Photons plus ultrasound: 25 years at Photonics West

In 2003, Lihong Wang, then at Texas A&M University, US, delivered images of brains in living rats that well and truly moved the field of photoacoustic imaging into the fast lane. Revealing intricate networks of blood vessels in the rat cerebral cortex - acquired with the skin and skull intact - the tomograms represented a massive leap forward for functional imaging using photoacoustics, also known as optoacoustics, and stunned researchers worldwide.

"This was a real breakthrough, and Wang had even induced changes in blood flow by stimulating the rats' whiskers," says Paul Beard, founder of the Photoacoustic Imaging Group at University College, London (UCL), UK. "I saw his images and thought, well, this is the standard we have to meet from now on."

Fellow optoacoustic imaging pioneer and CEO of US-based TomoWave Laboratories, Alexander Oraevsky, was equally impressed. "The worldwide community looked at these images and said, ‘they're just so beautiful', and everyone finally realized this was something very real," he recalls. "We could see functional information that just wasn't available from any other technology; it was the key milestone and made everybody notice."

Photoacoustic images of abdominal skin microvasculature in a mouse. Image: Paul Beard/UCL.

In fact, the effect of sound generation by light was first reported in 1880 - only for the phenomenon to disappear again quickly, and for a very long time. Alexander Graham Bell had discovered that shining rapidly modulated sunlight on an optically absorbing medium induced an acoustic wave, and proposed a ‘photophone' based on this effect. But his results were crude, the idea shelved, and photoacoustics slid into oblivion for almost a century, while technology caught up.

By the 1980s, sensitive acoustic receivers and intense laser light sources were available. As a result, optoacoustic sensing and photoacoustic spectroscopy applications were proliferating. At the same time, physicist and radiologist Theodore Bowen from the University of Arizona was using ionizing radiation and microwaves to excite acoustic waves in materials, in an attempt to retrieve depth-resolved spatial information.

As UCL's Beard points out, Bowen wasn't using light, but his results have been important to the field of photoacoustic imaging. "He didn't really produce an image, and his results were some way from the real genesis of the technique that came in the 1990s," he says. "But this was one of the starting points of photoacoustic imaging, and shouldn't be forgotten."

By the early 1990s, the field of photoacoustic imaging was taking shape. Oraevsky, who by now had been working on laser-induced acoustic waves for several years, was on the cusp of unveiling results that would demonstrate the effect's potential.

"It hadn't been clear to us how deep into real tissue you could actually see," he remembers. "So we started to perform experiments using many stacks of chicken breasts with a little piece of liver underneath."

Optoacoustic image of human breast vasculature, captured using TomoWave's "LOISA3D" clinical imaging system. Image: TomoWave Laboratories.

Using his then-new technique based on time-resolved detection of laser-induced stress transients, he could visualize the depth distribution of absorbed optical energy in the tissue layers. In 1994 Oraevsky reported his results in SPIE Proceedings, and his technology was catapulted towards practical laser optoacoustic imaging systems for pre-clinical research and breast imaging. As he highlights: "We had clearly seen the signal, and at this ‘aha' moment realized that yes, it was possible to see deep into the tissue."

The same year Robert Kruger, then at the Indiana University Medical Center and also CEO of Optosonics, published similar experimental results in the same SPIE Proceedings. And only two years later, Beard - as part of his doctoral studies - was using photoacoustic methods to characterize arterial tissue for detecting atherosclerosis. "In the course of this work we ‘accidentally' obtained a photoacoustic waveform that we later realized represented the structure of the tissue specimen in the depth direction," Beard highlights.

Following these early demonstrations the field expanded, with more and more researchers joining the field. By 1999, Kruger had developed the world's first RF-induced thermoacoustic computed tomography scanner for detecting breast cancer, equipment that could differentiate soft tissue with 2-5 mm resolution, up to a depth of 40 mm. At the same time, Wang published the first microwave-induced thermoacoustic tomography based on scanning a focused ultrasonic transducer.

By 2001, Oraevsky had unveiled his prototype of a clinical laser optoacoustic imaging system (LOIS), again, for detecting breast cancer. The system used a single optical fiber to deliver laser pulses and an arc-shaped array of piezoelectric transducers to detect the acoustic signal. Crucially, later clinical trials of the equipment on breast cancer patients showed enhanced contrast between normal tissues and cancerous tumors when compared with X-ray mammography, plus the ability to differentiate cancer from benign masses more accurately, with image resolution on par with regular ultrasound.

Indeed, in 2005, researchers from the University of Twente, The Netherlands, unveiled the ‘Twente Photoacoustic Mammoscope'. At the same time, US-based Seno Medical Instruments bought Oraevsky's intellectual property to commercialize optoacoustic imaging for breast cancer diagnosis.

Tipping point
Like many other researchers in the field, Wang and colleagues had been devising image reconstruction algorithms, and by 2001 had developed an algorithm that delivered, in Wang's words, ‘beautiful images'.

Next, they set out to demonstrate in vivo functional imaging of small animal brains, using laser-induced photoacoustic tomography, and in 2003 made imaging history with the seminal results published in Nature Biotechnology. The team had mapped brain structures in rats, as well as functional changes in blood vessels as the rats responded to whisker stimulation.

"This advance was so exciting," Wang says. "The paper is now the most cited in the field, and we have experienced exponential growth since this publication."

With results in hand, the next challenge was to improve spatial resolution. By 2004 the Wang team had developed the first photoacoustic microscope for in vivo imaging. They used it to image angiogenesis - the formation of new blood vessels that feed tumor growth - as well as melanoma and hemoglobin oxygen saturation of single blood vessels, another hallmark of cancer, in small animals. They also imaged total hemoglobin concentration in humans, reporting the results in 2006. "With these and our 2003 results, we were the first to take 3D photoacoustic imaging to in vivo and the micron-level," points out Wang. "This 2006 paper is the second most cited paper in photoacoustic imaging, so maybe it helped to accelerate growth as well."

Another breakthrough followed in 2009, when Wang unveiled the first photoacoustic endoscope, based on a miniaturized imaging probe. Integrating an optical fiber, ultrasound sensor, and mechanical scanning unit at the end of the endoscope, it enabled high-resolution imaging of soft tissue at depths that even photoacoustic microscopy couldn't reach. Key results included imaging the gastrointestinal tract of a rat.

3D optoacoustic tomography showing an entire mouse body, including blood vessels and organs. Image: Alexander Oraevsky.

Importantly, in the same year Oraevsky and colleagues also unveiled a 3D optoacoustic tomography system capable of imaging an entire mouse body, for applications in pre-clinical research. The system included a motor to rotate the mouse within an array of ultrasound transducers, as well as an optical module to illuminate tissues of the entire mouse body evenly.

Oraevsky's results, and his images of mouse blood vessels, kidneys, liver and spleen - with 0.5 mm spatial resolution - caught the attention of many researchers.

Beard describes those images as ‘just amazing'. Oraevsky adds: "This was so important to pre-clinical research as you could, for example, image cancer progression from onset to metastatic activity for months and months, as well as study what happens when you introduce a therapeutic drug."

The road to clinical applications
In the years that have followed, Oraevsky has developed his 3D photoacoustic tomography system to support clinical breast imaging. The equipment combines ultrasound with optoacoustic imaging to provide both anatomical and morphological information, adding molecular detail on blood.

From the outset, Oraevsky has been confident of the impact that optoacoustic imaging could have on cancer detection and diagnosis. Crucially, optoacoustics can estimate oxygen saturation in hemoglobin, which correlates with hypermetabolism, a key characteristic of cancer. As Oraevsky explains: "Aggressively growing cancer also requires an increased blood supply, and creates its own microvasculature network. "Optoacoustic imaging, based on the difference in optical contrast of blood hemoglobin and oxyhemoglobin, is uniquely suited to detect breast vasculature and tumor microvasculature."

Indeed, a host of companies now offer pre-clinical and/or clinical systems that can directly detect such microvascular hallmarks of cancer to a depth of around 4 cm, in vivo, a capability previously only available through contrast-enhanced MRI.

Key companies involved in the sector include Oraevsky's Tomowave Laboratories as well as Seno Medical, the Canada-based FujiFilm subsidiary VisualSonics, Germany's iThera Medical, US-based Endra Life Sciences, and Canon Medical Systems in Japan - most of whom are presenting papers at this year's Photonics West. US-based CalPACT has also licensed technology from the Caltech Optical Imaging Laboratory where Wang is now a professor. Oraevsky says that while pre-clinical systems have already reached the market, the first clinical systems are currently undergoing regulatory approvals - and should be diagnosing patients soon.

"I believe we will see the first system from Seno Medical reach the market in 2019," he predicts. "Our technology is not in clinical [settings] yet, but we are getting there, and the next breakthrough will come when systems reach clinical markets and hospitals start to use them."

The technology can also be used to detect blood vessel damage associated with diabetes and cardiovascular disease. "We can study microvasculature to see the onset of these diseases, and potentially prevent a stroke or a heart attack," Oraevsky says. "There are so many applications of this technology where doctors need to detect the properties of blood and circulation."


In-vivo 3D photoacoustic tomographic image of a human breast. Image: Lihong V. Wang/Caltech.

Wang agrees. Thanks to faster imaging speeds, he and colleagues have developed a single-breath-hold photoacoustic computed tomography system that reveals detailed angiographic structures in human breasts - without any imaging artifacts caused by breathing motion. Recently they also unveiled a photoacoustic flow cytography method to detect single circulating tumor cells in mice. And as Wang highlights, the technique can also be applied to melanoma detection, as well as prostate, gastrointestinal tract and colon cancer screening, plus neonatal brain imaging.

"Adult brain imaging is probably one of the most challenging areas in photoacoustic tomography, so we'd like to start with neonatal brain imaging - as the cranial bones are still soft and the fontanelle may be open," Wang explains.

Real-time 3D image acquisition
Alongside these extensive developments, Beard has been pioneering the use of ultra-high-sensitivity, wideband Fabry-Perot (FP) ultrasound sensors. Serving as an alternative to traditional, narrowband piezoelectric transducers, these offer higher-resolution images of structures, whatever their size.

Crucially, Beard's UCL team has also slashed the time it takes for these sensors to acquire a 3D image - from a lengthy five minutes to less than one second, thus matching the acquisition times of piezoelectric sensors, but with improved image quality.

"We increased image acquisition speeds by parallelizing detection and using higher repetition rate excitation lasers," he says. "With these faster image acquisition rates, we now hope to present an imaging system that will give real-time 3D images, bringing us into the realm of clinical imaging."

Using a similar technology platform, Beard and colleagues have also developed endoscopic probes, and combined the technology with a mobile arm for bedside patient imaging. "We've just started our first clinical studies," he says. "It is very rewarding to have seen the technology develop from first principles to pre-clinical and now clinical imaging. I never thought we'd reach the image acquisition speeds necessary for this."


In-vivo photoacoustic tomographic images of a mouse. Image: Lihong V. Wang/Caltech.

So what next for the world of optoacoustic/photoacoustic imaging? Determined to continue driving the technology further into the molecular realm, and working with Martin Pule at UCL's Cancer Institute, Beard recently imaged a genetically encoded probe - tyrosinase - for tagging specific cells in photoacoustic imaging. Tyrosinase is an enzyme that generates the pigment eumelanin, which provides good contrast when imaging cells in vivo - as Wang first demonstrated back in 2011.

With colleagues at his California Institute of Technology (Caltech) laboratory, he has now developed a technique called "single-impulse panoramic photoacoustic computed tomography." Remarkably, it can acquire a cross-sectional image through the entire body of a small animal within tens of microseconds, at high resolution. "I have seen a lot of interest [from the pharmaceutical industry] in this technology for drug discovery," Wang reports.
Meanwhile, Oraevsky is pursuing temperature imaging, and has shown his first images in live animals. "If we collect a series of optoacoustic images under conditions of changing tissue temperature and constant optical properties, we can observe that the brightness of the image is proportional to temperature," he points out. "So, we can guide doctors performing thermal therapy of cancer ... by generating 3D images of temperature distribution in and around tumors."

These are just a few of the recent breakthroughs, and a look at this year's "Photons Plus Ultrasound: Imaging and Sensing" conference sessions confirms that the field is expanding as fast as ever. From assessing the aggressiveness of prostate cancer, to super-resolution photoacoustic imaging, laparoscopic surgery, and guiding injections of stem cells into spinal cords, the wide-ranging nature of the applications offers a glimpse of what lies ahead.

As Wang, who co-chairs the popular conference with Oraevsky, says: "I have always believed that to fully understand biology you've got to have information across all the relevant length-scales, from organelles to organisms. It has taken us a long time to get this far, but photoacoustic imaging provides this multi-scale continuum, and I know from here we will continue to grow."

Rebecca Pool is a UK-based freelance writer. A version of this article appeared in the Photonics West Show Daily.