U-M engineers advance a new way to “listen” for light inside the body
A scalable optical ultrasound sensor array could help make photoacoustic imaging sharper, smaller and more clinically practical
6–9 minutes
A team of University of Michigan researchers has demonstrated a new optical sensing approach that could improve how scientists and clinicians image biological tissue deep inside the body.
Led by Xueding Wang, Jonathan Rubin Collegiate Professor, Biomedical Engineering, and Professor, Department of Radiology and Director, Optical Imaging Laboratory, along with Guan Xu, Associate Professor, Ophthalmology and Visual Sciences, and L. Jay Guo, Emmett Leith Collegiate Professor of Electrical Engineering and Computer Science, Professor, EECS – Electrical and Computer Engineering, and Professor (courtesy), Applied Physics; Macromolecular Science & Engineering; and Mechanical Engineering, the team developed a high-performance array of tiny optical sensors that can detect ultrasound waves generated by light inside tissue. The advance represents an important step toward making photoacoustic tomography, or PAT, more powerful, scalable and clinically practical.
“This is excellent, very exciting work,” Dr. Wang said. “It is a joint effort involving three labs, and it brings together expertise in biomedical imaging, ophthalmology, photonics and engineering.”
Photoacoustic tomography is an imaging technique that combines the strengths of optical imaging and ultrasound. In PAT, short laser pulses are delivered into tissue. When molecules such as hemoglobin absorb that light, they heat up very slightly and rapidly expand, producing ultrasound waves. Those sound waves can then be detected and converted into images.
In simpler terms, PAT uses light to create sound — and then uses that sound to see inside the body.
That combination is powerful because light can reveal biological information such as blood content, oxygenation and tissue composition, while ultrasound can travel deeper through tissue than light alone. This makes PAT especially promising for biomedical applications such as cancer detection, where changes in blood vessel growth, oxygen levels and tissue structure can be early indicators of disease.
But one major challenge has limited the field: detecting ultrasound signals with enough sensitivity, speed and scalability.
Most ultrasound and photoacoustic systems rely on piezoelectric transducers, which convert sound waves into electrical signals. While widely used, these conventional detectors can face limitations in bandwidth, miniaturization and how densely they can be packed into arrays. Those factors are important because sharper biomedical images often require many small, highly sensitive detectors working together.
The U-M team addressed this challenge using microring resonators — microscopic ring-shaped optical devices that can detect tiny changes caused by incoming ultrasound waves. When ultrasound reaches the sensor, it slightly changes the optical properties of the microring. By monitoring light traveling through the ring, researchers can detect the ultrasound signal with high sensitivity.
“It is a purely optical device,” Dr. Wang said. “That means we can avoid using electrical cables and instead use optical fibers and optical components to achieve detection. It is also very sensitive and broadband, which is important for ultrasound and photoacoustic imaging.”
The microring resonator-based acoustic detector was originally invented by Dr. Guo’s group at U-M, Dr. Wang noted. Since then, research groups around the world have explored the technology. In the new study, the U-M team moved the concept forward by building not just a single detector, but an array containing more than 40 microring sensing elements.
“What is unique here is that we used an array of microrings, not just one,” Dr. Wang said. “There are many technical challenges in implementing 40 microrings together. But when you have many detectors in an array, you can acquire images with faster speed and higher resolution.”
The array was fabricated using nanoimprint lithography, a manufacturing method that can create extremely small features over relatively large areas in a cost-effective and repeatable way. That scalability is important because, for PAT to move closer to clinical use, researchers need sensor arrays that are not only sensitive but also practical to manufacture.
Using nanoimprint lithography, the team precisely controlled the size of each microring at the nanometer scale. That allowed the sensors to have distinct optical resonances within a narrow spectral range, enabling multiple sensors to be read out efficiently while maintaining the optical quality needed for sensitive ultrasound detection.
When integrated into a PAT system, the microring array detected ultrasound signals across a broad bandwidth exceeding 170 megahertz. That wide bandwidth is significant because higher-frequency ultrasound signals carry information about smaller structures. In imaging, that can translate into finer detail.
“Resolution is very important because it allows you to see details,” Dr. Wang said. “With this approach, we can potentially achieve very high-resolution imaging.”
To demonstrate the platform’s biomedical potential, the researchers used it to image ex vivo mouse prostate tissue. The images showed strong agreement with known tissue structures, including regions associated with blood vessels. The team also analyzed the frequency content of the photoacoustic signals and found that the system could help distinguish between normal and cancerous prostate tissues.
That result points to one of PAT’s most promising advantages: it may provide not only structural images, but also functional and pathological information about tissue.
For Dr. Wang, one of the most exciting possibilities is that microring-based arrays could eventually be made small enough to fit inside needles, endoscopes or other minimally invasive clinical tools.
“Traditional ultrasound probes are relatively large, so they are usually placed on the surface of the body,” he said. “A microring-based array can be very small and flexible. It could be integrated into a needle, an endoscope or an intravascular imaging device to image inside the body.”
One potential application is image-guided biopsy. In current clinical practice, biopsy needles are often guided by external imaging, but clinicians may still have limited information about the exact tissue being sampled.
“Right now, when a needle is inserted into the prostate or breast to take tissue samples, it can be like working in the dark,” Dr. Wang said. “You do not always know what kind of tissue you are taking. If we can put this imaging system into the needle, we can image the tissue before harvesting it.”
That capability could help physicians better target suspicious tissue while avoiding unnecessary sampling of healthy tissue.
“It could help physicians say, ‘This is the tissue we should sample because we are in the cancer, not normal tissue,’” Dr. Wang noted.
The same concept could also apply to endoscopic procedures, including imaging and biopsy guidance in the gastrointestinal tract. For diseases such as Crohn’s disease or other inflammatory bowel conditions, a miniature imaging system integrated into an endoscope could help clinicians identify inflamed or abnormal regions more precisely.
Another promising application is ocular imaging, where the sensor array could be integrated into a contact lens for continuous, real-time monitoring of ocular metabolism. Such a platform may facilitate the assessment of eye health during extreme physiological conditions, including long-duration spaceflight.
Although the current study was performed in a research setting, the broader impact lies in the fabrication and sensing strategy. By combining polymer photonics, nanoimprint lithography and photoacoustic imaging, the team demonstrated a pathway toward compact, high-resolution optical ultrasound arrays that could be produced more efficiently than devices made with more traditional fabrication approaches.
The technology may also have uses beyond medical imaging. Microring resonators are important building blocks in integrated photonic systems, including optical communications and advanced sensing platforms. A scalable method for manufacturing dense, high-quality microring arrays could therefore benefit multiple fields.
For biomedical imaging, however, the promise is especially clear: better ultrasound detection could lead to photoacoustic systems that are sharper, more sensitive and more practical to build.
The team is already looking ahead to larger arrays. While the 40-element array demonstrated feasibility, Dr. Wang said future systems could include 128, 256 or more elements, expanding the field of view and improving imaging performance.
“Forty microring elements is really the first step,” he said. “We will not limit ourselves to 40 rings. This is a good first step toward large-element arrays.”
Significant engineering challenges remain before the technology can be translated into routine clinical use, Wang noted. But the study demonstrates that scalable, high-performance optical ultrasound arrays are possible.
“There are still many challenges in front of us to make this practical in the clinic,” Wang said. “But this is a very important step forward because we demonstrate the feasibility here.”
By advancing optical ultrasound arrays, the U-M team has opened a pathway toward next-generation imaging tools that can reveal biological detail deep inside tissue — helping researchers better understand disease and, ultimately, supporting improved diagnosis and treatment.