A Comprehensive Characterization of Parameters Affecting High-Frequency Irreversible Electroporation Lesions Tyler Miklovic1, Eduardo L. Latouche1, Matthew R. DeWitt1, Rafael V. Davalos1, Michael B. Sano 2,3,* 1. 2.
Virginia Tech – Wake Forest School of Biomedical Engineering and Sciences Stanford University School of Medicine, Department of Radiation Oncology 3. UNC – NCSU Joint Department of Biomedical Engineering
Abbreviated Title: Comprehensive Characterization of H-FIRE Lesions *Correspondences should be addressed to Michael B. Sano at UNC – NCSU Joint Department of Biomedical Engineering Electronic mail:
[email protected]
Supplemental Data Calculation of Power Spectrum The power spectrum for IRE and H-FIRE waveforms were created using MATLAB (Version R2017a, MathWorks Inc.,
Natick, MA). For both IRE and H-FIRE an entire treatment was recorded using a high speed data acquisition system and saved to a spreadsheet. Each file consisted of approximately 756,000 data points collected with a time step between 0.010 and 0.012 µs. A standard algorithm for computing the Fast Fourier Transform (fft) was used to calculate the power spectrum for each treatment. Data was then normalized to 1 by dividing each data point by the maximum value in each set. The IRE treatment consisted of 100x monopolar pulses each 50 µs in duration. The H-FIRE treatment consisted of 100x bipolar bursts using the 1-1-1 waveform which was energized for a total of 50 µs. The data presented in Supplemental Figure 1 A show that IRE and H-FIRE treatments have unique power spectrums with IRE primarily delivering energy below 10 kHz and H-FIRE above 100 kHz. The electrical conductivities of mammalian tissues are dynamic and a function of the electrical frequency 1. When normalized to the conductivity of healthy kidney tissue, the electrical conductivity of skin, tendons, and blood vessels begin to converge as frequency increases towards 1 MHz (Supplemental Figure 1 B). This converging of electrical
Supplemental Figure 1: IRE and H-FIRE deliver energy within different regions of the frequency spectrum. (A) The Fourier transform of the 50 µs IRE (black) and 1 µs H-FIRE (red) treatment recordings show their respective power spectrums. IRE pulses deliver most of their energy in the spectrum below 10 kHz while HFIRE pulses deliver most of their energy between 100 kHz and 1 MHz. (B) The electrical conductivity of skin, fat, blood vessels, and tendons relative to kidney tissue as a function of frequency. The electrical properties of these tissues (excluding fat) begin to converge around 1 MHz. These connective tissues are less likely to create distortions in the electric field distribution when H-FIRE waveforms are delivered, simplifying treatment planning. Data in (B) adapted from Gabriel et al.1
Supplemental Figure 2: Ablation zones did not grow substantially over time due to melanin diffusion. Ablations were evaluated at (A) 30 and then (B) 36 hours post treatment. No change in the outer boundary of the ablation was observed, indicating that there is minimal diffusion of reactive species outside of the originally defined lesion boundary. Tissue darkening at the center of the ablation occurred rapidly between 30 and 36 hours as those regions were now exposed to additional oxygen and browning of the untreated tissue also occurred.
The effect of conductivity changes due to electroporation were modeled using a changing conductivity function using values determined experimentally by Neal et al.5 with the initial and final conductivity set to 0.4113 and 0.927 S/m, respectfully. To estimate the potential effects of high voltage pulses, a voltage of 5000 V was applied to the boundaries of the top electrode on the BP probe and the bottom electrode boundaries were set to ground (0 V). Previous studies indicate that the threshold for inducing muscle contractions with a 100 µs duration pulse is approximately 2 V/cm while 1 µs pulses have an approximate threshold of 100 V/cm 6. For IRE pulses (Supplemental Figure 4A), a very large volume of the simulated abdominal cavity is exposed to an electric field which is sufficiently high to induce muscle contractions. In contrast, for H-FIRE pulses, a relatively small volume of tissue is elevated above the 100 V/cm threshold for inducing muscle contractions (Supplemental Figure 4B). The significant increase in muscle contraction threshold paired with a relatively small increase in lethal threshold indicates that clinically relevant ablations can be created without inducing the extreme muscle contractions seen in typical IRE procedures. This may also mitigate the likelihood of inducing cardiac arrhythmias and enable treatment without the need for chemical paralytics or cardiac synchronization. Supplemental Figure 3: Lesion volume was calculated by representing the lesion as an ellipsoid. 48 hours after treatment, the potatoes were sliced along the electrode insertion paths (A,C) and the length and height of the ablation was measured. Then, they were sliced perpendicular to the electrode insertion path at the widest point (C,D) to measure the depth of the ablation. This data was used to calculate the ablation volume and to determine the lethal electric field in each dimension.
properties theoretically minimizes distortions in the electric field created by tissue heterogeneities2 which may drastically simplify treatment planning for H-FIRE treatments. Numerical Prediction of Muscle Stimulation Visualizations of the estimated volume of tissue stimulated by IRE and H-FIRE pulses were created using finite element models presented previously3-4. Briefly, models were constructed in COMSOL Multiphysics (V5.2a, COMSOL Inc., Palo Alto, CA). A two dimensional axisymmetric geometry was used to simulate a three dimensional domain (Supplemental Figure 3). A cylindrical geometry representing an abdominal cavity had a radius of 20 cm and height of 40 cm. 1.26 mm diameter cylindrical geometries were used to simulate the probe body (σ = 1x10-12 S/m) and exposed electrodes (σ = 4.032x106 S/m) a clinically available electrode for the NanoKnife (AngioDynamics Inc., Latham, NY) ablation system: the “bipolar probe” (BP probe) which contains two active 0.7 cm long electrodes separated by 0.8 cm of insulating material.
Supplemental Figure 4: H-FIRE waveforms stimulate a smaller volume of muscle tissue than traditional IRE pulses. Finite element simulations show the volume of tissue above the muscle contraction threshold for (A) 100 µs monopolar pulses (2 V/cm) and (B) 1 µs bipolar pulses (100 V/cm) when 5kV is applied to the NanoKnife Bipolar Probe in a 40 cm diameter tissue model. Red areas indicate tissue which is above the muscle contraction threshold for these pulse durations. The 1 µs pulse group has a significantly smaller volume of tissue which would be stimulated and in general it is likely that this volume would likely remain inside the organ being treated.
Supplemental References 1. Gabriel, C. Compilation of the Dielectric Properties of Body Tissues at RF and Microwave Frequencies; DTIC Document: 1996. 2. Arena, C. B.; Sano, M. B.; Rylander, M. N.; Davalos, R. V., Theoretical considerations of tissue electroporation with high-frequency bipolar pulses. IEEE Trans Biomed Eng 2011, 58 (5), 1474-82. 3. Sano, M. B.; Fan, R. E.; Xing, L., Asymmetric Waveforms Decrease Lethal Thresholds in High Frequency Irreversible Electroporation Therapies. Scientific Reports 2017, 7. 4. Sano, M. B.; Fan, R. E.; Hwang, G. L.; Sonn, G. A.; Xing, L., Production of Spherical Ablations Using Nonthermal Irreversible Electroporation: A Laboratory Investigation Using a Single Electrode and Grounding Pad. Journal of vascular and interventional radiology : JVIR 2016, 27 (9), 1432-1440 e3. 5. Neal, R. E.; Millar, J. L.; Kavnoudias, H.; Royce, P.; Rosenfeldt, F.; Pham, A.; Smith, R.; Davalos, R. V.; Thomson, K. R., In vivo characterization and numerical simulation of prostate properties for non‐thermal irreversible electroporation ablation. The Prostate 2014, 74 (5), 458-468. 6. Rogers, W. R.; Merritt, J. H.; Comeaux, J. A.; Kuhnel, C. T.; Moreland, D. F.; Teltschik, D. G.; Lucas, J. H.; Murphy, M. R., Strength-duration curve for an electrically excitable tissue extended down to near 1 nanosecond. IEEE transactions on plasma science 2004, 32 (4), 1587-1599.
