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Optical detection of ultrasound for photoacoustic imaging provides large bandwidth and high sensitivity at high acoustic frequencies. Higher spatial resolutions can therefore be achieved using Fabry-P\'erot cavity sensors, as compared to conventional piezoelectric detection. However, fabrication constraints during deposition of the sensing polymer layer require a precise control of the interrogation beam wavelength to provide optimal sensitivity. This is commonly achieved by employing slowly tunable narrowband lasers as interrogation sources, hence limiting the acquisition speed. We propose instead to use a broadband source and a fast tunable acousto-optic filter to adjust the interrogation wavelength at each pixel within a few microseconds. We demonstrate the validity of this approach by performing photoacoustic imaging with a highly inhomogeneous Fabry-P\'erot sensor., Comment: 9 pages, 4 figures

International audience; Is there a bright spot in the shadow of an opaque disk? Nearly 200 years ago, Augustin Fresnel and François Arago’s remarkable answer to this question validated the wave theory of light and inaugurated the modern theories of diffraction. Today, their renowned experiment can be easily reproduced using lasers and cameras. Far beyond its historical interest, the experiment is a versatile platform to illustrate the main concepts of optical physics, including diffraction, interference, speckle, and Fourier optics.

Depuis le début des années 2000, l’imagerie photoacoustique connait un formidable essor en tant que technique d’imagerie optique biomédicale. Cet essor, qui se traduit à la fois par les nombreuses recherches menées par un nombre d’équipes de plus en plus important dans le monde et par les multiples applications qui en découlent, est lié aux performances uniques de cette modalité d’imagerie optique en termes de résolution et de contraste.

The resolution of photoacoustic imaging of blood vasculature is limited at depth by the acoustic diffraction limit. In this work, we propose to exploit the fluctuations caused by flowing absorbers (such as red blood cells in blood vessels) to perform photoacoustic imaging beyond the acoustic diffraction limit: following the super-resolution optical fluctuation imaging (SOFI) method, we analyze the n-th order statistics from the temporal photoacoustic fluctuations induced by flowing particles. We performed a proof-of-concept experiment in a 5-channel microfluidic silicon-based circuit flown with a suspension of RBC-mimicking 10 µm red-tainted polymer spheres (Microparticles, GmbH, Berlin, Germany). The sample was illuminated with a 5 ns pulsed ND-YAG laser (532 nm, Innolas, Krailling, Germany) with a fluence of 3 mJ/cm^2 and imaged at a 20 Hz rate using a L22-8v probe (128 elements, Verasonics, Redmond, WA, USA) coupled to a Verasonics Vantage 256 ultrasound scanner. Whereas the resolution of conventional photoacoustic imaging was too low to resolve individual channels, the nth order statistical analysis of the photoacoustic fluctuations provided images with a resolution enhancement scaling as n^{1/2}, in agreement with the SOFI theory and with numerical simulations. As opposed to our previous work which exploited speckle-based photoacoustic fluctuations to increase the resolution, the approach proposed here based on sample fluctuations do not require coherent light and can be readily applied to conventional photoacoustic imaging setup. Furthermore, in order to discard the oscillatory behavior of the photoacoustic point-spread-function, we extended in this work the SOFI theory to complex-valued photoacoustic images.

We propose a SOFI-based high-order statistical analysis of photoacoustic signal fluctuations resulting from motion of optical absorbers for super-resolution photoacoustic imaging. This technique can be potentially used for in vivo imaging without contrast agents.

Manipulating the propagation of light through scattering media remains a major challenge for many applications, including astronomy, biomedical imaging and colloidal optics. Wavefront shaping is one of the most promising ways to mitigate scattering and focus through inhomogeneous samples. However, wavefront correction remains accurate over only a limited spatial extent within the scattering medium - a correlation range referred to as the optical memory effect. Here, by selecting only the weakly scattered light for wavefront shaping, we show that the addition of temporal degrees of freedom enhances this correlation range. We investigate spatial scattering correlations by digitally time-gating the early arriving light in the spectral domain. We demonstrate that the range of the translational memory effect for the early arriving light is increased almost fourfold, paving the way for a range of scattering media imaging applications.

Understanding how distributed neuronal circuits integrate sensory information and generate behavior is a central goal of neuroscience. However, it has been difficult to study neuronal networks at single-cell resolution across the entire adult brain in vertebrates because of their size and opacity. We address this challenge here by introducing the fish Danionella translucida to neuroscience as a potential model organism. This teleost remains small and transparent even in adulthood, when neural circuits and behavior have matured. Despite having the smallest known adult vertebrate brain, D. translucida displays a rich set of complex behaviors, including courtship, shoaling, schooling, and acoustic communication. In order to carry out optical measurements and perturbations of neural activity with genetically encoded tools, we established CRISPR-Cas9 genome editing and Tol2 transgenesis techniques. These features make D. translucida a promising model organism for the study of adult vertebrate brain function at single-cell resolution.

In deep tissue photoacoustic imaging the spatial resolution is inherently limited by the acoustic wavelength. Recently, it was demonstrated that it is possible to surpass the acoustic diffraction limit by analyzing fluctuations in a set of photoacoustic images obtained under unknown speckle illumination patterns. Here, we purpose an approach to boost reconstruction fidelity and resolution, while reducing the number of acquired images by utilizing a compressed sensing computational reconstruction framework. The approach takes into account prior knowledge of the system response and sparsity of the target structure. We provide proof of principle experiments of the approach and demonstrate that improved performance is obtained when both speckle fluctuations and object priors are used. We numerically study the expected performance as a function of the measurement's signal to noise ratio and sample spatial-sparsity. The presented reconstruction framework can be applied to analyze existing photoacoustic experimental data sets containing dynamic fluctuations.

In deep-tissue photoacoustic imaging, optical-contrast images of deep-lying structures are formed by recording acoustic waves that are generated by optical absorption. Although photoacoustics is perhaps the leading technique for high-resolution deep-tissue optical imaging, its spatial resolution is fundamentally limited by the acoustic wavelength, which is orders of magnitude longer than the optical diffraction limit. Here, we present an approach for surpassing the acoustic diffraction limit in photoacoustics by exploiting inherent temporal fluctuations in the photoacoustic signals due to sample dynamics, such as those induced by the flow of absorbing red blood cells. This was achieved using a conventional photoacoustic imaging system by adapting concepts from super-resolution fluorescence fluctuation microscopy to the statistical analysis of acoustic signals from flowing acoustic emitters. Specifically, we experimentally demonstrate that flow of absorbing particles and whole human blood yields super-resolved photoacoustic images, and provides static background reduction. By generalizing the statistical analysis to complex-valued signals, we demonstrate super-resolved photoacoustic images that are free from common photoacoustic imaging artifacts caused by band-limited acoustic detection. The presented technique holds potential for contrast-agent-free microvessel imaging, as red blood cells provide a strong endogenous source of naturally fluctuating absorption.

In deep photoacoustic imaging, resolution is inherently limited by acoustic diffraction, and ultrasonic frequencies cannot be arbitrarily increased because of attenuation in tissue. Here we report on the use of multiple speckle illumination to perform super resolution photoacoustic imaging. We show that the analysis of speckle-induced second-order fluctuations of the photoacoustic signal combined with deconvolution enables to resolve optically absorbing structures below the acoustic diffraction limit.

Wavefront control allows controlling interferences of coherent scattered light at depth in scattering tissues. Combined with photoacoustics imaging, I will show it allows photoacoustic-guided focusing in depth, and improved photoacoustic imaging.

This presentation will illustrate how optical speckle patterns, a manifestation of coherent light discarded by the transport theory for energy, may be exploited to enhance photoacoustic imaging, in particular in terms of visibility and super-resolution. Article not available.

The version of this paper originally published contained errors in reference citations: in the first paragraph of the Results section, the text "This extent of optical clarity probably results from the absence of skull above the brain22. In our specimens, Nissl-stained coronal sections through the head showed that the skull surrounds the brain only laterally and ventrally” should have read “This extent of optical clarity probably results from the absence of skull above the brain21. In our specimens, Nissl-stained coronal sections through the head22 showed that the skull surrounds the brain only laterally and ventrally.” In addition, the unit abbreviation “μm” was incorrectly divided at a line break in the third paragraph of the Discussion, which might have led to some confusion. These errors have been corrected in the PDF and HTML versions of the article.

International audience; We study the potential of photoacoustic guidance for light focusing through scattering samples via wavefront-shaping and iterative optimization. We experimentally demonstrate that the focusing efficiency on an extended absorber can be improved by iterative optimization of the high frequency components of the broadband photoacous-tic signal detected with a spherically focused transducer. We demonstrate more than 12-fold increase in the photoacoustic signal generated by a 30 μm wire using a narrow frequency band around 60 MHz. By monitoring the speckle pattern evolution during the optimization process with a CCD camera, we experimentally confirm that such optimization leads to a smaller optical focus than what would be obtained by optimizing lower frequencies of the photoacoustic feedback.

We report the implementation of the photo-acoustic transmission-matrix (PATM) method with a linear ultrasound array. The PATM allows optimizing light delivery to specified regions of the image and revealing features otherwise hidden by limited view.

We investigate experimentally the use of speckle illumination for photoacoustic imaging. In particular, we demonstrate that otherwise invisible features are revealed through high-frequency signals fluctuations from different speckle realizations.

International audience; We implement the photoacoustic transmission matrix approach on a two-dimensional photoacoustic imaging system, using a 15 MHz linear ultrasound array. Using a black leaf skeleton as a complex absorbing structure, we demonstrate that the photoacoustic transmission matrix approach allows to reveal structural features that are invisible in conventional photoacoustic images, as well as to selectively control light focusing on absorbing targets, leading to a local enhancement of the photoacoustic signal.

International audience; Optical wavefront shaping has emerged as a powerful tool for manipulating light in strongly scattering media. It enables diffraction-limited focusing and imaging at depths where conventional microscopy techniques fail. However, to date, most examples of wavefront shaping have relied on direct access to the targets or implanted probes, and the challenge is to apply it non-invasively inside complex samples. Recently, ultrasonic-tagging techniques have been utilized successfully, but these allow only small acoustically tagged volumes to be addressed at each measurement. Here, we introduce an approach that allows the non-invasive measurement of an optical transmission matrix over a large volume, inside complex samples, using a standard photoacoustic imaging set-up. We demonstrate the use of this matrix for detecting, localizing and selectively focusing light on absorbing targets through diffusive samples, as well as for extracting the scattering medium properties. Combining the transmission-matrix approach with the advantages of photoacoustic imaging opens a path towards deep-tissue imaging and light delivery utilizing endogenous optical contrast.

In high-frequency photoacoustic imaging with uniform illumination, homogeneous photo-absorbing structures may be invisible because of their large size or limited-view issues. Here we show that, by exploiting dynamic speckle illumination, it is possible to reveal features which are normally invisible with a photoacoustic system comprised of a 20MHz linear ultrasound array. We demonstrate imaging of a 5 mm diameter absorbing cylinder and a 30 micrometer black thread arranged in a complex shape. The hidden structures are directly retrieved from photoacoustic images recorded for different random speckle illuminations of the phantoms by assessing the variation in the value of each pixel over the illumination patterns., Comment: 4 pages, 3 figures, Accepted in Optics letters