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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).

View larger version (103K): [in a new window]   Figure 1. Experimental apparatus. An internally cooled RF electrode (white arrow) has been inserted into a NaCl gel–filled well within an agar phantom. The RF electrode has been placed in a saline bath at a fixed distance from the grounding pad (G). Thermocouple probes (solid black arrows) have been inserted to measure temperature. An acrylic guide (open arrow) ensures proper positioning of the thermocouple. The RF generator and a temperature measurement device can be seen in the background.

View larger version (27K): [in a new window]   Figure 2. Graph of all experimental data from the agar phantoms depicts effect of background tissue conductivity on RF heating. Negative exponential relationships between temperature and background NaCl concentration are seen for each different inner compartment NaCl concentration. T2cm(°C) represents the temperature 2.0 cm from the electrode 12 minutes into RF ablation.

View larger version (24K): [in a new window]   Figure 3a. Results of computer-simulated RF heating. (a) Graph shows effect of background heating [(O)] in siemens (S) per meter on T2cm for three different inner conductivities [(I)]. (b) Graph shows radius of several isotherms generated around the inner compartment that had an inner conductivity of 1.7 siemen/m and demonstrates the distance at which various thermal isotherms can be found in relation to the midpoint of the RF electrode. Both T2cm and the isotherms follow a negative power function based on the inner and outer electrical conductivity.

View larger version (23K): [in a new window]   Figure 3b. Results of computer-simulated RF heating. (a) Graph shows effect of background heating [(O)] in siemens (S) per meter on T2cm for three different inner conductivities [(I)]. (b) Graph shows radius of several isotherms generated around the inner compartment that had an inner conductivity of 1.7 siemen/m and demonstrates the distance at which various thermal isotherms can be found in relation to the midpoint of the RF electrode. Both T2cm and the isotherms follow a negative power function based on the inner and outer electrical conductivity.

View larger version (24K): [in a new window]   Figure 4. Comparison of experimental and computer-generated data. Background electrical conductivity (normalized in terms of a relative based on data in the Table) is compared with T2cm for three inner compartment NaCl concentrations. Left: 0.3% NaCl (inner conductivity [(I)], 1.7 siemen [S]/meter). Middle: 1.0% NaCl (inner conductivity, 4.5 siemen/m). Right: 36.0% NaCl (inner conductivity, 45 siemen/m). The error bars for experimental data represent standard deviations. For the 0.3% and 1.0% inner compartment NaCl concentrations, most data points fall within a single standard deviation. The curves represent the power function expressing the experimental data.

View larger version (38K): [in a new window]   Figure 5a. (a) Graph shows electrical field distributions and temperatures around an RF electrode for two-compartment RF ablation. For all three cases, the electrical conductivity of the 1.0-cm-radius inner compartment [(I)] is held constant at 4.5 siemen (S)/m (or 1.0% NaCl). When the background tissue conductivity [(O)] is equivalent to that of the inner compartment at 4.5 siemen/m, there is a smooth continuous decrease in the electrical field distribution around the electrode (green line with open symbols). However, with decreasing background electrical conductivity, a second electrical field peak (arrow) is identified at the interface between the inner and outer electrical conductivity boundaries. This increased electrical field distribution is associated with increasing temperatures (as shown by the curves with solid symbols). (b) In this graph, for the magenta curves, the inner compartment electrical conductivity has been maximized at 45 siemen/m, whereas the outer background conductivity has been minimized at 0.2 siemen/m. This causes the secondary interface electrical peak to nearly double in intensity, at a cost of reducing the inner electrical conductivity peak (a phenomenon requiring further study). This shift in electrical energy distribution alters the thermal profile, as demonstrated by a much shorter but much wider thermal distribution. (c) In this graph, there is reversal of the electrical conductivity parameters so that outer conductivity is markedly elevated compared with inner compartment conductivity. This results in a negative inflection at the interface between the compartments that reduces temperature deeper in the tissue. For all graphs, open data points represent the electrical field, whereas the solid data points represent the temperature distribution.

View larger version (36K): [in a new window]   Figure 5b. (a) Graph shows electrical field distributions and temperatures around an RF electrode for two-compartment RF ablation. For all three cases, the electrical conductivity of the 1.0-cm-radius inner compartment [(I)] is held constant at 4.5 siemen (S)/m (or 1.0% NaCl). When the background tissue conductivity [(O)] is equivalent to that of the inner compartment at 4.5 siemen/m, there is a smooth continuous decrease in the electrical field distribution around the electrode (green line with open symbols). However, with decreasing background electrical conductivity, a second electrical field peak (arrow) is identified at the interface between the inner and outer electrical conductivity boundaries. This increased electrical field distribution is associated with increasing temperatures (as shown by the curves with solid symbols). (b) In this graph, for the magenta curves, the inner compartment electrical conductivity has been maximized at 45 siemen/m, whereas the outer background conductivity has been minimized at 0.2 siemen/m. This causes the secondary interface electrical peak to nearly double in intensity, at a cost of reducing the inner electrical conductivity peak (a phenomenon requiring further study). This shift in electrical energy distribution alters the thermal profile, as demonstrated by a much shorter but much wider thermal distribution. (c) In this graph, there is reversal of the electrical conductivity parameters so that outer conductivity is markedly elevated compared with inner compartment conductivity. This results in a negative inflection at the interface between the compartments that reduces temperature deeper in the tissue. For all graphs, open data points represent the electrical field, whereas the solid data points represent the temperature distribution.

View larger version (30K): [in a new window]   Figure 5c. (a) Graph shows electrical field distributions and temperatures around an RF electrode for two-compartment RF ablation. For all three cases, the electrical conductivity of the 1.0-cm-radius inner compartment [(I)] is held constant at 4.5 siemen (S)/m (or 1.0% NaCl). When the background tissue conductivity [(O)] is equivalent to that of the inner compartment at 4.5 siemen/m, there is a smooth continuous decrease in the electrical field distribution around the electrode (green line with open symbols). However, with decreasing background electrical conductivity, a second electrical field peak (arrow) is identified at the interface between the inner and outer electrical conductivity boundaries. This increased electrical field distribution is associated with increasing temperatures (as shown by the curves with solid symbols). (b) In this graph, for the magenta curves, the inner compartment electrical conductivity has been maximized at 45 siemen/m, whereas the outer background conductivity has been minimized at 0.2 siemen/m. This causes the secondary interface electrical peak to nearly double in intensity, at a cost of reducing the inner electrical conductivity peak (a phenomenon requiring further study). This shift in electrical energy distribution alters the thermal profile, as demonstrated by a much shorter but much wider thermal distribution. (c) In this graph, there is reversal of the electrical conductivity parameters so that outer conductivity is markedly elevated compared with inner compartment conductivity. This results in a negative inflection at the interface between the compartments that reduces temperature deeper in the tissue. For all graphs, open data points represent the electrical field, whereas the solid data points represent the temperature distribution.

View larger version (24K): [in a new window]   Figure 6a. Graphs show correlation of E-peaks (the second electrical field peak identified at the interface between inner and outer compartments of varied electrical conductivity) to tissue temperatures during RF ablation. (a) For this comparison of E-peak to temperatures at a fixed 2 cm from the electrode, there is divergence of the slopes of the linear correlation. (b) For this comparison of E-peak to the 50°C isotherm, the slopes converge. This difference can be attributed to the complex heating patterns generated during RF ablation in a system that has two or more compartments of varied electrical conductivity (Fig 5). S = siemen, (I) = inner conductivity.

View larger version (23K): [in a new window]   Figure 6b. Graphs show correlation of E-peaks (the second electrical field peak identified at the interface between inner and outer compartments of varied electrical conductivity) to tissue temperatures during RF ablation. (a) For this comparison of E-peak to temperatures at a fixed 2 cm from the electrode, there is divergence of the slopes of the linear correlation. (b) For this comparison of E-peak to the 50°C isotherm, the slopes converge. This difference can be attributed to the complex heating patterns generated during RF ablation in a system that has two or more compartments of varied electrical conductivity (Fig 5). S = siemen, (I) = inner conductivity.