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Radiofrequency Ablation: Importance of Background Tissue Electrical Conductivity—An Agar Phantom and Computer Modeling Study¹

¹ From the Minimally-Invasive Tumor Therapy Laboratory, Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 1 Deaconess Rd, WCC 308B, Boston, MA 02215. Received June 1, 2004; revision requested August 9; revision received September 20; accepted October 20. Supported by a grant from the National Cancer Institute, National Institutes of Health, Bethesda, Md (RO1-CA87992-01A1). Address correspondence to S.N.G. (e-mail: sgoldber{at}caregroup.harvard.edu).

PURPOSE: To determine whether radiofrequency (RF)-induced heating can be correlated with background electrical conductivity in a controlled experimental phantom environment mimicking different background tissue electrical conductivities and to determine the potential electrical and physical basis for such a correlation by using computer modeling.

MATERIALS AND METHODS: The effect of background tissue electrical conductivity on RF-induced heating was studied in a controlled system of 80 two-compartment agar phantoms (with inner wells of 0.3%, 1.0%, or 36.0% NaCl) with background conductivity that varied from 0.6% to 5.0% NaCl. Mathematical modeling of the relationship between electrical conductivity and temperatures 2 cm from the electrode (T_2cm) was performed. Next, computer simulation of RF heating by using two-dimensional finite-element analysis (ETherm) was performed with parameters selected to approximate the agar phantoms. Resultant heating, in terms of both the T_2cm and the distance of defined thermal isotherms from the electrode surface, was calculated and compared with the phantom data. Additionally, electrical and thermal profiles were determined by using the computer modeling data and correlated by using linear regression analysis.

RESULTS: For each inner compartment NaCl concentration, a negative exponential relationship was established between increased background NaCl concentration and the T_2cm (R² = 0.64–0.78). Similar negative exponential relationships (r² > 0.97%) were observed for the computer modeling. Correlation values (R²) between the computer and experimental data were 0.9, 0.9, and 0.55 for the 0.3%, 1.0%, and 36.0% inner NaCl concentrations, respectively. Plotting of the electrical field generated around the RF electrode identified the potential for a dramatic local change in electrical field distribution (ie, a second electrical peak ["E-peak"]) occurring at the interface between the two compartments of varied electrical background conductivity. Linear correlations between the E-peak and heating at T_2cm (R² = 0.98–1.00) and the 50°C isotherm (R² = 0.99–1.00) were established.

CONCLUSION: These results demonstrate the strong relationship between background tissue conductivity and RF heating and further explain electrical phenomena that occur in a two-compartment system.

© RSNA, 2005

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