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PIER PIER B PIER C PIER M PIER Letters PIER Photonics
2026-08-11
Advanced Computational Framework for Terahertz Inverse Imaging in Stratified Biomedical Media: A Physics-Informed FDTD Model with Dielectric Gradient Reconstruction
By
Khushi D. Thakkar,

    Dr. Khushi D. Thakkar

    Department of Electronics and Communication Engineering

    Dharmsinh Desai University

    India

    Homepage

Rizwan Habibbhai Alad,

    Dr. Rizwan Habibbhai Alad

    Department of Electronics and Communication, Faculty of Technology

    Dharmsinh Desai University

    India

    Homepage

Purvang D. Dalal

    Dr. Purvang D. Dalal

    Department of Electronics and Communication

    Dharmsinh Desai University

    India

    Homepage

Progress In Electromagnetics Research C, Vol. 172, 262-274, 2026
doi:10.2528/PIERC26010902 | Google Scholar

Abstract
Terahertz Time-Domain Spectroscopy (THz-TDS) enables noninvasive skin hydration monitoring, but existing techniques rely on one-dimensional fitting, which cannot resolve lateral heterogeneity. This paper presents a physics-informed twodimensional finite-difference time-domain (2D-FDTD) forward modeling framework coupled with an adjoint-based optimization algorithm to reconstruct spatially resolved permittivity maps of hydrated skin. The forward model employs a Dual-Debye dielectric dispersion formulation with relaxation times τ1 = 8.3 ps (free water) and τ2 = 0.3 ps (bound water), capturing the frequency-dependent response in the range εr = 6.34-8.71 corresponding to physiological hydration fractions φ = 0.20-0.70. An adjointbased gradient descent algorithm with total variation (TV) regularization recovers the 2D permittivity distribution from reflected THz-TDS signals, achieving 0.03% root-mean-square (RMS) reconstruction error at the 30th iteration. Frequencydomain decomposition of the absorption coefficient is validated against synthetic hydrogel phantom data, confirming that bound water contributes 82 ± 4% of total absorption at 0.5 THz. The framework distinguishes healthy skin, psoriasis, and chronic wound edema with p < 10-17 statistical significance. To the best of the authors' knowledge, this constitutes an early demonstration of a 2D physics-informed inverse imaging framework demonstrated on both depth-resolved gradient phantoms and laterally heterogeneous pore phantoms of the 6-9 permittivity band with sub-percent accuracy in the THz biomedical domain; experimental validation of in-vivo tissue remains a critical next step.
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Citation
Khushi D. Thakkar, Rizwan Habibbhai Alad, and Purvang D. Dalal, "Advanced Computational Framework for Terahertz Inverse Imaging in Stratified Biomedical Media: A Physics-Informed FDTD Model with Dielectric Gradient Reconstruction," Progress In Electromagnetics Research C, Vol. 172, 262-274, 2026.
doi:10.2528/PIERC26010902
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