Computed Tomography-Based Modeling of Water Vapor-Induced Changes in Permittivity During Microwave Ablation

Objective: Measurements of tissue permittivity with small open-ended coaxial probes during microwave tissue heating have been plagued by high variability as tissue water becomes vaporized. Analysis of such variability has been hampered by a lack of direct visualization of the measurement volume. The objective of this study was to determine if X-ray computed tomography (CT) could be used to visualize the measurement volume and then predict dielectric permittivity based on the visualized tissue composition. Methods: CT attenuation at 120 kVp was measured at the end of an open-ended coaxial probe during microwave ablation of ex vivo liver (2.45 GHz at 50 W delivered for 5 minutes). Tissue composition was estimated from attenuation maps and used to predict dielectric properties based on established mixture equations. Predicted permittivity was then compared to measured values using error metrics and linear regression. Results: There was a good agreement between measured and modeled permittivity during 5-minute ablations at 2.45 GHz (r = 0.94, p <; .001). Normalized root mean squared errors were below 17% in permittivity modeling at 2.45 GHz. Conclusion and significance: CT-based model predictions of dielectric properties are feasible in ex vivo liver. The models may facilitate real-time imaging-based permittivity mapping.