Overcoming the acoustic diffraction limit in photoacoustic imaging by localization of flowing absorbers

The resolution of photoacoustic imaging deep inside scattering media is limited by the acoustic diffraction limit. In this work, taking inspiration from super-resolution imaging techniques developed to beat the optical diffraction limit, we demonstrate that the localization of individual optical absorbers can provide super-resolution photoacoustic imaging well beyond the acoustic diffraction limit. As a proof-of-principle experiment, photoacoustic cross-sectional images of microfluidic channels were obtained with a 15 MHz linear CMUT array while absorbing beads were flown through the channels. The localization of individual absorbers allowed to obtain super-resolved cross-sectional image of the channels, by reconstructing both the channel width and position with an accuracy better than $\lambda/10$. Given the discrete nature of endogenous absorbers such as red blood cells, or that of exogenous particular contrast agents, localization is a promising approach to push the current resolution limits of photoacoustic imaging

Super-resolution photoacoustic imaging via flow induced absorption fluctuations

In deep tissue photoacoustic imaging the spatial resolution is inherently limited by the acoustic wavelength. We present an approach for surpassing the acoustic diffraction limit by exploiting temporal fluctuations in the sample absorption distribution, such as those induced by flowing particles. In addition to enhanced resolution, our approach inherently provides background reduction, and can be implemented with any conventional photoacoustic imaging system. The considerable resolution increase is made possible by adapting notions from super-resolution optical fluctuations imaging (SOFI) developed for blinking fluorescent molecules, to flowing acoustic emitters. By generalizing SOFI mathematical analysis to complex valued signals, we demonstrate super-resolved photoacoustic images that are free from oscillations caused by band-limited detection. The presented technique holds potential for contrast-agent free micro-vessels imaging, as red blood cells provide a strong endogenous source of naturally fluctuating absorption

Photoacoustic fluctuation imaging: theory and application to blood flow imaging

Photoacoustic fluctuation imaging, which exploits randomness in photoacoustic generation, provides enhanced images in terms of resolution and visibility, as compared to conventional photoacoustic images. While a few experimental demonstrations of photoacoustic fluctuation imaging have been reported, it has to date not been described theoretically. In the first part of this work, we propose a theory relevant to fluctuations induced either by random illumination patterns or by random distributions of absorbing particles. The theoretical predictions are validated by Monte Carlo finite-difference time-domain simulations of photoacoustic generation in random particle media. We provide a physical insight into why visibility artefacts are absent from second-order fluctuation images. In the second part, we demonstrate experimentally that harnessing randomness induced by the flow of red blood cells produce photoacoustic fluctuation images free of visibility artefacts. As a first proof of concept, we obtain two-dimensional images of blood vessel phantoms. Photoacoustic fluctuation imaging is finally applied in vivo to obtain 3D images of the vascularization in a chicken embryo

Super-resolution photoacoustic and ultrasound imaging with sparse arrays

It has previously been demonstrated that model-based reconstruction methods relying on a priori knowledge of the imaging point spread function (PSF) coupled to sparsity priors on the object to image can provide super-resolution in photoacoustic (PA) or in ultrasound (US) imaging. Here, we experimentally show that such reconstruction also leads to super-resolution in both PA and US imaging with arrays having much less elements than used conventionally (sparse arrays). As a proof of concept, we obtained super-resolution PA and US cross-sectional images of microfluidic channels with only 8 elements of a 128-elements linear array using a reconstruction approach based on a linear propagation forward model and assuming sparsity of the imaged structure. Although the microchannels appear indistinguishable in the conventional delay-and-sum images obtained with all the 128 transducer elements, the applied sparsity-constrained model-based reconstruction provides super-resolution with down to only 8 elements. We also report simulation results showing that the minimal number of transducer elements required to obtain a correct reconstruction is fundamentally limited by the signal-to-noise ratio. The proposed method can be straigthforwardly applied to any transducer geometry, including 2D sparse arrays for 3D super-resolution PA and US imaging

Molecular electrometer and binding of cations to phospholipid bilayers

Despite the vast amount of experimental and theoretical studies on the binding affinity of cations -especially the biologically relevant Na+ and Ca2+ - for phospholipid bilayers, there is no consensus in the literature. Here we show that by interpreting changes in the choline headgroup order parameters according to the 'molecular electrometer' concept [Seelig et al., Biochemistry, 1987, 26, 7535], one can directly compare the ion binding affinities between simulations and experiments. Our findings strongly support the view that in contrast to Ca2+ and other multivalent ions, Na+ and other monovalent ions (except Li+) do not specifically bind to phosphatidylcholine lipid bilayers at sub-molar concentrations. However, the Na+ binding affinity was overestimated by several molecular dynamics simulation models, resulting in artificially positively charged bilayers and exaggerated structural effects in the lipid headgroups. While qualitatively correct headgroup order parameter response was observed with Ca2+ binding in all the tested models, no model had sufficient quantitative accuracy to interpret the Ca2+: lipid stoichiometry or the induced atomistic resolution structural changes. All scientific contributions to this open collaboration work were made publicly, using nmrlipids. blogspot.fi as the main communication platform.Peer reviewe

Imagerie photoacoustique au-delà de la limite de diffraction acoustique

Wave imaging remained diffraction-limited for centuries, until methods, such as PALM, STORM and STED were proposed in optics. Thanks to such methods, imaging with a precision much better than the wavelength became possible.Inspired by these super-resolution methods, the present PhD study is focused on pushing the diffraction limit in acoustic-resolution photoacoustics. In the frames of this study, super-resolution is demonstrated experimentally in vitro with such techniques as super-localisation, fluctuation-based analysis and model-based reconstruction. For each method, the resolution limit is identified, strong and weak points are analysed, prospects are discussed. In addition, super-resolution by sparsity-based reconstruction is demonstrated in relation to sparse-array imaging. At the end, all studied super-resolution methods are compared. An extra chapter is devoted to overcoming visibility problems in photoacoustic imaging by means of fluctuation analysis.Avec l’avènement des méthodes de super-résolution, telles que PALM, STORM et STED, l’imagerie au-delà de la limite de diffraction est devenue réalité.Inspirée de ces approches de super-résolution introduites en optique, cette thèse a pour objectif d'aller au delà de la limite de diffraction acoustique en imagerie photoacoustique en profondeur. Dans le cadre de cette thèse, des méthodes telles que la super-localisation, l'exploitation des fluctuations et la reconstruction par problème inverse sont développées pour obtenir des images photoacoustiques super-résolues à partir de données expérimentales.Pour chaque méthode la limite de résolution est déterminée. En outre, une discussion est menée sur les avantages et inconvénients ainsi que les perspectives éventuelles pour chaque technique. La super-résolution sous contrainte de parcimonie est aussi exploitée dans le contexte de l’imagerie par des transducteurs à nombre d’éléments réduit (“sparse arrays”). Une approche exploitant les fluctuations est également mis en œuvre dans le but de résoudre les problèmes de visibilité en imagerie photoacoustique

Photoacoustic imaging beyond the acoustic diffraction limit

Avec l’avènement des méthodes de super-résolution, telles que PALM, STORM et STED, l’imagerie au-delà de la limite de diffraction est devenue réalité.Inspirée de ces approches de super-résolution introduites en optique, cette thèse a pour objectif d'aller au delà de la limite de diffraction acoustique en imagerie photoacoustique en profondeur. Dans le cadre de cette thèse, des méthodes telles que la super-localisation, l'exploitation des fluctuations et la reconstruction par problème inverse sont développées pour obtenir des images photoacoustiques super-résolues à partir de données expérimentales.Pour chaque méthode la limite de résolution est déterminée. En outre, une discussion est menée sur les avantages et inconvénients ainsi que les perspectives éventuelles pour chaque technique. La super-résolution sous contrainte de parcimonie est aussi exploitée dans le contexte de l’imagerie par des transducteurs à nombre d’éléments réduit (“sparse arrays”). Une approche exploitant les fluctuations est également mis en œuvre dans le but de résoudre les problèmes de visibilité en imagerie photoacoustique.Wave imaging remained diffraction-limited for centuries, until methods, such as PALM, STORM and STED were proposed in optics. Thanks to such methods, imaging with a precision much better than the wavelength became possible.Inspired by these super-resolution methods, the present PhD study is focused on pushing the diffraction limit in acoustic-resolution photoacoustics. In the frames of this study, super-resolution is demonstrated experimentally in vitro with such techniques as super-localisation, fluctuation-based analysis and model-based reconstruction. For each method, the resolution limit is identified, strong and weak points are analysed, prospects are discussed. In addition, super-resolution by sparsity-based reconstruction is demonstrated in relation to sparse-array imaging. At the end, all studied super-resolution methods are compared. An extra chapter is devoted to overcoming visibility problems in photoacoustic imaging by means of fluctuation analysis

Resolution, visibility and contrast enhancement in 2D and 3D photoacoustic imaging: a unified approach based on absorption fluctuations analysis

International audiencePhotoacoustic imaging provides optical contrast at depth beyond the optical transport mean free path. From the generation of ultrasound by light absorption, images can be reconstructed at the acoustic resolution (~100 �m) with a penetration of a few cm. Improving the resolution to bring it closer to the cellular scale is a major challenge. Moreover, the design of imaging systems often leads to limited view artifacts, where a part of the information needed for a complete reconstruction of the objects is missing. We studied multiple approaches for improving the resolution, visibility and contrast of photoacoustic imaging. We will show that a dynamic approach based on the analysis of fluctuations induced by a flow of absorbers can significantly improve resolution. The fluctuation approach, which has the advantage to use the native contrast of blood, also solves the visibility problem in 2D imaging of flow phantoms using a linear transducer (128 elements, 15 MHz). We will show this approach is effective using a sparse array of detectors for 3D imaging. Using a sparse spherical array (256 elements, 8 MHz), 3D imaging experiments in the chicken embryo vasculature model evidenced the presence of clutter around the reconstructed objects due to the low number of channels, resulting in a poor contrast. This clutter was greatly reduced by the fluctuation approach. We will further discuss photoacoustic fluctuations and ultrasound power Doppler similarities. Thus, photoacoustic fluctuation imaging overcomes many limitations of conventional imaging and will be further evaluated for in-vivo imaging

Imagerie photoacoustique biomédicale

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

Visualization3.mp4

Microscope movie of the samples with flowing whole blood, area near the input of the circuit with slower flow allows the visualization of single RB