology that is more robust, technician-independent, and, of course, less sticky.

--Kendra Redmond

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