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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.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.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.CT-based model predictions of dielectric properties are feasible in ex vivo liver. The models may facilitate real-time imaging-based permittivity mapping.

The purpose of this study was to develop a technique to measure the dielectric properties of biological tissues with an interstitial dipole antenna based upon previous efforts for open-ended coaxial probes. The primary motivation for this technique is to facilitate treatment monitoring during microwave tumor ablation by utilizing the heating antenna without additional intervention or interruption of the treatment. The complex permittivity of a tissue volume surrounding the antenna was calculated from reflection coefficients measured after high-temperature microwave heating by using a rational function model of the antenna’s input admittance. Three referencing liquids were needed for measurement calibration. The dielectric measurement technique was validated ex vivo in normal and ablated bovine livers. Relative permittivity and effective conductivity were lower in the ablation zone when compared to normal tissue, consistent with previous results. The dipole technique demonstrated a mean 10% difference of permittivity values when compared to open-ended coaxial cable measurements in the frequency range of 0.5–20 GHz. Variability in measured permittivities could be smoothed by fitting to a Cole–Cole dispersion model. Further development of this technique may facilitate real-time monitoring of microwave ablation treatments through the treatment applicator.

Although a surgical procedure is performed by visual inspection with histopathological assessment of the excised tumor and margins, percutaneous and noninvasive thermal ablation is performed strictly with the aid of imaging. Applicator guidance into the target zone, treatment monitoring and verification, and clinical follow-up rely on effective imaging. Detailed discussion of imaging is beyond the scope of this article, but the influence of imaging on the choice of thermal ablation or procedural approach will be discussed as needed. More information on imaging for interventional therapies can be found in other articles in this issue of IEEE Pulse.