Papers by Alexandre Aubry
Zenodo (CERN European Organization for Nuclear Research), Oct 4, 2023
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific r... more HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Non-invasive Retrieval of the Transmission Matrix for Optical Imaging Deep Inside a Multiple Scattering Medium
arXiv (Cornell University), Mar 10, 2023
Ultrasound in Medicine & Biology
In the paper, we succeeded to freeze-dry monodisperse microbubbles without degrading their size a... more In the paper, we succeeded to freeze-dry monodisperse microbubbles without degrading their size and acoustic properties. We used microfluidic technology to generate highly monodisperse (coefficient of variation, CV<5%) microbubbles and optimized their formulation along with a cryoprotectant. By using a specific technique of retrieval of the bubble, we showed that freeze-drying the microbubbles does not alter their size distribution (CV≤6). To compare the fundamental resonance properties of the bubbles, we performed backscattered acoustic characterization measurements. Our experimental results revealed that the freeze-drying process conserved the acoustic properties of the bubbles. The maximum backscattering power amplitude of fresh and freeze-dried monodisperse PVA bubbles was around ten eight times higher than that of SonoVue at a similar concentration in vitro. By solving the question of storage and transportation of monodisperse bubbles, our work facilitates their penetration in the domain of UCAs, for performing new tasks and developing novel non-invasive measurements, such as pressure, unaccessible to the existing commercialized bubbles.
Scattering reflection matrix approach to ultra-deep imaging through biological media (Conference Presentation)
Adaptive Optics and Wavefront Control for Biological Systems IV, 2018
When imaging with classical waves, multiple scattering (MS) is often seen as an unavoidable obsta... more When imaging with classical waves, multiple scattering (MS) is often seen as an unavoidable obstacle. The diffraction-limited resolution obtainable with methods such as microscopy requires that single-scattering (SS) dominates; for depths where MS processes become important, such methods result in an image without any connection to the reflectivity of the medium. Conversely, techniques such as diffuse optical tomography take advantage of the diffuse nature of light, but their resolution power is limited. To do better, methods such as wavefront shaping and adaptive optics have been developed. Focussing through a thick diffusive layer was demonstrated using a transmission matrix approach consisting of the measurement of Green’s functions between each pixel of a spatial light modulator (SLM) and of a charge-coupled device (CCD) camera across the medium. To image inside a multiple-scattering medium, we present a matrix approach based on the experimental measurement of a reflection matrix from the medium. An analysis based on the geometric and statistical properties of this reflection matrix can enhance the SS contribution which would otherwise be swamped by MS at large depths, and correct the resulting image for aberration effects induced by the turbid medium itself. The correction does not require the presence of bright scatterers, does not rely on any feedback loop and works even at depths where the field-of-view contains several isoplanatic patches. Here we present the application of our reflection matrix approach to optical imaging in biological tissues. Compared to OCT and related methods, we demonstrate an extension of the current imaging-depth limit.
Method and probing device by wave propagation
A method of sample wave propagation, wherein an incident wave is transmitted in a medium (1) comp... more A method of sample wave propagation, wherein an incident wave is transmitted in a medium (1) comprises diffusers (2) able to reflect the waves, and then picks up signals representative of a reflected wave reverberated by the environment from the incident wave, the sensed signals being the sum of a single scattering component and a multicast component. Treating the signals picked up by separating multiple scattering component of the single scattering component.
Gaussian beams and Legendre polynomials as invariants of the time reversal operator for a large rigid cylinder
The Journal of the Acoustical Society of America, 2006
The DORT method (French acronym for decomposition of the time reversal operator) is an active rem... more The DORT method (French acronym for decomposition of the time reversal operator) is an active remote sensing technique using an array of antennas for the detection and localization of scatterers. This method is based on the singular value decomposition of the interelement response matrix. In this paper an analytical expression of the singular vectors due to the reflection from a large rigid cylinder is provided. Depending on the array aperture, two asymptotic regimes are described. It is shown that the singular vectors correspond to Hermite-Gaussian modes for large apertures and Legendre polynomials for small ones. Using perturbation theory, the corresponding singular values are deduced. Theoretical predictions are in good agreement with experimental results.
The Journal of the Acoustical Society of America, 2008
We present a new imaging technique dedicated to the detection of a target embedded in a strongly ... more We present a new imaging technique dedicated to the detection of a target embedded in a strongly scattering medium. Classical imaging techniques such as echography fail in this kind of configuration because of multiply scattered echoes and aberration distorsions. The experimental set up consists in an array of programmable transducers placed in front of a collection of steel rods randomly distributed. The slab thickness is three times the mean free path. Behind this strongly diffusive slab, we set an echogene steel cylinder that we want to detect and localize. The impulse responses between each couple of transducers are measured and form the interelement matrix. Our technique separates the single-scattered echoes from the multiple scattering background. This is possible because of a deterministic coherence of single-scattering signals along the antidiagonals of the array response matrix, whatever the distribution of scatterers. Once this operation is performed, the detection of the ...
The Journal of the Acoustical Society of America, 2008
Waves scattered by a weakly scattering random medium contain a predominant single scattering cont... more Waves scattered by a weakly scattering random medium contain a predominant single scattering contribution as well as a multiple scattering contribution which is usually neglected. But its investigation can be fruitful for characterization purposes because it provides measurements of statistical parameters such as the scattering mean free path. Our aim is to extract the multiple scattering contribution in a weakly scattering random medium. The experimental set up consists in an array of programmable transducers placed in front of the sample. The impulse responses between each couple of transducers are measured and form the interelement matrix. Our technique allows the extraction of multiple scattering signals by taking advantage of their randomness, contrary to single scattering contributions which are shown to exhibit a deterministic coherence along the antidiagonals of the array response matrix, whatever the distribution of scatterers. To illustrate the interest of this technique, ...
The Journal of the Acoustical Society of America, 2008
We investigate, both experimentally and theoretically, the spectral properties of the backscatter... more We investigate, both experimentally and theoretically, the spectral properties of the backscattering operator in random media. The experimental set up consists in an array of 64 programmable ultrasonic transducers placed in front of a random scattering medium. The impulse responses between each couple of transducers are measured and form the interelement matrix.The evolution of the singular values with time and frequency is computed by means of a short-time Fourier analysis. Our aim is to compare the mean distribution of the singular values with the predictions of Random Matrix Theory (RMT). Two kinds of random media have been investigated: a highly scattering medium in which multiple scattering predominates and a weakly scattering medium. In both cases, residual correlations that may exist between matrix elements are shown to be a key parameter. The mean distribution of singular values exhibits, surprisingly, very different behaviors in the single and multiple scattering regimes. T...
Decomposition of the time reversal operator applied to subwavelength localization of scatterers
The Journal of the Acoustical Society of America, 2004
The decomposition of the time reversal operator (DORT method in French) is an ultrasonic detectio... more The decomposition of the time reversal operator (DORT method in French) is an ultrasonic detection method based the analysis of the whole pulse echo responses of an array. The eigenvalues and the eigenvectors of the time reversal operator (i.e., singular values and singular vectors of the array response matrix) provides information on localization of scatterers in the insonified medium. Recent experiments shows how DORT can also be used as a characterization method of subwavelength scatterers as cylinders [Minonzio et al., J. Acoust. Soc. Am. accepted May (2004)]. It has been shown that multiple singular values are associated with a single scatterer and that the singular vectors are combinations of the normal modes (monopole, dipole) of vibrations. In this study, interactions between two subwavelength cylinders less than a wavelength apart are described. Multiple scattering affects the singular value distribution. Experiments and calculations have been carried out on nylon and metal...
Applied Physics Letters, 2008
We present local measurements of the diffusion constant for ultrasonic waves undergoing multiple ... more We present local measurements of the diffusion constant for ultrasonic waves undergoing multiple scattering. The experimental setup uses a coherent array of programmable transducers. By achieving Gaussian beamforming at emission and reception, an array of virtual sources and receivers located in the near field is constructed. A matrix treatment is proposed to separate the incoherent intensity from the coherent backscattering peak. Local measurements of the diffusion constant D are then achieved. This technique is applied to a real case: a sample of human trabecular bone for which the ultrasonic characterization of multiple scattering is an issue.
Sounding Method and Device using Wave Propagation
In this paper, the interaction of a dipole emitter (e.g molecule or quantum dot) with complex pla... more In this paper, the interaction of a dipole emitter (e.g molecule or quantum dot) with complex plasmonic nanostructures is investigated by means of transformation optics. The fluorescence enhancement as well as the quantum efficiency are derived analytically. Their spectral and spatial properties are analyzed in the perspective of future experiments.
In geophysics, volcanoes are particularly difficult to image because of the multiscale heterogene... more In geophysics, volcanoes are particularly difficult to image because of the multiscale heterogeneities of fluids and rocks that compose them and their complex non-linear dynamics. By exploiting seismic noise recorded by a sparse array of geophones, we are able to reveal the magmatic and hydrothermal plumbing system of La Soufrière volcano in Guadeloupe. Spatio-temporal crosscorrelation of seismic noise actually provides the impulse responses between virtual geophones located inside the volcano. The resulting reflection matrix can be exploited to numerically perform an auto-focus of seismic waves on any reflector of the underground. An unprecedented view on the volcano's inner structure is obtained at a half-wavelength resolution. This innovative observable provides fundamental information for the conceptual modeling and highresolution monitoring of volcanoes. In everyday life, a multitude of sensors surround us to monitor our environment. In wave physics, those sensors can be active and work together to control the wave-field at will whether 1 it be for focusing (1) or communication (2) purposes. For imaging, the problem is often illposed because of the medium complexity and/or the sensor array sparsity. This is particularly the case in seismology, where the topography of the site under investigation can be so irregular that it is illusory to deploy a large and dense network of geophones. This paper addresses the issue of seismic imaging in complex areas such as volcanoes or fault zones based on data recorded by a sparse array of seismometers. The goal is to provide high spatial resolution and in-depth imaging of such critical areas that are of paramount importance for Earth sciences. To that aim, we will build on a matrix imaging approach imported from other fields than geophysics, such as medical ultrasonics (3, 4) and optical microscopy (5, 6) that were designed for scales ranging from a few centimeters for ultrasonic waves to a few hundreds of nanometers for light. In contrast with concurrent seismic methods such as full waveform inversion ( ), the strength of matrix imaging lies in the fact that: (i) it does not rely on a sophisticated wave velocity model whose knowledge is often limited and uncertain in geophysics; (ii) it is robust with respect to data quality which is a frequent issue in seismology. Matrix imaging relies on the array response matrix that contains the set of impulse responses between each seismometer. Although a geophone is purely passive, cross-correlation of seismic noise received at two stations is known to converge toward the Green's function between receiving stations (8, 9), as if one of them had been used as source, thus paving the way to passive matrix imaging (10-12). As surface waves dominate ambient noise, most past studies on the topic aimed at extracting surface wave properties from ambient noise correlations (13). However, they also contain the contribution of body waves reflected by deep structures ( ) and fluid reservoirs (10). As a proof-of-concept, we here exploit seismic noise recorded by a sparse geophone network deployed at the surface of the La Soufrière volcano of Guadeloupe (15,. The covariance matrix of this seismic noise provides the reflection matrix that contains all the available information
IEEE Transactions on Medical Imaging
This is the second article in a series of two which report on a matrix approach for ultrasound im... more This is the second article in a series of two which report on a matrix approach for ultrasound imaging in heterogeneous media. This article describes the quantification and correction of aberration, i.e. the distortion of an image caused by spatial variations in the medium speed-of-sound. Adaptive focusing can compensate for aberration, but is only effective over a restricted area called the isoplanatic patch. Here, we use an experimentally-recorded matrix of reflected acoustic signals to synthesize a set of virtual transducers. We then examine wave propagation between these virtual transducers and an arbitrary correction plane. Such wave-fronts consist of two components: (i) An ideal geometric wave-front linked to diffraction and the input focusing point, and; (ii) Phase distortions induced by the speed-of-sound variations. These distortions are stored in a so-called distortion matrix, the singular value decomposition of which gives access to an optimized focusing law at any point. We show that, by decoupling the aberrations undergone by the outgoing and incoming waves and applying an iterative strategy, compensation for even high-order and spatially-distributed aberrations can be achieved. As a proof-of-concept, ultrasound matrix imaging (UMI) is applied to the in-vivo imaging of a human calf. A map of isoplanatic patches is retrieved and is shown to be strongly correlated with the arrangement of tissues constituting the medium. The corresponding focusing laws yield an ultrasound image with an optimal contrast and a transverse resolution close to the ideal value predicted by diffraction theory. UMI thus provides a flexible and powerful route towards computational ultrasound.
IEEE Transactions on Medical Imaging
This is the first article in a series of two dealing with a matrix approach for aberration quanti... more This is the first article in a series of two dealing with a matrix approach for aberration quantification and correction in ultrasound imaging. Advanced synthetic beamforming relies on a double focusing operation at transmission and reception on each point of the medium. Ultrasound matrix imaging (UMI) consists in decoupling the location of these transmitted and received focal spots. The response between those virtual transducers form the so-called focused reflection matrix that actually contains much more information than a confocal ultrasound image. In this paper, a time-frequency analysis of this matrix is performed, which highlights the single and multiple scattering contributions as well as the impact of aberrations in the monochromatic and broadband regimes. Interestingly, this analysis enables the measurement of the incoherent input-output point spread function at any pixel of this image. A fitting process enables the quantification of the single scattering, multiple scattering and noise components in the image. From the single scattering contribution, a focusing criterion is defined, and its evolution used to quantify the amount of aberration throughout the ultrasound image. In contrast to the state-of-the-art coherence factor, this new indicator is robust to multiple scattering and electronic noise, thereby providing a contrasted map of the focusing quality at a much better transverse resolution. After a validation of the proof-of-concept based on time-domain simulations, UMI is applied to the in-vivo study of a human calf. Beyond this specific example, UMI opens a new route for speed-of-sound and scattering quantification in ultrasound imaging.
Physical Review B
We report on anomalous light transport in the strong scattering regime. Using low-coherence inter... more We report on anomalous light transport in the strong scattering regime. Using low-coherence interferometry, we measure the reflection matrix of titanium dioxide powders, revealing crucial features of strong optical scattering which can not be observed with transmission measurements: (i) a subdiffusive regime of transport at early times of flight that is a direct consequence of predominant recurrent scattering loops, and (ii) a crossover to a conventional, but extremely slow, diffusive regime at long times. These observations support previous predictions that near-field coupling between scatterers prohibits Anderson localization of light in three-dimensional disordered media.
Optics Express
We report on a theoretical model for image formation in full-field optical coherence tomography (... more We report on a theoretical model for image formation in full-field optical coherence tomography (FFOCT). Because the spatial incoherence of the illumination acts as a virtual confocal pinhole in FFOCT, its imaging performance is equivalent to a scanning time-gated coherent confocal microscope. In agreement with optical experiments enabling a precise control of aberrations, FFOCT is shown to have nearly twice the resolution of standard imaging at moderate aberration level. Beyond a rigorous study on the sensitivity of FFOCT with respect to aberrations, this theoretical model paves the way towards an optimized design of adaptive optics and computational tools for high-resolution and deep imaging of biological tissues.
This document provides supplementary information on: (i) the experimental set up used for the spo... more This document provides supplementary information on: (i) the experimental set up used for the spoke target experiment; (ii) the comparison between theory and experiment for resolution target images under astigmatism and coma; (iii) the configuration of the gold nano-bead experiment.
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Papers by Alexandre Aubry