Elie Bretin

Photoacoustic Imaging for Attenuating Acoustic Media

The aim of this chapter is to consider two challenging problems in photo-acoustic imaging. We consider extended optical sources in an attenuating acoustic background. We provide algorithms to correct the effects of imposed boundary conditions and that of attenuation as well. By testing our measurements against an appropriate family of functions, we show that we can access the Radon transform of the initial condition in the acoustic wave equation, and thus recover quantitatively the absorbing energy density. We also show how to compensate the effect of acoustic attenuation on image quality by using the stationary phase theorem.

Localization, Stability, and Resolution of Topological Derivative Based Imaging Functionals in Elasticity ∗

The focus of this work is on rigorous mathematical analysis of the topological derivative based detection algorithms for the localization of an elastic inclusion of vanishing characteristic size. A filtered quadratic misfit is considered and the performance of the topological derivative imaging functional resulting therefrom is analyzed. Our analysis reveals that the imaging functional may not attain its maximum at the location of the inclusion. Moreover, the resolution of the image is below the diffraction limit. Both phenomena are due to the coupling of pressure and shear waves propagating with different wave speeds and polarization directions. A novel imaging functional based on the weighted Helmholtz decomposition of the topological derivative is, therefore, introduced. It is thereby substantiated that the maximum of the imaging functional is attained at the location of the inclusion and the resolution is enhanced and it proves to be the diffraction limit. Finally, we investigate the stability of the proposed imaging functionals with respect to measurement and medium noises.

Time-reversal algorithms in viscoelastic media

In this paper, we consider the problem of reconstructing sources in a homogeneous viscoelastic medium from wavefield measurements using time-reversal algorithms. Our motivation is the recent advances on hybrid methods in biomedical imaging. We first present a modified time-reversal imaging algorithm based on a weighted Helmholtz decomposition and justify mathematically that it provides a better approximation than by simply time reversing the displacement field. Then, we investigate the source inverse problem in an elastic attenuating medium. We provide a regularized time-reversal imaging which corrects the attenuation effect at the first order.

Mathematical Methods in Elasticity Imaging

This book is the first to comprehensively explore elasticity imaging and examines recent, important developments in asymptotic imaging, modeling, and analysis of deterministic and stochastic elastic wave propagation phenomena. It derives the best possible functional images for small inclusions and cracks within the context of stability and resolution, and introduces a topological derivative-based imaging framework for detecting elastic inclusions in the time-harmonic regime. For imaging extended elastic inclusions, accurate optimal control methodologies are designed and the effects of uncertainties of the geometric or physical parameters on stability and resolution properties are evaluated. In particular, the book shows how localized damage to a mechanical structure affects its dynamic characteristics, and how measured eigenparameters are linked to elastic inclusion or crack location, orientation, and size. Demonstrating a novel method for identifying, locating, and estimating inclusions and cracks in elastic structures, the book opens possibilities for a mathematical and numerical framework for elasticity imaging of nanoparticles and cellular structures.

Noise Source Localization in an Attenuating Medium

In this paper we consider the problem of reconstructing the spatial support of noise sources from boundary measurements using cross correlation techniques. We consider media with and without attenuation and provide efficient imaging functionals in both cases. We also discuss the case where the noise sources are spatially correlated. We present numerical results to show the viability of the different proposed imaging techniques.

Consistency result for a non monotone scheme for anisotropic mean curvature flow

In this paper, we propose a new scheme for anisotropic motion by mean curvature in

Regularization of Discrete Contour by Willmore Energy

\R^d

Phase-field approximations of the Willmore functional and flow

. The scheme consists of a phase-field approximation of the motion, where the nonlinear diffusive terms in the corresponding anisotropic Allen-Cahn equation are linearized in the Fourier space. In real space, this corresponds to the convolution with a kernel of the form \[ K_{\phi,t}(x) = \F^{-1}\left[ e^{-4\pi^2 t \phi^o(\xi)} \right](x). \] We analyse the resulting scheme, following the work of Ishii-Pires-Souganidis on the convergence of the Bence-Merriman-Osher algorithm for isotropic motion by mean curvature. The main difficulty here, is that the kernel

On the Green function in visco‐elastic media obeying a frequency power‐law

K_{\phi,t}

Multiphase mean curvature flows with high mobility contrasts: A phase-field approach, with applications to nanowires

is not positive and that its moments of order 2 are not in