Expanded modeling of temperature-dependent dielectric properties for microwave thermal ablation

Abstract

Microwaves are a promising source for thermal tumor ablation due to their ability to rapidly heat dispersive biological tissues, often to temperatures in excess of 100 °C. At these high temperatures, tissue dielectric properties change rapidly and, thus, so do the characteristics of energy delivery. Precise knowledge of how tissue dielectric properties change during microwave heating promises to facilitate more accurate simulation of device performance and helps optimize device geometry and energy delivery parameters. In this study, we measured the dielectric properties of liver tissue during high-temperature microwave heating. The resulting data were compiled into either a sigmoidal function of temperature or an integration of the time–temperature curve for both relative permittivity and effective conductivity. Coupled electromagnetic–thermal simulations of heating produced by a single monopole antenna using the new models were then compared to simulations with existing linear and static models, and experimental temperatures in liver tissue. The new sigmoidal temperature-dependent model more accurately predicted experimental temperatures when compared to temperature–time integrated or existing models. The mean percent differences between simulated and experimental temperatures over all times were 4.2% for sigmoidal, 10.1% for temperature–time integration, 27.0% for linear and 32.8% for static models at the antenna input power of 50 W. Correcting for tissue contraction improved agreement for powers up to 75 W. The sigmoidal model also predicted substantial changes in heating pattern due to dehydration. We can conclude from these studies that a sigmoidal model of tissue dielectric properties improves prediction of experimental results. More work is needed to refine and generalize this model.

Review of the mathematical functions used to model the temperature dependence of electrical and thermal conductivities of biological tissue in radiofrequency ablation

“…As regards , some studies have considered a temperature-dependence based on a polynomial relation derived from NaCl solutions [21,35]. Ji and Brace [36] recently have suggested that variations in liver electrical conductivity at temperatures near 100ºC are best modeled using a sigmoid function for microwave thermal ablation. Watanabe et al [37] studied and modeled k temperature-dependence using three different piecewise functions to those used in our study.…”

Exploiting Microwave Imaging Methods for Real-Time Monitoring of Thermal Ablation

“…To this end, experimental results on the electromagnetic properties variations obtained in an ex vivo liver sample during the ablation were exploited [7][8][9].…”

“…The idea of adopting MWT for MTA monitoring relies on the above-mentioned experimental studies reporting the changes of dielectric properties of tissues during the ablation process [7][8][9]. In particular, these studies evidenced an irreversible temperature-dependent change in the dielectric parameters that can be characterized in terms of complex permittivity:…”

“…However, besides electromagnetic compatibility issues, magnetic resonance involves high cost equipment with obvious drawbacks in terms of economic sustainability, whereas the actual effectiveness of ultrasounds when temperature increases is still an open issue, since transducers are blinded by a hyperechogenic cloud caused by water vaporization as temperature increases up to about 100 ∘ C. The above reported limitations of existing imaging modalities motivate the interest in alternative techniques, and a possible candidate in this respect is microwave tomography (MWT), which images the electromagnetic properties of the tissue using low-cost and portable equipment. In fact, recent works have put into evidence tangible changes of both dielectric permittivity and electric conductivity of the tissue linked to the very high temperature increases (up to 120 ∘ C) achieved during the microwave thermal ablation procedures [7][8][9]. According to these measures, a decrease of both relative permittivity and electric conductivity occurs with the temperature.…”

Development of a Novel Switched-Mode 2.45 GHz Microwave Multiapplicator Ablation System

“…The applicator has been designed using the CST Microwave Studio Electromagnetic Simulation software (CSTComputer Simulation Technology-https://www.cst.com/). An optimization method has been employed in order to guarantee the maintenance of a good input matching (S 11 < −15 dB at 2.45 GHz) during the entire ablative procedure, taking into account the variation of the tissue (liver, kidney, and muscle) complex permittivity with temperature and time [27].…”