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High-Power Generator for Radiofrequency Ablation: Larger Electrodes and Pulsing Algorithms in Bovine ex Vivo and Porcine in Vivo Settings¹

¹ From the Laboratory for Minimally Invasive Tumor Therapy Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, Boston, MA 02215. Received December 14, 2005; revision requested February 9, 2006; revision received April 25; accepted May 31; final version accepted July 5. Supported by grants from the National Cancer Institute, National Institutes of Health, Bethesda, Md (RO1-CA87992-01A1), and Valleylab, Boulder, Colo. Address correspondence to S.N.G. (e-mail: sgoldber{at}caregroup.harvard.edu).

Purpose: To prospectively maximize the extent of tissue coagulation by using a high-power (1000-W, 4000-mA) radiofrequency (RF) generator to optimize pulsing algorithms.

Materials and Methods: The institutional animal care and use committee approved the use of the animal model in the in vivo portion of this study. RF ablations (n = 258) were performed in ex vivo bovine livers by using a 500-kHz high-power generator. Through internally cooled 3.0-cm single and 2.5- and 4.0-cm cluster electrodes, RF energy was applied for 12 minutes. For each electrode, simplex optimization was used to determine the pulsing algorithms to be used (ie, 5–50-second "on" [energy application] and 10–50-second "off" [cooling without RF heating] periods). Three-dimensional contour maps expressing the relationship between pulsing parameters and resultant coagulation were constructed. Then, 31 RF ablations were performed with optimal settings in vivo in porcine livers, and the results were compared with those obtained in control ablations performed by using a 2000-mA commercial generator. Finally, in 108 experiments, RF energy was applied in ex vivo livers for 6, 12, and 20 minutes with maximum current settings (1000–4000 mA) by using the optimal on and off settings for all three electrodes, and the results were analyzed with multivariate analysis of variance (MANOVA).

Results: For all three electrodes, a relationship between the on and off times during the pulsing cycle and the resultant coagulation was established (P < .01). With 3.0-cm single electrodes, maximum coagulation (mean, 5.2 cm ± 0.1 [standard deviation] ex vivo and 3.6 cm ± 0.2 in vivo) was achieved with pulse settings of 10–18 seconds on and 11–20 seconds off. With cluster electrodes, greater coagulation was achieved (mean, 6.5 cm ± 0.6 ex vivo and 3.9 cm ± 0.3 in vivo with 2.5-cm tip; 8.3 cm ± 0.3 ex vivo and 5.2 cm ± 0.8 in vivo with 4.0-cm tip) with optimal pulse settings. Thus, use of the high-power generator yielded substantially increased tissue coagulation in vivo compared with the coagulation achieved with the standard generator. MANOVA revealed that increased maximum current and RF ablation durations of up to 20 minutes were associated with greater coagulation, the size of which also varied according to electrode type (P < .01).

Conclusion: Markedly larger coagulation zones can be achieved with optimized high-power RF ablation. This may require longer pulsing intervals compared with those previously used.

© RSNA, 2007