Electromagnetic Transmission Coefficient-Based Assessment of Tissue State During Microwave Ablation

Background: During microwave ablation (MWA), applicators are positioned within/around the target under image guidance, however, real-time monitoring of the ablation zone growth is limited. The tissue dielectric properties change with increasing temperature; therefore, changes in the complex-valued electromagnetic transmission coefficient (s21) between applicators may provide a parameter for ablation monitoring. Objective: To relate the physical state of tissue to the energy transmitted between a pair of directional MWA applicators. Method: We implemented a method for sequential measurement of broadband s21 between two directional MWA applicators operating at 2.45GHz. In each experiment, the input of each applicator was periodically switched between a microwave generator (“heating mode”) and a vector network analyzer (VNA, “monitoring mode”). S-parameters and tissue images after ablation were collected from 19 experiments in fresh ex vivo liver tissue for heating times 53-1200s. From gathered s21, we computed the average transmission coefficient (ATC) and group delay (GD) and related them to tissue state. Results: Both ATC and GD decrease with time and saturate towards the completion of ablation at ~75% and ~96% of the initial value respectively. Conclusion: We demonstrated the potential of in-procedure tissue state assessment by monitoring s21 between two directional MWA applicators in ex vivo liver.