Transmission Coefficient-Based Monitoring of Microwave Ablation: Development and Experimental Evaluation in Ex Vivo...

To assess the feasibility of monitoring transient evolution of thermal ablation zones with a microwave transmission coefficient-based technique.

Microwave ablation was performed in ex vivo bovine liver with two 2.45 GHz directional antennas. A custom apparatus was developed to enable periodic switching between "heating mode" when power from the generator was coupled to the antennas, and "monitoring mode", when antennas were coupled to a network analyzer for broadband transmission coefficient ( s₂₁) measurements. Experiments were performed with applied powers ranging between 30-50 W per antenna for 53-1219 s. Transient s₂₁ spectra over the course of ablations were analyzed to determine feasibility of predicting extent of ablation zones and compared against ground truth assessment from images of sectioned tissue. A linear regression-based mapping between the two datasets was derived to predict ablation extent.

Normalized average transmission coefficient initially rapidly decreased and then increased before asymptotically approaching steady state, with the transition time ranging between 53 s (45 W) and 109 s (30 W). Analysis of ground truth ablation zone images indicated time to complete ablation of 230-350 s. The relative prediction error for time to complete ablation derived from the s₂₁ data was in the range of 1.6%-2.3% compared to ground truth.

We have demonstrated the feasibility of monitoring transient evolution of thermal ablation zones using microwave transmission coefficient measurements in ex vivo tissue.

The presented technique has potential to contribute towards addressing the clinical need for a method of monitoring evolution of thermal ablation zones.