Liver tissue deformation and thermal effect investigation during microwave ablation - Ex vivo and numerical modeling...

Microwave ablation (MWA) has been demonstrated to induce tissue deformation through thermal effects, critically impacting surgical outcomes. However, a comprehensive representation of deformation fields remains absent to provide preoperative guidance.

To capture tissue deformation in a relatively fine temporal-spatial resolution during MWA; to analyze temperature-dependent deformation behavior with the aid of a thermal simulation model; and to evaluate the impact of tissue deformation on the geometry of the target region, thereby altering the required thermal ablation coverage.

This study investigated tissue deformation in ex vivo bovine liver during MWA (100 W/10 min) by tracking the displacement of grid-arranged markers. Computed Tomography (CT) imaging monitored the spatial positions of markers across two 40 $\mathrm{mm}\times$ 55 mm areas, collecting data from 96 locations at 1-min intervals. The symmetry of the displacement was analyzed to illustrate deformation patterns. Alongside, a numerical model was established to simulate the temperature field and validated through ex vivo temperature measurements, facilitating a temporal-spatial correlation analysis between tissue temperature and deformation. Finally, the deformation and simulated temperature fields were jointly analyzed to assess the spatial correspondence between the thermal ablation zone and the deformed target region.

Displacement symmetry around the antenna is high ( $\sim$ 0.8), indicating rotational characteristics in the 3D tissue deformation around the antenna's central axis. The established deformation fields did not show the characteristics of horizontal symmetry, the maximum radial contraction was 3.2 mm (21.4%) and longitudinal expansion was 4.8 mm (21.6%). Temperature differences between simulated and actual measurements were 0.6 ${}^{\circ }C$ to 2.9 ${}^{\circ }C$ . Notably, a strong correlation between temperature and deformation around the antenna feed was observed and contraction was prominent when the temperature reached 60 ${}^{\circ }C$ . Different relative positions between the antenna and the local target region resulted in severely different deformation outcomes (1.2% vs. 41.1%). Accounting for tissue deformation led to adjustments in the estimated thermal coverage required to maintain margin adequacy, reduced in contraction-dominant cases and increased in expansion-dominant cases, corresponding to changes in the planned ablation duration under 100 W by 15.1%-38.4%.

MWA leads to substantial tissue deformation, with the temperature distribution partially indicating its extent. Quantitatively incorporating tissue deformation into preoperative planning may improve the accuracy of thermal coverage estimation and support better assessment of treatment adequacy.