Researchers routinely image biological tissue using optical techniques that make use of the intrinsic light emission of nanoparticles in the near-infrared range of the electromagnetic spectrum (which lies between 700 and 900 nm). Short-wave infrared (SWIR) frequencies of light, which have wavelengths of between 1000 and 2000 nm, are better in many ways for imaging biological tissue, however. This is because photons are much less scattered than those in the 700-900 nm window when traversing tissue and are not significantly absorbed by water either. All in all, this allows SWIR light to penetrate deeper into tissue and makes it almost as transparent as glass at these frequencies.

“We knew that the SWIR imaging mode would be better than the NIR one, but we were lacking high-quality emitters – light-emitting materials that could produce these precise wavelengths,” explains team member and lead author of the new study Oliver Bruns, who is in the Department of Chemistry at the Massachusetts Institute of Technology (MIT). Developing and applying those emitters for novel imaging approaches is the focus of our new paper.”

Indium-arsenide-based quantum dots

The researchers, led by Moungi Bawendi of MIT, developed versions of indium-arsenide-based quantum dots that emit at wavelengths of between 1000 and 2000 nm. Once injected into the body and excited by a 808-nm laser, the dots emit light that is bright enough to be then easily detected through the surrounding skin and muscle tissue. And that is not all: thanks to their intense brightness, the light emitted can be captured on very short time scales. “This makes it possible to produce not just single images but a video that captures details of motion, such as the flow of blood, making it possible to distinguish between veins and arteries, for example,” Bruns tells

The researchers first described these new particles in a Nature Communications doi:10.1038/ncomms12749 paper least year. They report their new work in Nature Biomedical Engineering doi:10.1038/s41551-017-0056.

Tracking blood flow

“Among other applications, our new technique can not only determine the direction in which blood flows, it is detailed enough to track individual particles within that flow,” adds Bruns. “We can track the flow in each and every capillary at high speed and obtain a quantitative measure of this flow. What is more, we can perform such measurements at very high resolution over large areas.

“Such imaging could potentially be used to study how the blood flow pattern in a tumour changes as the tumour develops, which might lead to new ways of monitoring disease progression or responsiveness to a particular drug treatment,” he continues. “This would give a good indication of how treatments are working that was not possible before.”

Developing safer versions

The researchers say they have successfully tested out their technique on mice. “Our SWIR-emitting particles are the first that are bright enough to allow imaging of internal organs in these animals when they are awake and moving – as opposed to previous techniques that required them to be anaesthetized,” says Bruns. He does stress, however, that initial applications would be for preclinical research in animals only since the compounds used in the quantum dots contain some materials that are unlikely to be approved for humans. “We are currently working on developing versions that would be safer in this context.”

The team also includes members from MIT's departments of Chemical Engineering, Biological Engineering and Mechanical Engineering, as well as from Harvard Medical School, the Harvard TH Chan School of Public Health, Raytheon Vision Systems, and University Medical Center in Hamburg, Germany.