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

Objectives:
To assess the feasibility of monitoring transient evolution of thermal ablation zones with a microwave transmission coefficient-based technique.
Methods:
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 (
${\mathbf{s}}_{21}$
) measurements. Experiments were performed with applied powers ranging between 30–50 W per antenna for 53–1219 s. Transient
${\mathbf{s}}_{21}$
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.
Results:
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
${\mathbf{s}}_{21}$
data was in the range of 1.6%–2.3% compared to ground truth.
Conclusion:
We have demonstrated the feasibility of monitoring transient evolution of thermal ablation zones using microwave transmission coefficient measurements in ex vivo tissue.
Significance:
The presented technique has potential to contribute towards addressing the clinical need for a method of monitoring evolution of thermal ablation zones.