(L-R) Roger Zemp and Parsin Hajireza took an unexpected photoacoustic signal seriously. Their curiosity paid off.
(Edmonton) It's a bit of a coup, getting published in Nature's Light: Science & Applications, a top-ranked journal about optics, and it almost didn't happen.
But by relentlessly investigating an anomaly, University of Alberta researchers have devised new non-contact medical imaging technology that delivers razor sharp pictures to health care practitioners.
Three years ago, Parsin Hajireza was more than a year into his PhD in biomedical engineering under the supervision of Roger Zemp, working on some nanostructured ultrasound transducers, investigating a technique called photoacoustic imaging.
Hajireza was in the lab, his bench crowded with coils of black wire and imaging equipment. He was aiming a couple of laser beams, set at different wavelengths, through the ultrasound transducer at some carbon fibres. To his surprise, he saw a signal on his monitor after he had backed the transducer away from his target.
It must be some kind of environmental noise, he thought. But when he repeated the steps, he came up with the same signal.
As he considered the possible causes of the signal, he kept coming back to one idea: it represented a non-contact detection of photoacoustic signals, similar to an ultrasound image. He took the problem to his supervisor, Roger Zemp.
"That doesn't make sense," Zemp told him, for good reason.
In 1888, Alexander Graham Bell discovered the photoacoustic effect, in which laser light is absorbed and converted to ultrasound waves. Anything that absorbs light, such as blood or even DNA for example, generates an ultrasound wave. Photoacoustic imaging can generate high-resolution contrast but, like the more familiar ultrasound, it requires the imaging device to be in contact with the subject. Hajireza was seeing signals that were generated photoacoustically from centimetres away, through the air.
Zemp and Hajireza devised experiments to explain the mechanism behind the mysterious signals, developed a model to describe it, and built a system around it. They called the technique Photoacoustic Remote Sensing (PARS) microscopy. The system trains two laser beams at a target: one is a visible pulsed beam to generate reflectivity, the other is a near-infrared beam to detect it.
"It's kind of like double-bouncing a friend on a trampoline," says Zemp. "Our pulsed beam creates a change in the reflectivity of a sample, which creates a large bounce. Our near-infrared beam detects it. The effect is much larger than we anticipated."
The magic is in the method, and it allows them to see stunning images. They can even follow a single red blood cell as it travels through a capillary in real time. The new system lets them see structures that absorb light, rather than scatter or emit light, providing them previously unavailable information.
Zemp and Hajireza, with co-authors Wei Shi, Kevan Bell and Robert Paproski, submitted the work to Light: Science & Applications. While they were delighted by their acceptance and publication on January 9, it shouldn't come as a surprise.
The technique, which the pair is commercializing in a company called illumiSonics, will be useful in clinical applications where imaging cannot contact the tissue, such as wounds, burns, tissue during surgery, brain imaging, dental cavity imaging and early cancer detection, when small tumours are just starting to build their network of blood vessels. And new developments are enabling them to see deeper into tissues, letting them measure the oxygen in blood and allowing them to glimpse gene expression.
"This paper is just the first step," Hajireza says. "There is a lot of interest from biologists, surgeons and oncologists." The team is developing new applications based on the technology.
"Sometimes scientists focus on what they expect to see and don't consider other possibilities. I think the reason we were successful, was that we took what I saw seriously and with open minds," Hajireza says. "Then we spent two years to build a system around it."
The research was funded by the Canadian Cancer Society, the Natural Sciences and Engineering Research Council of Canada, the U of A, the Canadian Institutes of Health Research, Alberta Innovates - Health Solutions and Alberta Innovates - Technology Futures.