arXiv subjects

Guillaume Bal

Publications and source records attributed to Guillaume Bal.

At least 91 records · Page 5Linked to original sources

arXiv2013

Local Inversions in Ultrasound Modulated Optical Tomography

Ultrasound modulated optical tomography, also called acousto-optics tomography, is a hybrid imaging modality that aims to combine the high contrast of optical waves with the high resolution of ultrasound. We follow the model of the influence of ultrasound modulation on the light intensity measurements developed in [Bal Schotland PRL 2010]. We present sufficient conditions ensuring that the absorption and diffusion coefficients modeling light propagation can locally be uniquely and stably reconstructed from the corresponding available information. We present an iterative procedure to solve such a problem based on the analysis of linear elliptic systems of redundant partial differential equations.

math.AP↗

arXiv2013

Linearized internal functionals for anisotropic conductivities

This paper concerns the reconstruction of an anisotropic conductivity tensor in an elliptic second-order equation from knowledge of the so-called power density functionals. This problem finds applications in several coupled-physics medical imaging modalities such as ultrasound modulated electrical impedance tomography and impedance-acoustic tomography. We consider the linearization of the nonlinear hybrid inverse problem. We find sufficient conditions for the linearized problem, a system of partial differential equations, to be elliptic and for the system to be injective. Such conditions are found to hold for a lesser number of measurements than those required in recently established explicit reconstruction procedures for the nonlinear problem.

math.AP↗

arXiv2012

The Levenberg-Marquardt Iteration for Numerical Inversion of the Power Density Operator

In this paper we develop a convergence analysis in an infinite dimensional setting of the Levenberg-Marquardt iteration for the solution of a hybrid conductivity imaging problem. The problem consists in determining the spatially varying conductivity $σ$ from a series of measurements of power densities for various voltage inductions. Although this problem has been very well studied in the literature, convergence and regularizing properties of iterative algorithms in an infinite dimensional setting are still rudimentary. We provide a partial result under the assumptions that the derivative of the operator, mapping conductivities to power densities, is injective and the data is noise-free. Moreover, we implemented the Levenberg-Marquardt algorithm and tested it on simulated data.

math.AP↗

arXiv2012

Convergence to SPDE of the Schrodinger equation with large, random potential

We study the asymptotic behavior of solutions to the Schr{ö}dinger equation with large-amplitude, highly oscillatory, random potential. In dimension $d<\mathfrak{m}$, where $\mathfrak{m}$ is the order of the leading operator in the Schrödinger equation, we construct the heterogeneous solution by using a Duhamel expansion and prove that it converges in distribution, as the correlation length $\varepsilon$ goes to 0, to the solution of a stochastic differential equation, whose solution is represented as a sum of iterated Stratonovich integral, over the space $C([0,+\infty),\mathcal{S}')$. The uniqueness of the limiting solution in a dense space of $L^2(Ω\times\mathbb{R}^d)$ is shown by verifying the property of conservation of mass for the Schrödinger equation. In dimension $d>\mathfrak{m}$, the solution to the Schr{ö}dinger equation is shown to converge in $L^2(Ω\times\mathbb{R}^d)$ to a deterministic Schr{ö}dinger solution in \cite{ZB-12}.

math.AP↗

arXiv2012

Boundary control of elliptic solutions to enforce local constraints

We present a constructive method to devise boundary conditions for solutions of second-order elliptic equations so that these solutions satisfy specific qualitative properties such as: (i) the norm of the gradient of one solution is bounded from below by a positive constant in the vicinity of a finite number of prescribed points; and (ii) the determinant of gradients of $n$ solutions is bounded from below in the vicinity of a finite number of prescribed points. Such constructions find applications in recent hybrid medical imaging modalities. The methodology is based on starting from a controlled setting in which the constraints are satisfied and continuously modifying the coefficients in the second-order elliptic equation. The boundary condition is evolved by solving an ordinary differential equation (ODE) defined so that appropriate optimality conditions are satisfied. Unique continuations and standard regularity results for elliptic equations are used to show that the ODE admits a solution for sufficiently long times.

math.AP↗

arXiv2012

Radiative Transport Limit of Dirac Equations with Random Electromagnetic Field

This paper concerns the kinetic limit of the Dirac equation with random electromagnetic field. We give a detailed mathematical analysis of the radiative transport limit for the phase space energy density of solutions to the Dirac equation. Our derivation is based on a martingale method and a perturbed test function expansion. This requires the electromagnetic field to be a space-time random field. The main mathematical tool in the derivation of the kinetic limit is the matrix-valued Wigner transform of the vector-valued Dirac solution. The major novelty compared to the scalar (Schrödinger) case is the proof of convergence of cross-modes to 0 weakly in space and almost surely in probability. The propagating modes are shown to converge in an appropriate strong sense to their deterministic limit.

math.AP↗

arXiv2012

Inverse anisotropic conductivity from power densities in dimension $n\ge 3$

We investigate the problem of reconstructing a fully anisotropic conductivity tensor $γ$ from internal functionals of the form $\nabla u\cdotγ\nabla u$ where $u$ solves $\nabla\cdot(γ\nabla u) = 0$ over a given bounded domain $X$ with prescribed Dirichlet boundary condition. This work motivated by hybrid medical imaging methods covers the case $n\ge 3$, following the previously published case $n=2$ \cite{Monard2011}. Under knowledge of enough such functionals, and writing $γ= τ\tilde γ$ ($\det \tildeγ= 1$) with $τ$ a positive scalar function, we show that all of $γ$ can be explicitely and locally reconstructed, with no loss of scales for $τ$ and loss of one derivative for the anisotropic structure $\tildeγ$. The reconstruction algorithms presented require rank maximality conditions that must be satisfied by the functionals or their corresponding solutions, and we discuss different possible ways of ensuring these conditions for $\C^{1,α}$-smooth tensors ($0<α<1$).

math.AP↗

arXiv2012

Inverse diffusion problems with redundant internal information

This paper concerns the reconstruction of a scalar diffusion coefficient $σ(x)$ from redundant functionals of the form $H_i(x)=σ^{2α}(x)|\nabla u_i|^2(x)$ where $α\in\Rm$ and $u_i$ is a solution of the elliptic problem $\nabla\cdot σ\nabla u_i=0$ for $1\leq i\leq I$. The case $α=\frac12$ is used to model measurements obtained from modulating a domain of interest by ultrasound and finds applications in ultrasound modulated electrical impedance tomography (UMEIT) as well as ultrasound modulated optical tomography (UMOT). The case $α=1$ finds applications in Magnetic Resonance Electrical Impedance Tomography (MREIT). We present two explicit reconstruction procedures of $σ$ for appropriate choices of $I$ and of traces of $u_i$ at the boundary of a domain of interest. The first procedure involves the solution of an over-determined system of ordinary differential equations and generalizes to the multi-dimensional case and to (almost) arbitrary values of $α$ the results obtained in two and three dimensions in \cite{CFGK} and \cite{BBMT}, respectively, in the case $α=\frac12$. The second procedure consists of solving a system of linear elliptic equations, which we can prove admits a unique solution in specific situations.

math.AP↗

arXiv2012

Inverse anisotropic diffusion from power density measurements in two dimensions

This paper concerns the reconstruction of an anisotropic diffusion tensor $γ=(γ_{ij})_{1\leq i,j\leq 2}$ from knowledge of internal functionals of the form $γ\nabla u_i\cdot\nabla u_j$ with $u_i$ for $1\leq i\leq I$ solutions of the elliptic equation $\nabla \cdot γ\nabla u_i=0$ on a two dimensional bounded domain with appropriate boundary conditions. We show that for I=4 and appropriately chosen boundary conditions, $γ$ may uniquely and stably be reconstructed from such internal functionals, which appear in coupled-physics inverse problems involving the ultrasound modulation of electrical or optical coefficients. Explicit reconstruction procedures for the diffusion tensor are presented and implemented numerically.

math.AP↗

arXiv2012

Inverse diffusion from knowledge of power densities

This paper concerns the reconstruction of a diffusion coefficient in an elliptic equation from knowledge of several power densities. The power density is the product of the diffusion coefficient with the square of the modulus of the gradient of the elliptic solution. The derivation of such internal functionals comes from perturbing the medium of interest by acoustic (plane) waves, which results in small changes in the diffusion coefficient. After appropriate asymptotic expansions and (Fourier) transformation, this allow us to construct the power density of the equation point-wise inside the domain. Such a setting finds applications in ultrasound modulated electrical impedance tomography and ultrasound modulated optical tomography. We show that the diffusion coefficient can be uniquely and stably reconstructed from knowledge of a sufficient large number of power densities. Explicit expressions for the reconstruction of the diffusion coefficient are also provided. Such results hold for a large class of boundary conditions for the elliptic equation in the two-dimensional setting. In three dimensions, the results are proved for a more restrictive class of boundary conditions constructed by means of complex geometrical optics solutions.

math.AP↗

arXiv2012

Reconstructions for some coupled-physics inverse problems

This letter announces and summarizes results obtained in arXiv:1111.5051 and considers several natural extensions. The aforementioned paper proposes a procedure to reconstruct coefficients in a second-order, scalar, elliptic equation from knowledge of a sufficiently large number of its solutions. We present this derivation and extend it to show which parameters may or may not be reconstructed for several hybrid (also called coupled physics) imaging modalities including photo-acoustic tomography, thermo-acoustic tomography, transient elastography, and magnetic resonance elastography. Stability estimates are also proposed.

math.AP↗

arXiv2012

Homogenization of the Schroedinger equation with large, random potential

We study the behavior of solutions to a Schr{ö}dinger equation with large, rapidly oscillating, mean zero, random potential with Gaussian distribution. We show that in high dimension $d>\mathfrak{m}$, where $\mathfrak{m}$ is the order of the spatial pseudo-differential operator in the Schr{ö}dinger equation (with $\mathfrak{m}=2$ for the standard Laplace operator), the solution converges in the $L^2$ sense uniformly in time over finite intervals to the solution of a deterministic Schr{ö}dinger equation as the correlation length $\varepsilon$ tends to 0. This generalizes to long times the convergence results obtained for short times and for the heat equation. The result is based on a careful decomposition of multiple scattering contributions. In dimension $d<\mathfrak{m}$, the random solution converges to the solution of a stochastic partial differential equation.

math.AP↗

arXiv2012

Explicit Reconstructions in QPAT, QTAT, TE, and MRE

Photo-acoustic Tomography (PAT) and Thermo-acoustic Tomography (TAT) are medical imaging modalities that combine the high contrast of radiative properties of tissues with the high resolution of ultrasound. In both modalities, a first step concerns the reconstruction of the radiation-induced source of ultrasound. Transient Elastography (TE) and Magnetic Resonance Elastography (MRE) combine the high elastic contrast of tissues with the high resolution of ultrasound and magnetic resonance, respectively. In both modalities, a first step concerns the reconstruction of the elastic displacement. The result of this first step, which is not considered in this paper, is the availability of internal functionals of the unknown tissue properties. All imaging modalities are recast as the reconstruction of parameters in elliptic equations from knowledge of solutions to such equations. This paper provides a characterization of the parameters that may or may not be reconstructed from such internal functionals. We provide explicit reconstruction procedures and indicate how stable they are with respect to errors in the available measurements. The modalities PAT, TAT, TE, and MRE allow us to reconstruct high-contrast optical and elastic properties of tissues with the high resolution of ultrasound or magnetic resonance imaging. They provide a means to reconstruct second-order tensors modeling tissue anisotropy as well as complex-valued coefficients modeling absorbing and dissipative effects.

physics.med-ph↗

arXiv2012

A corrector theory for diffusion-homogenization limits of linear transport equations

This paper concerns the diffusion-homogenization of transport equations when both the adimensionalized scale of the heterogeneities $α$ and the adimensionalized mean-free path $\eps$ converge to 0. When $α=\eps$, it is well known that the heterogeneous transport solution converges to a homogenized diffusion solution. We are interested here in the situation where $0<\eps\llα\ll1$ and in the respective rates of convergences to the homogenized limit and to the diffusive limit. Our main result is an approximation to the transport solution with an error term that is negligible compared to the maximum of $α$ and $\frac\epsα$. After establishing the diffusion-homogenization limit to the transport solution, we show that the corrector is dominated by an error to homogenization when $α^2\ll\eps$ and by an an error to diffusion when $\eps\llα^2$. Our regime of interest involves singular perturbations in the small parameter $η=\frac\epsα$. Disconnected local equilibria at $η=0$ need to be reconnected to provide a global equilibrium on the cell of periodicity when $η>0$. This reconnection between local and global equilibria is shown to hold when sufficient {\em no-drift} conditions are satisfied. The Hilbert expansion methodology followed in this paper builds on corrector theories for the result developed in \cite{NBAPuVo}.

math.AP↗

arXiv2012

Cauchy problem for Ultrasound Modulated EIT

Ultrasound modulation of electrical or optical properties of materials offers the possibility to devise hybrid imaging techniques that combine the high electrical or optical contrast observed in many settings of interest with the high resolution of ultrasound. Mathematically, these modalities require that we reconstruct a diffusion coefficient $σ(x)$ for $x\in X$, a bounded domain in $\Rm^n$, from knowledge of $σ(x)|\nabla u|^2(x)$ for $x\in X$, where $u$ is the solution to the elliptic equation $-\nabla\cdotσ\nabla u=0$ in $X$ with $u=f$ on $\partial X$. This inverse problem may be recast as a nonlinear equation, which formally takes the form of a 0-Laplacian. Whereas $p-$Laplacians with $p>1$ are well-studied variational elliptic non-linear equations, $p=1$ is a limiting case with a convex but not strictly convex functional, and the case $p<1$ admits a variational formulation with a functional that is not convex. In this paper, we augment the equation for the 0-Laplacian with full Cauchy data at the domain's boundary, which results in a, formally overdetermined, nonlinear hyperbolic equation. The paper presents existence, uniqueness, and stability results for the Cauchy problem of the 0-Laplacian. In general, the diffusion coefficient $σ(x)$ can be stably reconstructed only on a subset of $X$ described as the domain of influence of the space-like part of the boundary $\partial X$ for an appropriate Lorentzian metric. Global reconstructions for specific geometries or based on the construction of appropriate complex geometric optics solutions are also analyzed.

math.AP↗

arXiv2011

Reconstruction of coefficients in scalar second-order elliptic equations from knowledge of their solutions

This paper concerns the reconstruction of possibly complex-valued coefficients in a second-order scalar elliptic equation posed on a bounded domain from knowledge of several solutions of that equation. We show that for a sufficiently large number of solutions and for an open set of corresponding boundary conditions, all coefficients can be uniquely and stably reconstructed up to a well characterized gauge transformation. We also show that in some specific situations, a minimum number of such available solutions equal to $I_n=\frac12n(n+3)$ is sufficient to uniquely and globally reconstruct the unknown coefficients. This theory finds applications in several coupled-physics medical imaging modalities including photo-acoustic tomography, transient elastography, and magnetic resonance elastography.

math.AP↗

arXiv2011

Hybrid inverse problems and internal functionals

This paper reviews recent results on hybrid inverse problems, which are also called coupled-physics inverse problems of multi-wave inverse problems. Inverse problems tend to be most useful in, e.g., medical and geophysical imaging, when they combine high contrast with high resolution. In some settings, a single modality displays either high contrast or high resolution but not both. In favorable situations, physical effects couple one modality with high contrast with another modality with high resolution. The mathematical analysis of such couplings forms the class of hybrid inverse problems. Hybrid inverse problems typically involve two steps. In a first step, a well-posed problem involving the high-resolution low-contrast modality is solved from knowledge of boundary measurements. In a second step, a quantitative reconstruction of the parameters of interest is performed from knowledge of the point-wise, internal, functionals of the parameters reconstructed during the first step. This paper reviews mathematical techniques that have been developed in recent years to address the second step. Mathematically, many hybrid inverse problems find interpretations in terms of linear and nonlinear (systems of) equations. In the analysis of such equations, one often needs to verify that qualitative properties of solutions to elliptic linear equations are satisfied, for instance the absence of any critical points. This paper reviews several methods to prove that such qualitative properties hold, including the method based on the construction of complex geometric optics solutions.

math.AP↗

arXiv2011

Inverse transport with isotropic time-harmonic sources

This paper concerns the reconstruction of the scattering coefficient in a two-dimensional transport equation from angularly averaged measurements when the probing source is isotropic and time-harmonic. This is a practical setting in the medical imaging modality called Optical Tomography. As the modulation frequency of the source increases, we show that the reconstruction of the scattering coefficient improves. More precisely, as the frequency $ω$ increases, we show that all frequencies of the scattering coefficient lower than $b$ are reconstructed stably with an accuracy that improves as $ω$ increases and $b$ decreases. The proofs are based on an analysis of the single scattering singularities of the transport equation and on careful analyses of oscillatory integrals by stationary phase arguments.

math.AP↗