Optimization of a single insertion electrode array for the creation of clinically relevant ablations using high-frequency irreversible electroporation

Integrated Time Nanosecond Pulse Irreversible Electroporation (INSPIRE): Assessment of Dose, Temperature, and Voltage on Experimental and Clinical Treatment Outcomes

OBJECTIVE

This study sought to investigate a novel strategy using temperature-controlled delivery of nanosecond pulsed electric fields as an alternative to the 50-100 microsecond pulses used for irreversible electroporation.

METHODS

INSPIRE treatments were carried out at two temperatures in 3D tumor models using doses between 0.001 s and 0.1 s. The resulting treatment zones were quantified using viability staining and lethal electric field intensities were determined numerically. Computational modeling was then used to determine parameters necessary for INSPIRE treatments to achieve equivalent treatment zones to clinical electroporation treatments and evaluate the potential for these treatments to induce deleterious thermal damage.

RESULTS

Lethal thresholds between 1109 and 709 V/cm were found for nominal 0.01 s treatments with pulses between 350 ns and 2000 ns at physiological temperatures. Further increases in dose resulted in significant decreases in lethal thresholds. Given these experimental results, treatment zones comparable to clinical electroporation are possible by increasing the dose and voltage used with nanosecond duration pulses. Temperature-controlled simulations indicate minimal thermal cell death while achieving equivalent treatment volumes to clinical electroporation.

CONCLUSION

Nanosecond electrical pulses can achieve comparable outcomes to traditional electroporation provided sufficient electrical doses or voltages are applied. The use of temperature-controlled delivery may minimize thermal damage during treatment.

SIGNIFICANCE

Intense muscle stimulation and the need for cardiac gating have limited irreversible electroporation. Nanosecond pulses can alleviate these challenges, but traditionally have produced significantly smaller treatment zones. This study suggests that larger ablation volumes may be possible with the INSPIRE approach and that future in vivo studies are warranted.

Novel tetrapolar single-needle electrode for electrochemotherapy in bone cavities: Modeling, design and validation

Electrochemotherapy is a cancer treatment in which local pulsed electric fields are delivered through electrodes. The effectiveness of the treatment depends on exposing the tumor to a threshold electric field. Electrode geometry plays an important role in the resulting electric field distribution, especially in hard-to-reach areas and deep-seated tumors. We designed and developed a novel tetrapolar single-needle electrode for proper treatment in bone cavities. In silico and in vitro experiments were performed to evaluate the electric field and electric current produced by the electrode. In addition, tomography images of a real case of nasal cavity tumor were segmented into a 3D simulation to evaluate the electrode performance in a bone cavity. The proposed electrode was validated and its operating range was set up to 650 V. In the nasal cavity tumor, we found that the electrode can produce a circular electric field of 3 mm with an electric current of 14.1 A at 500 V, which is compatible with electrochemotherapy standards and commercial equipment.

Investigation of integrated time nanosecond pulse irreversible electroporation against spontaneous equine melanoma

Introduction

Integrated time nanosecond pulse irreversible electroporation (INSPIRE) is a novel tumor ablation modality that employs high voltage, alternating polarity waveforms to induce cell death in a well-defined volume while sparing the underlying tissue. This study aimed to demonstrate the in vivo efficacy of INSPIRE against spontaneous melanoma in standing, awake horses.

Methods

A custom applicator and a pulse generation system were utilized in a pilot study to treat horses presenting with spontaneous melanoma. INSPIRE treatments were administered to 32 tumors across 6 horses and an additional 13 tumors were followed to act as untreated controls. Tumors were tracked over a 43-85 day period following a single INSPIRE treatment. Pulse widths of 500ns and 2000ns with voltages between 1000 V and 2000 V were investigated to determine the effect of these variables on treatment outcomes.

Results

Treatments administered at the lowest voltage (1000 V) reduced tumor volumes by 11 to 15%. Higher voltage (2000 V) treatments reduced tumor volumes by 84 to 88% and eliminated 33% and 80% of tumors when 500 ns and 2000 ns pulses were administered, respectively.

Discussion

Promising results were achieved without the use of chemotherapeutics, the use of general anesthesia, or the need for surgical resection in regions which are challenging to keep sterile. This novel therapeutic approach has the potential to expand the role of pulsed electric fields in veterinary patients, especially when general anesthesia is contraindicated, and warrants future studies to demonstrate the efficacy of INSPIRE as a solid tumor treatment.

Ultra-Low Intensity Post-Pulse Affects Cellular Responses Caused by Nanosecond Pulsed Electric Fields

High-intensity nanosecond pulse electric fields (nsPEF) can preferentially induce various effects, most notably regulated cell death and tumor elimination. These effects have almost exclusively been shown to be associated with nsPEF waveforms defined by pulse duration, rise time, amplitude (electric field), and pulse number. Other factors, such as low-intensity post-pulse waveform, have been completely overlooked. In this study, we show that post-pulse waveforms can alter the cell responses produced by the primary pulse waveform and can even elicit unique cellular responses, despite the primary pulse waveform being nearly identical. We employed two commonly used pulse generator designs, namely the Blumlein line (BL) and the pulse forming line (PFL), both featuring nearly identical 100 ns pulse durations, to investigate various cellular effects. Although the primary pulse waveforms were nearly identical in electric field and frequency distribution, the post-pulses differed between the two designs. The BL's post-pulse was relatively long-lasting (~50 µs) and had an opposite polarity to the main pulse, whereas the PFL's post-pulse was much shorter (~2 µs) and had the same polarity as the main pulse. Both post-pulse amplitudes were less than 5% of the main pulse, but the different post-pulses caused distinctly different cellular responses. The thresholds for dissipation of the mitochondrial membrane potential, loss of viability, and increase in plasma membrane PI permeability all occurred at lower pulsing numbers for the PFL than the BL, while mitochondrial reactive oxygen species generation occurred at similar pulsing numbers for both pulser designs. The PFL decreased spare respiratory capacity (SRC), whereas the BL increased SRC. Only the PFL caused a biphasic effect on trans-plasma membrane electron transport (tPMET). These studies demonstrate, for the first time, that conditions resulting from low post-pulse intensity charging have a significant impact on cell responses and should be considered when comparing the results from similar pulse waveforms.

Numerical Estimation of SAR and Temperature Distributions inside Differently Shaped Female Breast Tumors during Radio-Frequency Ablation

Radio-frequency (RF) ablation is a reliable technique for the treatment of deep-seated malignant tumors, including breast carcinoma, using high ablative temperatures. The paper aims at a comparative analysis of the specific absorption rate and temperature distribution during RF ablation with regard to different female breast tumors. In the study, four tumor models equivalent to an irregular tumor were considered, i.e., an equivalent sphere and ellipsoid with the same surfaces and volumes as the irregular tumor and an equivalent sphere and ellipsoid inscribed in the irregular tumor. An RF applicator with a specific voltage, operating at 100 kHz inserted into the anatomically correct female breast, was applied as a source of electromagnetically induced heat. A conjugated Laplace equation with the modified Pennes equation was used to obtain the appropriate temperature gradient in the treated area. The levels of power dissipation in terms of the specific absorption rate (SAR) inside the naturalistically shaped tumor, together with the temperature profiles of the four simplified tumor models equivalent to the irregular one, were determined. It was suggested that the equivalent tumor models might successfully replace a real, irregularly shaped tumor, and the presented numeric methodology may play an important role in the complex therapeutic RF ablation process of irregularly shaped female breast tumors.

Modeling of a single bipolar electrode with tines for irreversible electroporation delivery

Irreversible electroporation (IRE) is a non-thermal tumor ablation technology employed to treat solid tumors not amenable to resection or thermal ablation. The IRE systems currently in clinical use deliver electrical pulses via multiple monopolar electrodes. This approach can present significant technical challenges due to the requirement for accurate placement of multiple electrodes and maintenance of parallel electrode alignment during pulse delivery. In this study, we sought to evaluate a novel IRE electrode configuration consisting of a single bipolar electrode with deployable tines. Using commercial finite element software predicted ablation outcomes, thermal damage, ablation sphericity, and energy delivery were calculated for existing monopolar and bipolar electrodes, and bipolar electrodes with either 4 or 8 deployable tines. The bipolar electrodes with tines generated larger predicted ablations compared to existing monopolar (>100%) and bipolar (>10%) arrangements, and the ablation shape using bipolar electrodes with tines were more spherical than those modeled for bipolar electrodes. Thermal damage modeled for bipolar electrodes and bipolar electrodes with tines was less than that of monopolar electrodes (using identical pulse parameters), and bipolar electrodes with tines delivered less energy than monopolar or bipolar electrodes. These studies using a single point of device insertion suggest the potential for developing alternative IRE delivery techniques, and may simplify clinical use and increase the predicted ablation shape/volume.

Feasibility of Concentric Electrodes in Contact Irreversible Electroporation for Superficial Lesion Treatment

<i>Objective:</i> Contact irreversible electroporation (IRE) is a method for ablating cells by applying electric pulses via surface electrodes in contact with a target tissue. To facilitate the application of the contact IRE to superficial lesion treatment, this study further extended the ablation depth, which had been limited to a 400-m depth in our previous study, by using concentric electrodes. <i>Methods:</i> A prototype device of concentric electrodes was manufactured using a Teflon-coated copper wire inserted in a copper tube. The ablation area was experimentally determined using a tissue phantom comprising 3D cultured fibroblasts and compared with the electric field distribution obtained using numerical analyses. </i>Results:</i> Experiments showed that cells 540 m from the surface of the tissue phantom were necrotized by the application of 150 pulses at 100 V. The outline of the ablation area agreed well with the contour line of 0.4 kV/cm acquired by the analyses. The ablation depth predicted for the concentric electrode using this critical electric field was 1.4 times deeper than that for the parallel electrode. For the actual application of treatment, a multiple-electrode device that bundles several pairs of concentric electrodes was developed, and confirmed that to be effective for treating wide areas with a single treatment. <i>Conclusion:</i> The electric field estimated by the analyses with the experimentally determined threshold confirmed that concentric electrodes could attain a deeper ablation than parallel electrodes. <i>Significance:</i> Using the concentric electrodes, we were able to localize ablation to specific target cells with much less damage to neighboring cells.

Experimental and Numerical Investigation of Parameters Affecting High-frequency Irreversible Electroporation for Prostate Cancer Ablation

While the primary goal of focal therapy for prostate cancer (PCa) is conserving patient quality of life by reducing oncological burden, available modalities use thermal energy or whole-gland radiation which can damage critical neurovascular structures within the prostate and increase risk of genitourinary dysfunction. High-frequency irreversible electroporation (H-FIRE) is a promising alternative ablation modality that utilizes bursts of pulsed electric fields (PEFs) to destroy aberrant cells via targeted membrane damage. Due to its non-thermal mechanism, H-FIRE offers several advantages over state-of-the-art treatments, but waveforms have not been optimized for treatment of PCa. In this study, we characterize lethal electric field thresholds (EFTs) for H-FIRE waveforms with three different pulse widths as well as three interpulse delays in vitro and compare them to conventional IRE. Experiments were performed in non-neoplastic and malignant prostate cells to determine the effect of waveforms on both targeted (malignant) and adjacent (non-neoplastic) tissue. A numerical modeling approach was developed to estimate the clinical effects of each waveform including extent of non-thermal ablation, undesired thermal damage, and nerve excitation. Our findings indicate that H-FIRE waveforms with pulse durations of 5 and 10 µs provide large ablations comparable to IRE with tolerable levels of thermal damage and minimized muscle contractions. Lower duration (2 µs) H-FIRE waveforms exhibit the least amount of muscle contractions but require increased voltages which may be accompanied by unwanted thermal damage.

An in vivo study of a custom-made high-frequency irreversible electroporation generator on different tissues for clinically relevant ablation zones

PURPOSE

To examine the ablation zone, muscle contractions, and temperature increases in both rabbit liver and kidney models in vivo for a custom-made high-frequency irreversible electroporation (H-FIRE) generator.

MATERIALS AND METHODS

A total of 18 New Zealand white rabbits were used to investigate five H-FIRE protocols (n = 3 for each protocol) and an IRE protocol (n = 3) for the performance of the designed H-FIRE device in both liver and kidney tissues. The ablation zone was determined by using histological analysis 72 h after treatment. The extent of muscle contractions and temperature change during the application of pulse energy were measured by a commercial accelerometer attached to animals and fiber optic temperature probe inserted into organs with IRE electrodes, respectively.

RESULTS

All H-FIRE protocols were able to generate visible ablation zones without muscle contractions, for both liver and kidney tissues. The area of ablation zone generated in H-FIRE pulse protocols (e.g., 0.3-1 μs, 2000 V, and 90-195 bursts) appears similar to that of IRE protocol (100 μs, 1000 V, and 90 pulses) in both liver and kidney tissues. No significant temperature increase was noticed except for the protocol with the highest pulse energy (e.g., 1 μs, 2000 V, and 180 bursts).

CONCLUSION

Our work serves to complement the current H-FIRE pulse waveforms, which can be optimized to significantly improve the quality of ablation zone in terms of precision for liver and kidney tumors in clinical setting.

Thermochromic Tissue Phantoms for Evaluating Temperature Distribution in Simulated Clinical Applications of Pulsed Electric Field Therapies

Background: Irreversible electroporation (IRE) induces cell death through nonthermal mechanisms, however, in extreme cases, the treatments can induce deleterious thermal transients. This study utilizes a thermochromic tissue phantom to enable visualization of regions exposed to temperatures above 60°C. Materials and Methods: Poly(vinyl alcohol) hydrogels supplemented with thermochromic ink were characterized and processed to match the electrical properties of liver tissue. Three thousand volt high-frequency IRE protocols were administered with delivery rates of 100 and 200 μs/s. The effect of supplemental internal applicator cooling was then characterized. Results: Baseline treatments resulted thermal areas of 0.73 cm2, which decreased to 0.05 cm2 with electrode cooling. Increased delivery rates (200 μs/s) resulted in thermal areas of 1.5 and 0.6 cm2 without and with cooling, respectively. Conclusions: Thermochromic tissue phantoms enable rapid characterization of thermal effects associated with pulsed electric field treatments. Active cooling of applicators can significantly reduce the quantity of tissue exposed to deleterious temperatures.

Algorithmically Controlled Electroporation: A Technique for Closed Loop Temperature Regulated Pulsed Electric Field Cancer Ablation

OBJECTIVE

To evaluate the effect of a closed-loop temperature based feedback algorithm on ablative outcomes for pulsed electric field treatments.

METHODS

A 3D tumor model of glioblastoma was used to assess the impact of 2 μs duration bipolar waveforms on viability following exposure to open and closed-loop protocols. Closed-loop treatments evaluated transient temperature increases of 5, 10, 15, or 22 °C above baseline.

RESULTS

The temperature controlled ablation diameters were conditionally different than the open-loop treatments and closed-loop treatments generally produced smaller ablations. Closed-loop control enabled the investigation of treatments with steady state 42 °C hyperthermic conditions which were not feasible without active feedback. Baseline closed-loop treatments at 20 °C resulted in ablations measuring 9.9 ± 0.3 mm in diameter while 37 °C treatments were 20% larger (p < 0.0001) measuring 11.8 ± 0.3 mm indicating that this protocol induces a thermally mediated biological response.

CONCLUSION

A closed-loop control algorithm which modulated the delay between successive pulse waveforms to achieve stable target temperatures was demonstrated. Algorithmic control enabled the evaluation of specific treatment parameters at physiological temperatures not possible with open-loop systems due to excessive Joule heating.

SIGNIFICANCE

Irreversible electroporation is generally considered to be a non-thermal ablation modality and temperature monitoring is not part of the standard clinical practice. The results of this study indicate ablative outcomes due to exposure to pulses on the order of one microsecond may be thermally mediated and dependent on local tissue temperatures. The results of this study set the foundation for experiments in vivo utilizing temperature control algorithms.

Electro-Thermal Therapy Algorithms and Active Internal Electrode Cooling Reduce Thermal Injury in High Frequency Pulsed Electric Field Cancer Therapies

Thermal tissue injury is an unintended consequence in current irreversible electroporation treatments due to the induction of Joule heating during the delivery of high voltage pulsed electric fields. In this study active temperature control measures including internal electrode cooling and dynamic energy delivery were investigated as a process for mitigating thermal injury during treatment. Ex vivo liver was used to examine the extent of thermal injury induced by 5000 V treatments with delivery rates up to five times faster than current clinical practice. Active internal cooling of the electrode resulted in a 36% decrease in peak temperature vs. non-cooled control treatments. A temperature based feedback algorithm (electro-thermal therapy) was demonstrated as capable of maintaining steady state tissue temperatures between 30 and 80 °C with and without internal electrode cooling. Thermal injury volumes of 2.6 cm³ were observed for protocols with 60 °C temperature set points and electrode cooling. This volume reduced to 1.5 and 0.1 cm³ for equivalent treatments with 50 °C and 40 °C set points. Finally, it was demonstrated that the addition of internal electrode cooling and active temperature control algorithms reduced ETT treatment times by 84% (from 343 to 54 s) vs. non-cooled temperature control strategies with equivalent thermal injury volumes.

Temperature Dependence of High Frequency Irreversible Electroporation Evaluated in a 3D Tumor Model

Electroporation is a bioelectric phenomenon used to deliver target molecules into cells in vitro and irreversible electroporation (IRE) is an emerging cancer therapy used to treat inoperable tumors in situ. These phenomena are generally considered to be non-thermal in nature. In this study, a 3D tumor model was used to investigate the correlation between temperature and the effectiveness of standard clinical IRE and high frequency (H-FIRE) protocols. It was found for human glioblastoma cells that in the range of 2 to 37 °C the H-FIRE lethal electric field threshold value, which describes the minimum electric field to cause cell death, is highly dependent on temperature. Increasing the initial temperature from 2 to 37 °C resulted in a significant decrease in lethal electric field threshold from 1168 to 507 V/cm and a 139% increase in ablation size for H-FIRE burst treatments. Standard clinical protocol IRE treatments resulted in a decrease in lethal threshold from 485 to 453 V/cm and a 7% increase in ablation size over the same temperature range. Similar results were found for pancreatic cancer cells which indicate that tissue temperature may be a significant factor affecting H-FIRE ablation size and treatment planning in vivo while lower temperatures may be useful in maintaining cell viability for transfection applications.

Electro-thermal therapy: Microsecond duration pulsed electric field tissue ablation with dynamic temperature control algorithms

Electro-thermal therapy (ETT) is a new cancer treatment modality which combines the use of high voltage pulsed electric fields, dynamic energy delivery rates, and closed loop thermal control algorithms to rapidly and reproducibly create focal ablations. This study examines the ablative potential and profile of pulsed electric field treatments delivered in conjunction with precise temperature control algorithms. An ex vivo perfused liver model was utilized to demonstrate the capability of 5000 V 2 μs duration bipolar electrical pulses and dynamic temperature control algorithms to produce ablations. Using a three applicator array, 4 cm ablation zones were created in under 27 min. In this configuration, the algorithms were able to rapidly achieve and maintain temperatures of 80 °C at the tissue-electrode interface. A simplified single applicator and grounding pad approach was used to correlate the measured ablation zones to electric field isocontours in order to determine lethal electric field thresholds of 708 V/cm and 867 V/cm for 45 °C and 60 °C treatments, respectively. These results establish ETT as a viable method for hepatic tumor treatment with ablation profiles equivalent to other energy based techniques. The single applicator and multi-applicator approaches demonstrated may enable the treatment of complex tumor geometries. The flexibility of ETT temperature control yields a malleable intervention which gives clinicians robust control over the ablation modality, treatment time, and safety profile.

Experimental High-Frequency Irreversible Electroporation Using a Single-Needle Delivery Approach for Nonthermal Pancreatic Ablation In Vivo

PURPOSE

To investigate the feasibility of single-needle high-frequency irreversible electroporation (SN-HFIRE) to create reproducible tissue ablations in an in vivo pancreatic swine model.

MATERIALS AND METHODS

SN-HFIRE was performed in swine pancreas in vivo in the absence of intraoperative paralytics or cardiac synchronization using 3 different voltage waveforms (1-5-1, 2-5-2, and 5-5-5 [on-off-on times (μs)], n = 6/setting) with a total energized time of 100 μs per burst. At necropsy, ablation size/shape was determined. Immunohistochemistry was performed to quantify apoptosis using an anticleaved caspase-3 antibody. A numerical model was developed to determine lethal thresholds for each waveform in pancreas.

RESULTS

Mean tissue ablation time was 5.0 ± 0.2 minutes, and no cardiac abnormalities or muscle twitch was detected. Mean ablation area significantly increased with increasing pulse width (41.0 ± 5.1 mm² [range 32-66 mm²] vs 44 ± 2.1 mm² [range 38-56 mm²] vs 85.0 ± 7.0 mm² [range 63-155 mm²]; 1-5-1, 2-5-2, 5-5-5, respectively; p < 0.0002 5-5-5 vs 1-5-1 and 2-5-2). The majority of the ablation zone did not stain positive for cleaved caspase-3 (6.1 ± 2.8% [range 1.8-9.1%], 8.8 ± 1.3% [range 5.5-14.0%], and 11.0 ± 1.4% [range 7.1-14.2%] cleaved caspase-3 positive 1-5-1, 2-5-2, 5-5-5, respectively), with significantly more positive staining at the 5-5-5 pulse setting compared with 1-5-1 (p < 0.03). Numerical modeling determined a lethal threshold of 1114 ± 123 V/cm (1-5-1 waveform), 1039 ± 103 V/cm (2-5-2 waveform), and 693 ± 81 V/cm (5-5-5 waveform).

CONCLUSIONS

SN-HFIRE induces rapid, predictable ablations in pancreatic tissue in vivo without the need for intraoperative paralytics or cardiac synchronization.

Simplified Non-Thermal Tissue Ablation With a Single Insertion Device Enabled by Bipolar High-Frequency Pulses

OBJECTIVE

To demonstrate the feasibility of a single electrode and grounding pad approach for delivering high frequency irreversible electroporation treatments (H-FIRE) in in-vivo hepatic tissue.

METHODS

Ablations were created in porcine liver under surgical anesthesia by adminstereing high frequency bursts of 0.5-5.0 μs pulses with amplitudes between 1.1-1.7 kV in the absence of cardiac synchronization or intraoperative paralytics. Finite element simulations were used to determine the electric field strength associated with the ablation margins (E_Lethal) and predict the ablations feasible with next generation electronics.

RESULTS

All animals survived the procedures for the protocol duration without adverse events. E_Lethal of 2550, 1650, and 875 V/cm were found for treatments consisting of 100x bursts containing 0.5 μs pulses and 25, 50, and 75 μs of energized-time per burst, respectively. Treatments with 1 μs pulses consisting of 100 bursts with 100 μs energized-time per burst resulted in E_Lethal of 650 V/cm.

CONCLUSION

A single electrode and grounding pad approach was successfully used to create ablations in hepatic tissue. This technique has the potential to reduce challenges associated with placing multiple electrodes in anatomically challenging environments.

SIGNIFICANCE

H-FIRE is an in situ tumor ablation approach in which electrodes are placed within or around a targeted region to deliver high voltage electrical pulses. Electric fields generated around the electrodes induce irrecoverable cell membrane damage leading to predictable cell death in the relative absence of thermal damage. The sparing of architectural integrity means H-FIRE offers potential advantages compared to thermal ablation modalities for ablating tumors near critical structures.

High-Frequency Irreversible Electroporation Using 5,000-V Waveforms to Create Reproducible 2- and 4-cm Ablation Zones-A Laboratory Investigation Using Mechanically Perfused Liver

PURPOSE

To investigate if high-frequency irreversible electroporation (H-FIRE) treatments can be delivered at higher voltages and with greater energy delivery rates than currently implemented in clinical irreversible electroporation protocols.

MATERIALS AND METHODS

Treatments using 3,000 V and 5,000 V were administered to mechanically perfused ex vivo porcine liver via a single applicator and grounding pad (A+GP) as well as a 4-applicator array (4AA). Integrated energized times (IET) 0.01-0.08 seconds and energy delivery rates 25-300 μs/s were investigated. Organs were preserved at 4°C for 10-15 hours before sectioning and gross analysis using a metabolic stain to identify the size and shape of ablation zones.

RESULTS

A+GP ablations measured between 1.6 cm and 2.2 cm, which did not increase when IET was increased from 0.02 seconds to 0.08 seconds (P > .055; range, 1.9-2.1 cm). Changes in tissue color and texture consistent with thermal damage were observed for treatments with energy delivery rates 50-300 μs/s, but not for treatments delivered at 25 μs/s. Use of the 4AA with a 3-cm applicator spacing resulted in ablations measuring 4.4-4.9 cm with energy delivery times of 7-80 minutes.

CONCLUSIONS

H-FIRE treatments can rapidly and reproducibly create 2-cm ablations using an A+GP configuration. Treatments without thermal injury were produced at the expense of extended treatment times. More rapid treatments resulted in ablations with varying degrees of thermal injury within the H-FIRE ablation zone. Production of 4-cm ablations is possible using a 4AA.