Effect of Pulse Widths and Cycles on Invasive, Bipolar, and Gated Radiofrequency-Induced Thermal Reactions in ex vivo Bovine Liver Tissue

Abstract

Background Radiofrequency (RF) oscillations generate thermal tissue reactions, the patterns of which vary depending on the mode and efficiency of energy delivery. The aim of our study was to analyze patterns of RF-induced thermal tissue reactions according to the modes of RF delivery, including continuous and gated modes, using an alternating current, invasive bipolar RF device. Methods RF energies at frequencies of 1 and 2 MHz were delivered at respective experimental settings into ex vivo bovine liver tissue at a 0.5-mm microneedle penetration depth. The tissue samples were then evaluated thermometrically. A histologic study was performed to evaluate RF-induced thermal tissue reactions at a 3.0-mm microneedle penetration depth. Results Thermal imaging study revealed homogenous, well-demarcated, square-shaped zones of RF-induced thermal reactivity on the treated area. Multivariate linear regression analysis revealed that higher temperature elevations immediately after RF treatment (∆T 1 ) were positively associated with RF frequency, power, conduction time/pulse pack, and off-time between pulse packs and negatively associated with total off time. In the 1-MHz experimental setting, higher ∆T 1 showed a positive association with power, conduction time/pulse pack, and off-time between pulse packs and a negative association with the number of pulse packs. In the 2-MHz setting, however, higher ∆T 1 was positively associated with only total treatment time. Conclusion Thermometric effects during bipolar and gated RF treatments are significantly associated with the frequency, power, and pulse widths and cycles of pulse packs.

Immediate and Late Effects of Pulse Widths and Cycles on Bipolar, Gated Radiofrequency-Induced Tissue Reactions in in vivo Rat Skin

“…Our previous thermometric pilot study using an ex vivo bovine liver model revealed that longer off-times between pulses increased tissue temperature and resulted in greater histological thermal tissue reactions than shorter off-times. 6 The thermometric study used RF conduction times ranging from 100 to 800 ms for each pulse pack. 6 In this study, we found that shorter conduction times with multiple pulse packs and longer off-times between pulse packs generated remarkable non-necrotic thermal tissue reactions, which were associated with increased collagen bundles in the upper and deeper dermis.…”

“… 6 The thermometric study used RF conduction times ranging from 100 to 800 ms for each pulse pack. 6 In this study, we found that shorter conduction times with multiple pulse packs and longer off-times between pulse packs generated remarkable non-necrotic thermal tissue reactions, which were associated with increased collagen bundles in the upper and deeper dermis. Particularly, only a few histological preparations exhibited peri-electrode coagulative necrosis zones.…”

“… 6 Therein, the effects of variables, including RF frequency, power, the number of pulse packs, conduction time/pulse, off-time between pulse packs, total conduction time, total off-time, and total treatment time, on RF-induced thermal tissue reactions were statistically analyzed using univariate and multivariate linear regression. 6 Interestingly, the thermometric data of an ex vivo bovine liver model treated with 1-MHz RF was significantly affected by regulating the level of power, the numbers of pulse packs, conduction time/pulse pack, and off-time between pulse packs. 6 Meanwhile, the experimental setting of total treatment time, which referred to the sum of total RF conduction time and total off-time, was the single most important factor for generating 2-MHz RF-induced thermal reactions in an ex vivo bovine liver tissue model.…”

“… 6 Meanwhile, the experimental setting of total treatment time, which referred to the sum of total RF conduction time and total off-time, was the single most important factor for generating 2-MHz RF-induced thermal reactions in an ex vivo bovine liver tissue model. 6 …”

“…Therefore, the RF frequency of 1 MHz was chosen because the thermometric outcomes of 1-MHz RF treatments were closely associated with the pulse widths and cycles of pulse packs as shown in our ex vivo pilot study. 6 Then, the total treatment time was experimentally regulated to 200, 400, 600, and 800 ms, and the number of pulse packs therein was set to 1, 4, 7, and 10. Once the microneedle electrodes were inserted into the in vivo rat skin using a step motor at the penetration depth of 1.5 mm, RF was delivered into the dermis with single to multiple cycles of pulse packs.…”