Tissue Electrical Properties As a Function of Thermal Dose for Use in a Finite Element Model

Abstract A finite element model for tissue heating by electromagnetic energy was used to predict temperatures over time intervals associated with clinical treatments. Thermal and electrical properties of tissue as well as perfusion are input as functions of temperature. The software recalculates power absorption and temperature distribution based on updates of tissue properties which may change due to the thermal dose history. A damage integral overlay was implemented to predict the extent of necrosis. At frequencies below 1 MHz, there were no published data for various tissue types regarding the change in electrical properties of tissue over a range of temperatures and times. To address this, an automated laboratory was set up to characterize electrical properties of brain, liver, muscle, fat, and blood over a range of time and temperatures at 500 kHz. Although brain, fat, and blood showed a monotonic increase in conductivity with increasing temperature and time, there were breakpoints. For muscle and liver, breakpoints were seen at 60°C and 70°C, respectively, with a decrease in conductivity thereafter. Finally, the importance of considering tissue changes will be demonstrated in devices intended for use in human clinical trials, modeled with the system.