Photoacoustic imaging is a way to look beneath the surface of a biological sample at high resolution. A laser is used to excite tissue, expanding it slightly and generating vibrations within it, which results in sound waves. It images better than ultrasound and at depths greater than optical techniques, revealing some pretty small objects that would be difficult to observe otherwise. Yet, just like light has a diffraction, the wavelengths of sound generated using photoacoustic imaging can be a fundamental barrier for ultrasound detectors. This acoustic diffraction limit prevents photoacoustics from seeing capillaries and other highly detailed parts of our anatomy, but researchers at Hebrew University of Jerusalem, Israel and Université Grenoble Alpes in Grenoble, France have overcome this limitation when imaging blood vessels and the flow within them.
The researchers focused their attention on how the photoacoustic signal is affected by red blood cells flowing within a vessel. As the red cells pass by, they lead to what seems like noise in the signal, which is caused by the cells absorbing only certain frequencies of light. By using a computer to run statistical algorithms that analyze these signal fluctuations, the researchers were able to tease out details that would otherwise remain invisible.
The technology doesn’t require any new hardware, making it easily applicable to existing photoacoustic setups and opening an opportunity for greater clinical use of the modality.
Some more details according to The Optical Society:
In a study published in Optica last year, Katz and his colleagues demonstrated the ability to surpass the acoustic diffraction limit using a SOFI-inspired photoacoustic imaging technique. That work had two main limitations. First, it required the use of a long-coherence laser, not a standard part of photoacoustic imaging systems, in order to form dynamic structured interference patterns called speckle to create the signal fluctuations. Second, due to their small dimensions, the use of speckles as dynamic illumination resulted in the fluctuations having a low amplitude with respect to the mean photoacoustic signal, which in turn made it difficult to resolve the specimen in question.
In the new Optica study, the researchers showed that they could overcome these limitations by applying the statistical analysis framework to the inherent signal fluctuations caused by the flow of red blood cells—so the researchers didn’t need to rely on coherent structured illumination—and furthermore demonstrated experimentally that they could perform super-resolution photoacoustic imaging using a conventional imaging system.
Study in journal Optica: Super-resolution photoacoustic imaging via flow-induced absorption fluctuations…
Via: The Optical Society…
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