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Super-resolution photoacoustic imaging

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Will in vivo optical imaging of deep tissues be possible soon with high resolution ? To make a deep optical image through turbid media such as biological tissues, photoacoustic imaging is a promising and rapidly developing technique. However, if the resolution allowed by this technique at depth in tissue is much better than that of optical microscopy methods, it nevertheless remains limited by the acoustic diffraction. Two experimental approaches inspired by optical super-resolution methods have just been implemented to push back the resolution limits of photoacoustic imaging.

In photoacoustic imaging, optical images can be obtained at depth by detecting the acoustic waves emitted following the absorption of light (via the so-called photoacoustic effect). Although photoacoustic imaging is one of the most advanced techniques for high-resolution deep optical imaging, the corresponding resolution remains fundamentally limited by the acoustic diffraction, resulting in a resolution well below the optical diffraction.

By adapting methods initially developed to bypass the optical diffraction limit, we have recently obtained super-resolved photoacoustic images, that is, with a resolution beyond the acoustic diffraction limit. In a first demonstration, we exploited the so-called localization approach, which consists of reconstructing a structure containing particles from the measurement of their positions. The measurement of a position is indeed not limited by diffraction, provided that it is possible to image a single particle at a time in each diffraction spot. We have implemented this approach on a dilute solution of absorbing flowing particles [1].

If the particles are too concentrated to be localized individually, one can nevertheless exploit the signal fluctuations caused by the very flow of the particles. We have adapted the concepts of super-resolved optical imaging of fluctuations (SOFI) to photoacoustic imaging. Experiments using a conventional photoacoustic imaging system have resulted in super-resolved photoacoustic images of the structure in which red blood cell flow occurs [2].

The proposed techniques could potentially enable in vivo imaging of micro-vascularization at depth with unprecedented resolution, without the need to resort to contrast agents, since the contrast can be here naturally provided by the absorption of light by the blood.

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[1] Vilov, S., Arnal, B., and Bossy, E. "Overcoming the acoustic diffraction limit in photoacoustic imaging by the localization of flowing absorbers". Optics Letters, 42(21), 2017.
[2] T. Chaigne, B. Arnal, S. Vilov, E. Bossy and O. Katz, "Super-resolution photoacoustic imaging via flow-induced absorption fluctuations," Optica 4, 1397-1404, 2017.