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Radiofrequency Ablation: Modeling the Enhanced Temperature Response to Adjuvant NaCl Pretreatment¹

¹ From the Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, Boston, MA 02215. Supported by grants from the National Cancer Institute, National Institutes of Health, Bethesda, Md (RO1-CA87992–01A1), and Radionics/Tyco Healthcare, Burlington, Mass. Received November 19, 2002; revision requested February 6, 2003; revision received March 5; accepted May 27. Address correspondence to S.N.G. (e-mail: sgoldber@caregroup.harvard.edu).

PURPOSE: To characterize the effects of volume and concentration of adjuvant NaCl pretreatment on radiofrequency (RF) ablation and to model these results to determine their applicability to in vivo systems.

MATERIALS AND METHODS: Standardized 1-L 5% agar phantoms were constructed with central wells of varying volume that were filled with a protein-based polymer gel of varying NaCl concentration (0%–35%). RF ablation to the maximum system current output (2,000 mA) was applied to internally cooled 2-cm electrodes placed in the center of the gel wells. Remote thermometry was performed 20 mm from the electrode. Temperatures generated within the phantom were then used to model the response surface by using regression analysis. The generated model was then applied to previously published in vivo data to determine its applicability to a porcine liver tissue model. Statistical analyses included one-way analysis of variance to compare the temperatures reached with different NaCl concentrations and volumes with those reached without NaCl. In addition, modeled functions were evaluated for goodness of fit and the statistical significance of their coefficients.

RESULTS: NaCl volume and concentration had significant effects on RF-generated heating of the agar phantoms. The mean maximum temperature, 91.4^oC ± 0.8 (SD), was reached with 3.5 mL of 10% NaCl gel. This was significantly higher than the mean temperature reached in phantoms containing 0% NaCl gel, 40.3^oC ± 4.9 (P < .001). Heat increases to the maximum temperature correlated strongly with the deposited RF energy, with maximum temperatures limited by the current output of the RF generator. The response surface was defined by a generator energy–dependent region and a generator current–limited region, which were best modeled by a modified gamma-variate function and an exponential function, respectively (r² = 0.92). This model correlated well with previously published in vivo data (r² = 0.86).

CONCLUSION: Modulation of electrical conductivity has different effects on RF ablation response that are dependent on generator capabilities and the volume and concentration of NaCl pretreatment.

© RSNA, 2004

Index terms: Computers, simulation • Experimental study • Neoplasms, therapy, _**.1299² • Radiofrequency (RF) ablation, _**.1299

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