Changes in dielectric properties at 460 kHz of kidney and fat during heating: importance for radio-frequency thermal therapy

We have developed a system to measure the changes due to heating to high temperatures in the dielectric properties of tissues in the radio-frequency range. A two-electrode arrangement was connected to a low-frequency impedance analyser and used to measure the dielectric properties of ex vivo porcine kidney and fat at 460 kHz. This frequency was selected as it is the most commonly used for radio-frequency thermal therapy of renal tumours. Tissue samples were heated to target temperatures between 48 and 78 deg C in a hot water bath and changes in dielectric properties were measured during 30 min of heating and 15 min of cooling. Results suggest a time-temperature dependence of dielectric properties, with two separate components: one a reversible, temperature-dependent effect and the other a permanent effect due to structural events (e.g. protein coagulation, fat melting) that occur in tissues during heating. We calculated temperature coefficients of 1.3 ± 0.1% deg. C-1 for kidney permittivity and 1.6% deg. C-1 for kidney conductivity, 0.9 ± 0.1% deg. C-1 for fat permittivity and 1.7 ± 0.1% deg C-1 for fat conductivity. An Arrhenius model was employed to determine the first-order kinetic rates for the irreversible changes in dielectric properties. The following Arrhenius parameters were determined: an activation energy of 57 ± 5 kcal mol-1 and a frequency factor of (6 ± 1) x 1034 s-1 for conductivity of kidney, an activation energy of 48 ± 2 kcal mol-1 and a frequency factor of 6 x 1028 s-1 for permittivity of kidney. A similar analysis led to an activation energy of 31 ± 4 kcal mol-1 and a frequency factor of (4.43 ±1) x 1016 s-1 for conductivity of fat, and an activation energy of 40 ± 4 kcal mol-1 and a frequency factor of 4 x 1022 s-1 for permittivity of fat. Structural events occurring during heating at different target temperatures as determined by histological analyses were correlated with the changes in the measured dielectric properties