Rationale and Objectives
To evaluate the ex vivo ablation zones created in hepatic tissue using monopolar and bipolar gas- and water-cooled radiofrequency (RF) applicators.
Materials and Methods
RF ablations were performed on ex vivo bovine liver tissue using closed circuit water-cooled and closed circuit cryogenically cooled (via CO 2 enthalpy) 15-ga linear-needle applicators. Both monopolar and bipolar electrode applicators were used, with the electric current administered ranging in 50-mA increments from 1100 to 1300 mA for the monopolar case, and from 500 to 700 mA for the bipolar case. Total ablation time was 15 minutes. Six tissue samples were ablated per setting. The ablated volumes were assumed to have a three-dimensional ellipsoid shape, with one long major axis and two smaller minor axes. Gross histology was used to measure the dimensions of the ablated regions to quantify the ablated volume, the dimensions of the axis, and the ratio between the long axis and the smallest minor axis, which was termed the ellipticity index.
Results
The gas-cooled monopolar applicator achieved the largest short-axis ablation diameter (4.05 ± 0.4 cm), followed by the water-cooled monopolar applicator (3.18 ± 0.29 cm). With the bipolar applicator, the gas-cooled applicators also achieved larger short-axis ablation diameters (3.02 ± 0.15 cm) than the water-cooled applicators (2.72 ± 0.29 cm). The gas-cooled monopolar applicator also provided the largest ablation volume (42.7 ± 10.7 mL) and the most spherically shaped lesions (ellipticity index: 1.21 ± 0.10). Lesion size increased with injected current up to a threshold current of 1200/1250 mA (monopolar water-/gas-cooled) and 600/650 mA (bipolar water-/gas-cooled), but dropped at greater values.
Conclusions
Gas-cooled monopolar applicators were superior to the other tested applicators in terms of both volume and sphericity of the ablation zone.
Because the goal of achieving large and reliable ablation of lesions has not been completely achieved, new ablation methods continue to be developed. We can assume that a typical lesion created by a linear ablation probe can be modeled as a three-dimensional spheroid, which has one major and two minor axes, with the major axis lying parallel to the probe shaft. Currently, the extent of the lesion’s long axis can be controlled by choosing between different lengths of electrodes, but controlling the short axis remains an issue that limits the success of ablation therapy, especially in the case of larger lesions. Indeed, studies have shown that local tumor recurrence is more frequently observed in lesions >3 cm which have been ablated . In radiofrequency (RF) ablation, an increase in the ablation volume can be accomplished through several methods: by increasing the electrode surface, which is achieved by increasing the probe gauge , by using several applicators in a cluster , by using a multiprobe array , or by using expandable applicators . Increasing the power output can also increase the ablated volume; this can be combined with strategies for reducing probe heating, such as pulsing the energy application . Another method is to influence the energy application at the interface between the applicator and the tissue by preventing tissue dehydration and carbonization using actively cooled applicators that use closed circuit water cooling , perfusion , or cryogenic cooling .
RF ablation can be delivered using monopolar or bipolar applicators. In a monopolar setting, part of the applied energy is lost between the electrodes and may lead to heating of metallic materials, such as pacemaker electrodes and surgical clips . In addition, skin burns may appear if the grounding pads are partially detached . Bipolar applicators do not require grounding pads and have been reported to be more energy efficient .
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Materials and methods
RF System
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Experimental Setup and Design
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Ablation Assessment
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Statistical Analysis
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Results
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Table 1
Ablation Zone Characteristics
Cooling (mA) Water-perfused Gas-cooled Short Axis (mm) Long Axis (mm) Ellipticity Index Ablation Duration (seconds) Regularity Index (TDmin/TDmax) Short Axis (mm) Long Axis (mm) Ellipticity Index Ablation Duration (seconds) Regularity Index (TDmin/TDmax) Bipolar radiofrequency system 500 19 ± 1 42 ± 1 2.18 900 ± 0 0.88 ± 0.07 17 ± 1 28 ± 2 1.64 900 ± 0 0.88 ± 0.05 550 21 ± 1 40 ± 1 1.89 880 ± 49 0.94 ± 0.04 22 ± 2 35 ± 3 1.61 883 ± 41 0.96 ± 0.05 600 27 ± 3 46 ± 2 1.69 872 ± 35 0.94 ± 0.13 27 ± 3 43 ± 2 1.60 847 ± 87 0.93 ± 0.06 650 25 ± 1 44 ± 1 1.80 795 ± 120 0.96 ± 0.33 30 ± 2 43 ± 1 1.54 825 ± 63 0.94 ± 0.04 700 22 ± 1 42 ± 2 1.96 540 ± 187 0.89 ± 0.08 28 ± 3 46 ± 2 1.64 655 ± 194 0.99 ± 0.11 Monopolar radiofrequency system 1100 29 ± 1 44 ± 1 1.52 692 ± 163 0.93 ± 0.03 30 ± 2 43 ± 2 1.41 757 ± 196 0.91 ± 0.04 1150 31 ± 3 41 ± 2 1.36 482 ± 61 0.94 ± 0.04 34 ± 1 44 ± 2 1.29 900 ± 0 0.92 ± 0.06 1200 32 ± 3 51 ± 4 1.60 610 ± 148 0.95 ± 0.04 39 ± 5 49 ± 4 1.28 745 ± 180 0.93 ± 0.06 1250 27 ± 2 43 ± 3 1.59 425 ± 142 0.95 ± 0.03 41 ± 5 49 ± 2 1.21 900 ± 0 0.93 ± 0.03 1300 27 ± 2 43 ± 3 1.64 375 ± 92 0.95 ± 0.04 33 ± 5 47 ± 5 1.45 522 ± 276 0.93 ± 0.04
Table 2
Mann–Whitney U Test: Levels of Significance
Short-axis Diameters ( P Value) Ellipticity Index ( P Value) Volume ( P Value) Monopolar gas-cooled versus monopolar water-cooled.002.015.030 Monopolar gas-cooled versus bipolar gas-cooled.002.002.002 Monopolar gas-cooled versus bipolar water-perfused.002.002.002 Monopolar water-perfused versus bipolar water-perfused.026.009.041 Monopolar water-perfused versus bipolar gas-cooled .345.041 .064 Bipolar gas-cooled versus bipolar water-perfused.034 .132 .064
P-Level < .05 in bold.
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Discussion
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