Dielectric Properties of Ex Vivo Porcine Liver Tissue Characterized at Frequencies Between 5 and 500 kHz When Heated at Different Rates

Objective: The released energy during radio frequency thermal ablation therapy changes the dielectric properties of biological tissues. Understanding changes of dielectric properties of biological tissues during heating is fundamental to suitably model the medical procedure. The aim of this work is to obtain the thermal dependences of conductivity and permittivity of ex vivo porcine liver tissue at six frequencies from 5 to 500 kHz, during heating from 37 °C to 100 °C at three heating rates of approximately 0.1, 3, and 10 °C/min. Methods: Two experimental setups using different heating sources and a four-needle electrode connected to an impedance analyzer were developed to evaluate the thermal dependencies. Results: The results at a body temperature of 37 °C show a good agreement with the data reported in the literature. The conductivity initially shows an increase followed by a decrease, whereas the permittivity increases before a subsequent sharp decrease. Above 60 °C, different trends are observed for the three heating rates studied. Conclusion: The electric conductivity and permittivity show a similar behavior at all evaluated frequencies and heating rates. The observed abrupt change of the slope near 45 °C at a slow heating rate may be used to identify the region of reversible changes in the tissue. Significance: These results confirm the connection among tissue dielectric properties, working frequency, and exposure time with thermal damage during heating.