An iterative method to reconstruct the refractive index of a medium from time-of-flight measurements

TL;DR

This paper presents an iterative numerical method to reconstruct the refractive index of a medium from ultrasound time-of-flight data, extending tomography techniques to inhomogeneous media with proven convergence and numerical validation.

Contribution

It introduces a new iterative solver for the nonlinear inverse problem, including an explicit backprojection operator in Riemannian geometry, and provides convergence analysis and numerical experiments.

Findings

The method yields stable approximations of the refractive index.

The backprojection operator is explicitly represented and numerically implemented.

Numerical experiments demonstrate the effectiveness of the approach.

Abstract

The article deals with a classical inverse problem: the computation of the refractive index of a medium from ultrasound time-of-flight (TOF) measurements. This problem is very popular in seismics but also for tomographic problems in inhomogeneous media. For example ultrasound vector field tomography needs a priori knowledge of the sound speed. According to Fermat's principle ultrasound signals travel along geodesic curves of a Riemannian metric which is associated with the refractive index. The inverse problem thus consists of determining the index of refraction from integrals along geodesics curves associated with the integrand leading to a nonlinear problem. In this article we describe a numerical solver for this problem scheme based on an iterative minimization method for an appropriate Tikhonov functional. The outcome of the method is a stable approximation of the sought index of refraction as well as a corresponding set of geodesic curves. We prove some analytical convergence results for this method and demonstrate its performance by means of several numerical experiments. Another novelty in this article is the explicit representation of the backprojection operator for the ray transform in Riemannian geometry and its numerical realization relying on a corresponding phase function that is determined by the metric. This gives a natural extension of the conventional backprojection from 2D computerized tomography to inhomogeneous geometries.