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

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Permittivity and conductivity change drastically during microwave heating in media with high water content due to vaporization. Tissues with ~70% water content present this change during microwave ablation of cancerous tissues.

Small open-ended coaxial probes, as the most common measurement tool for complex permittivity, 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 including the vapor deposition in the tissue. Mixture models have been introduced for modeling the complex permittivity of different types in the past. Here, we utilized these models to account for vapor content in regard to dielectric properties. There was a good agreement between measured and modeled permittivity and demonstrated that CT-based model predictions of dielectric properties are feasible in ex vivo liver. The models may facilitate real-time imaging-based permittivity mapping.

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