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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).

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 1: Schematic representation of RF pulsing algorithm. Current graphed over time demonstrates the application of RF energy with use of a pulsed algorithm. Region A denotes the period of energy application (on time), and region B denotes the cooling period (off time) that serves to prevent tissue boiling near the electrode. The successive on periods exhibit a programmed 100-mA current decrement when the on time is too short. For this particular example, a specified current of 2000 mA was programmed with a designated on time of 10 seconds. After the third pulse (A), the generator was unable to remain at maximum current for the specified on time owing to rapidly induced impedance rises. Thus, after the designated cooling time of 15 seconds (B), a current 100 mA below the maximum set current was applied to continue energy application and prevent tissue charring.

View larger version (42K): [in this window] [in a new window] [Download PPT slide]   Figure 2a: Effects of varying on times (x-axis values, in centimeters) and off times of RF application on coagulation size achieved with 2.5-cm cluster electrode. (a) Two-dimensional and (b) three-dimensional surface responses for the 2.5-cm cluster electrode demonstrate that coagulation diameters of 4.0–6.0 cm can be achieved, depending on the combination of on and off times selected. The 2.5-cm cluster electrode exhibits an optimal "ridge" at an on time range of 23–35 seconds and an off time range of 28–38 seconds. Use of this electrode yielded a mean maximum coagulation diameter of 6.5 cm ± 0.6 at pulse settings of 33 seconds on and 38 seconds off at 3000 mA.

View larger version (44K): [in this window] [in a new window] [Download PPT slide]   Figure 2b: Effects of varying on times (x-axis values, in centimeters) and off times of RF application on coagulation size achieved with 2.5-cm cluster electrode. (a) Two-dimensional and (b) three-dimensional surface responses for the 2.5-cm cluster electrode demonstrate that coagulation diameters of 4.0–6.0 cm can be achieved, depending on the combination of on and off times selected. The 2.5-cm cluster electrode exhibits an optimal "ridge" at an on time range of 23–35 seconds and an off time range of 28–38 seconds. Use of this electrode yielded a mean maximum coagulation diameter of 6.5 cm ± 0.6 at pulse settings of 33 seconds on and 38 seconds off at 3000 mA.

View larger version (18K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Effects of varying on and off times of RF application on the coagulation (radius denoted by color scale at right, in centimeters) achieved by using 4.0-cm cluster electrode. The surface response for this electrode demonstrates a zone of maximum coagulation comparable to that achieved with the 2.5-cm cluster electrode. The zone of maximum coagulation achieved with the 4.0-cm cluster electrode corresponds to an on time range of 21–27 seconds and an off time range of 22–31 seconds. Notably, this electrode produced drastically larger coagulation diameters, generating a mean maximum coagulation diameter of 8.3 cm ± 0.3 with pulse settings of 24 seconds on and 25 seconds off at 4000 mA.

View larger version (21K): [in this window] [in a new window] [Download PPT slide]   Figure 4: Effects of varying on and off times of RF application on the coagulation (radius denoted by color scale at right, in centimeters) achieved by using 3.0-cm single electrode. The surface response for this electrode exhibits a ridge structure similar to that seen for the 2.5- and 4.0-cm cluster electrodes. Smaller zones of coagulation were achieved. The zone of maximum coagulation corresponds to an on time range of 10–18 seconds and an off time range of 11–20 seconds. The 3.0-cm single electrode yielded a mean maximum coagulation diameter of 5.2 cm ± 0.1 with pulse settings of 15 seconds on and 16 seconds off at 2500 mA.