Optimization and characterization of a novel internally-cooled radiofrequency ablation system with optimized pulsing algorithm in an ex-vivo bovine liver

Abstract

Purpose: To prospectively characterize and optimize radiofrequency energy deposition to determine ideal parameters for achieving large ablation zones. Materials and methods: An internally-cooled RF system was used to perform 214 ablations in 72 exvivo bovine livers. Tip exposure (1-5 cm), electrode current (400-2500 mA), and application duration (3-15 min) were systematically varied. A pulsing algorithm optimized efficiency of RF deposition, including initial automatic ramping followed by adjustment in current, in response to changes in tissue impedance. Following the procedure ablation diameter and length were measured, sphericity calculated, and correlated with parameters of energy deposition and tissue temperatures. Results: Increasing electrode exposure from 1-5 cm produced linear increases in ablation diameter from 1.4 ± 0.1 to 5.3 ± 0.1 cm (y ¼ 1.1x-0.5; R 2 ¼ 0.93), and length (y ¼ 1.18x þ 0.34; R 2 ¼ 0.92). A sphericity index >0.85 was noted at optimal energy setting for electrode exposures of 1-4 cm. Maximum temperatures post-ablation increased with active tip length from 68.5 ± 4.9 C to 91.3 ± 1.5 C in a logarithmic (y ¼ 0.94ln(x)À2.75; R 2 ¼ 0.90) or power relationship between temperature and the resultant ablation diameter (y ¼ 0.27e 0.0295x ; R 2 ¼ 0.76). A tight exponential relationship (y ¼ 0.28x 0.38; R 2 ¼ 0.97) was also observed between total energy deposition and ablation diameter. Finally, a multifactor relationship of the diameter of ablation to electrode tip exposure and the time to first impedance rise was successfully modeled, with a root mean squared error of 1.9 mm and R 2 ¼ 0.95. Conclusion: Large, reproducible, and spherical ablation areas can be achieved with the novel system described, with efficient delivery of RF energy deposited into tissue. These findings may have important clinical relevance in regards to the clinical utility of RF ablation compared to other competitive forms of thermal tumor ablation.

Percutaneous radiofrequency ablation of HCC: reduced ablation duration and increased ablation size using single, internally cooled electrodes with an optimized pulsing algorithm

“…Energy and heating parameters, and the diameters (perpendicular and along the electrode axis), and sphericity of the resultant ablation were analyzed by electrode tip length as previously described [17]. The primary clinical endpoint of no evidence of local tumor progression was analyzed by tumor size ( 3 cm versus >3 cm) and time of ablation (12 min conventional standard versus shorter ablation times).…”

“…An internally cooled electrode RF system (Cambridge Interventional CRF System, Burlington, MA) was used for all patients. This consisted of a high power RF generator producing up to 2500 mA output into 50 X, a fluid pump, tubing, cables, and four large grounding pads [17]. To maximize the efficiency of energy deposition, the generator algorithm first endeavored to achieve a pre-set current based upon electrode tip length over 30 s [17], and then pulsed the electrode current in response to changes in tissue impedance.…”

“…This consisted of a high power RF generator producing up to 2500 mA output into 50 X, a fluid pump, tubing, cables, and four large grounding pads [17]. To maximize the efficiency of energy deposition, the generator algorithm first endeavored to achieve a pre-set current based upon electrode tip length over 30 s [17], and then pulsed the electrode current in response to changes in tissue impedance. While the impedance was stable, current was increased in 100 mA increments every 30 s. When impedance rose, current was halted for at least 20 s before it was resumed.…”

“…While the impedance was stable, current was increased in 100 mA increments every 30 s. When impedance rose, current was halted for at least 20 s before it was resumed. When impedance rose so rapidly that a current could not be maintained for at least 10 s, the current was reduced by 100 mA when energy deposition resumed [17].…”

“…Recently, more thorough and systematic optimization of RF pulsing algorithms has been reported in ex vivo tissues [17]. This optimization allows for the administration of a greater amount of current and provides dynamic energy deposition through an optimized pulsing algorithm using internally cooled electrode exposures of up to 5 cm.…”