Computer modelling of an impedance-controlled pulsing protocol for RF tumour ablation with a cooled electrode

Abstract

Purpose: To develop computer models to mimic the impedance-controlled pulsing protocol implemented in RF generators used for clinical practice of radiofrequency ablation (RFA), and to assess the appropriateness of the models by comparing the computer results with those obtained in previous experimental studies.Methods: A 12-minute RFA was modeled using a cooled electrode (17G, 3 cm tip) inserted in hepatic tissue. The short (transverse) diameter of the coagulation zone was assessed under in vivo (with blood perfusion and considering clamping) and ex vivo (at 21ºC) conditions.The computer results obtained by programming voltage pulses were compared with current pulses.Results: The differences between voltage and current pulses protocol were noticeable: using current instead of voltage allows larger coagulation zones to be created, due to the higher energy applied by current pulses. If voltage pulses are employed, the model can accurately predict number of roll-offs, although the waveform of the applied power is clearly not realistic. If current voltages are employed, the applied power waveform matches well with those reported experimentally, but there are significantly fewer roll-offs. Our computer results were overall into the ranges of experimental ones. Conclusions:The proposed models reproduce reasonably well the electrical-thermal performance and coagulation zone size obtained during an impedance-controlled pulsing protocol.

Analytical Solution for Electrical Problem Forced by a Finite‐Length Needle Electrode: Implications in Electrostimulation

“…The general electric behavior of any electrode is basically as follows: a voltage is uniformly set along the entire surface of the electrode, while both current density J and electric field E distributions are spatially non-uniform at the electrode surface, as they result from solving the Laplace equation (assuming electrical conductivity to be constant). This behavior has previously been correctly reproduced by mathematical models using numerical methods [7][8][9][10]. Since the numerical simulation has to be able to accurately reproduce the physical situation expressed by the partial differential equations that model the process, comparing their results with any existing benchmark analytical solutions would be one way of validating the numerical simulations.…”

Computational modelling of internally cooled wet (ICW) electrodes for radiofrequency ablation: impact of rehydration, thermal convection and electrical conductivity

“…The only difference in the modeling of all these cases was in the value of electrical conductivity and the corresponding modulated current value, so it was surprising that cases B, C and D presented differences with the control case but not with each other. A possible explanation for this fact may be found in [32], in which authors observed that differences in electrical performance were reduced when current was increased.…”

“…Electrical boundary conditions were: Zero current density in the transverse direction to the symmetry axis, in the tissue boundaries and inside the electrode, zero voltage in the dispersive electrode (bottom and top tissue surfaces) and current value of 1.5 A applied in the active electrode [32]. Once roll-off occurs, power is switched off for 15 s and then a new current pulse is applied until the next roll-off [33].…”

How coagulation zone size is underestimated in computer modeling of RF ablation by ignoring the cooling phase just after RF power is switched off

“…An impedance‐controlled pulsing protocol was modeled for a 240‐second ablation with a current constant of 1500 mA, which is aimed at increasing coagulation zone size, since periods of low current deposition alternate with higher peak current, allowing the tissue near the electrode to cool while the deeper tissue is heated . Radiofrequency power was switched off once the impedance exceeded a threshold, or roll‐off, which is usually 20 Ω higher than the initial value and was switched on again after 15 seconds …”

“…18 Radiofrequency power was switched off once the impedance exceeded a threshold, or roll-off, which is usually 20 Ω higher than the initial value and was switched on again after 15 seconds. 19 3 | RESULTS Figure 3 shows the mean value of the difference in damage computed along the z axis for the needle-type dry electrode and constant temperature protocol when the tissue characteristics were changed. Overall, the most important finding was that the differences between considering and ignoring the cooling phase in the computer simulations were always lower than 0.15 mm (for coagulation zone radius), which represented approximately 5% of the radius of the coagulation zone (around 3 mm).…”