arXiv subjects

Teemu Sahlström

Publications and source records attributed to Teemu Sahlström.

5 recordsLinked to original sources

arXiv2026

Estimation of Dielectric Parameters from Ultrasound Waves in Quantitative Thermoacoustic Tomography

Thermoacoustic tomography (TAT) is an imaging technique based on the thermoacoustic effect, combining electromagnetic contrast and high resolution of ultrasound imaging. In TAT, a short micro- or radio wave pulse is directed to the imaged target. Energy of this pulse is absorbed depending on the dielectric parameters of the target, resulting in a spatially varying pressure distribution via the thermoacoustic effect. This pressure, known as the initial pressure distribution, propagates as acoustic waves that are measured on the boundary of the target using ultrasound sensors. In the inverse problem of TAT, the initial pressure is estimated from the measured ultrasound waves. TAT can further be extended to quantitative TAT (QTAT), where the aim is to estimate the dielectric parameters of the target from the measured ultrasound waves, utilizing a model for electromagnetic wave propagation. In this work, we study the inverse problem of QTAT, and propose an approach for simultaneous estimation of electrical conductivity and permittivity from the ultrasound waves. This problem is approached in the framework of Bayesian inverse problems, enabling incorporation of prior and noise models. The forward model describing electromagnetic and acoustic wave propagation is based on the Maxwell's equations and the acoustic wave-equation, respectively. The approach is evaluated with numerical simulations. The results show that the dielectric parameters can be estimated using the proposed approach with good precision. However, the ultrasound sensor geometry and the number of electromagnetic pulses have a significant effect on the accuracy of the estimated parameters.

physics.comp-ph↗

arXiv2026

Utilising a learned forward operator in the inverse problem of photoacoustic tomography

We study the use of a learned forward operator in the inverse problem of photoacoustic tomography. The Fourier neural operator to approximate the photoacoustic wave propagation is used. Further, the inverse problem is solved using a gradient-based approach with automatic differentiation. The methodology is evaluated using numerical simulations, and the results are compared to a conventional approach, where the forward operator is approximated using the pseudospectral $k$-space method. The results show that the learned forward operator can be used to approximate the photoacoustic wave propagation with good accuracy, and that it can be utilised as a computationally efficient forward operator in solving the inverse problem of photoacoustic tomography.

physics.comp-ph↗

arXiv2025

Fast reconstruction approaches for photoacoustic tomography with smoothing Sobolev/Matérn priors

In photoacoustic tomography (PAT), the computation of the initial pressure distribution within an object from its time-dependent boundary measurements over time is considered. This problem can be approached from two well-established points of view: deterministically using regularisation methods, or stochastically using the Bayesian framework. Both approaches frequently require the solution of a variational problem. In the paper we elaborate the connection between these approaches by establishing the equivalence between a smoothing Mat{é}rn class of covariance operators and Sobolev embedding operator $E_s: H^s \hookrightarrow L^2$. We further discuss the use of a Wavelet-based implementation of the adjoint operator $E_s^*$ which also allows for efficient evaluations for certain Mat{é}rn covariance operators, leading to efficient implementations both in terms of computational effort as well as memory requirements. The proposed methods are validated with reconstructions for the photoacoustic problem.

math.NA↗

arXiv2024

Simultaneous estimation of electrical conductivity and permittivity in quantitative thermoacoustic tomography

In this work, the inverse problem of quantitative thermoacoustic tomography is studied. In quantitative thermoacoustic tomography, dielectric parameters of an imaged target are estimated from an absorbed energy density induced by an externally introduced electromagnetic excitation. In this work, simultaneous estimation of electrical conductivity and permittivity is considered. We approach this problem in the framework of Bayesian inverse problems. The dielectric parameters are estimated by computing maximum a posteriori estimates, and the reliability of the estimates is studied using the Laplace's approximation. The forward model to describe electromagnetic wave propagation is based on a vectorial Maxwell's equation, that is numerically approximated using a finite element method with edge elements. The proposed methodology is evaluated using numerical simulations utilising one or two electromagnetic excitations at multiple excitation frequencies. The results show that the electrical conductivity and permittivity can be simultaneously estimated in quantitative thermoacoustic tomography. However, the problem can suffer from non-uniqueness, which could be overcome using multiple electromagnetic excitations.

physics.comp-ph↗

arXiv2022

Utilizing variational autoencoders in the Bayesian inverse problem of photoacoustic tomography

There has been an increasing interest in utilizing machine learning methods in inverse problems and imaging. Most of the work has, however, concentrated on image reconstruction problems, and the number of studies regarding the full solution of the inverse problem is limited. In this work, we study a machine learning based approach for the Bayesian inverse problem of photoacoustic tomography. We develop an approach for estimating the posterior distribution in photoacoustic tomography using an approach based on the variational autoencoder. The approach is evaluated with numerical simulations and compared to the solution of the inverse problem using a Bayesian approach.

physics.comp-ph↗