Articles techniques et présentations

Background Microwave ablation (MWA) induces non-uniform temperature changes within tissue while simultaneously altering its thermal and electrical properties during the heating process. These changes can potentially be reflected through bioimpedance measurements, thereby providing an additional indicator of the tissue's thermal state during ablation. This study develops a multiphysics model in COMSOL Multiphysics to correlate microwave energy delivery, thermal field evolution, and the transfer impedance response of liver tissue. The primary objective is to investigate whether changes in the average temperature within the region of interest produce a systematic and quantifiable variation in bioimpedance.

COMSOL Setup A 2D axisymmetric model was constructed featuring a liver tissue domain measuring 30 mm ×80 mm and a coaxial antenna. The electromagnetic field was solved using the Electromagnetic Waves, Frequency Domain interface at a frequency of 2.45 GHz with an input power of 60 W delivered via a coaxial transverse electromagnetic port. The absorbed electromagnetic energy was coupled with the Bioheat Transfer interface via Electromagnetic Heating to calculate the time-dependent temperature field. Liver tissue properties-including electrical conductivity, relative permittivity, thermal conductivity, density, and specific heat capacity-were defined as temperature-dependent functions. In the current configuration, blood perfusion and metabolic heat generation terms were set to zero. The bioimpedance response was calculated using the Electric Currents interface at 20 kHz, employing a 0.5 mA excitation current and a tetrapolar configuration consisting of two current-excitation electrodes and two voltage-picked-up electrodes. Transfer impedance is determined as follow: Z=ΔV/I The two normal and reciprocal equations are also solved to obtain the sensitivity quantity S. The bioimpedance study was conducted using 61 thermal snapshots taken between 0 and 600 s, with a sampling interval of 10 s.

Results Over a 600-second simulation period, the average temperature of the region of interest (TROI) rose from 20.00°C to 56.27°C, representing an increase of approximately 36.27°C. Over the same period, the transfer impedance value predicted by the model increased monotonically from 5.325 Ω to 5.992 Ω, corresponding to a relative change of 12.51%. An analysis of 61 data points reveals a strong linear correlation between the transfer impedance and TROI, as follow: Z≈0.01992TROI+4.8237, R2=0.989, r=0.9881 Meanwhile, the integral sensitivity S stayed stable, indicating that sensitivity is better suited to characterizing current paths and sensitive regions rather than serving as the primary bioimpedance signal.

Conclusion The proposed COMSOL model establishes a multiphysics chain linking microwave heating, temperature-dependent tissue properties, and low-frequency transfer impedance. In the investigated configuration, transfer impedance varies systematically with the average temperature of the region of interest and exhibits a strong linear relationship. These results indicate that transfer bioimpedance has the potential to serve as a temperature-sensitive indicator for monitoring the thermal state of tissue during microwave ablation simulations, while also providing a foundation for future research on bioimpedance-based ablation monitoring.