Radiofrequency ablation of the porcine liver in vivo: increased coagulation with an internally cooled perfusion electrode

Linear radiofrequency ablation using dual switching-control mode achieves rapid and bloodless liver resection, an experimental research

BACKGROUND

Radiofrequency (RF)-assisted devices are widely used for hemostasis during liver resection. This study compared the use of dual switching (DS) versus single switching (SS) control modes for RF-based liver resections in a pig model.

METHODS

The RF-based system comprised a 200-W generator and three electrodes with 4-cm tips arranged in a linear configuration using an adaptor. Eight Lanyu pigs were used to assess ablation outcomes with electrode spacing of 2 or 3 cm, and ablation durations of 1.5, 2 or 3 min. All combinations were tested in DS and SS modes. Procedures were performed on left lateral, caudal and right anterior liver lobes, and after which transections were performed using a scalpel. Blood loss, complete ablation rate and ablation speed were compared.

RESULTS

DS mode was shown to induce significantly less blood loss than SS mode when the electrode spacing was set at 2 cm and the ablation duration was 2 min or 3 min (p=.010 and .012, respectively). Extended ablation duration and narrow electrode spacing tended to induce less blood loss, regardless of operating mode. Bloodless resection was achieved using DS mode with electrode spacing of 2 cm and ablation duration of 2-3 min. The highest rate of complete ablation (11.3 cm²/min) was achieved using DS mode with electrode spacing of 2 cm and ablation duration of 1.5 min.

CONCLUSION

RF-based hepatic resection using DS mode is safe and feasible, resulting in less blood loss than SS mode with a higher rate of complete ablation (i.e., superior ablation efficiency).

Pennes' bioheat equation vs. porous media approach in computer modeling of radiofrequency tumor ablation

The objective of this study was to compare three different heat transfer models for radiofrequency ablation of in vivo liver tissue using a cooled electrode and three different voltage levels. The comparison was between the simplest but less realistic Pennes' equation and two porous media-based models, i.e. the Local Thermal Non-Equilibrium (LTNE) equations and Local Thermal Equilibrium (LTE) equation, both modified to take into account two-phase water vaporization (tissue and blood). Different blood volume fractions in liver were considered and the blood velocity was modeled to simulate a vascular network. Governing equations with the appropriate boundary conditions were solved with Comsol Multiphysics finite-element code. The results in terms of coagulation transverse diameters and temperature distributions at the end of the application showed significant differences, especially between Pennes and the modified LTNE and LTE models. The new modified porous media-based models covered the ranges found in the few in vivo experimental studies in the literature and they were closer to the published results with similar in vivo protocol. The outcomes highlight the importance of considering the three models in the future in order to improve thermal ablation protocols and devices and adapt the model to different organs and patient profiles.

Role of saline concentration during saline-infused radiofrequency ablation: Observation of secondary Joule heating along the saline-tissue interface

Infusion of saline prior to radiofrequency ablation (RFA) is known to enlarge the thermal coagulation zone. The abundance of ions in saline elevate the electrical conductivity of the saline-saturated region. This promotes greater electric current flow inside the tissue, which increases the amount of RF energy deposition and subsequently enlarges the coagulation zone. In theory, infusion of higher concentration of saline should lead to larger coagulation zone due to the greater number of ions. Nevertheless, existing studies on the effects of concentration on saline-infused RFA have been conflicting, with the exact role of saline concentration yet to be fully elucidated. In this paper, computational models of saline-infused RFA were developed to investigate the role of saline concentration on the outcome of saline-infused RFA. The elevation in tissue electrical conductivity was modelled using the microscopic mixture model, while RFA was modelled using the coupled dual porosity-Joule heating model. Results obtained indicated that the presence of a concentration threshold to which no further elevation in tissue electrical conductivity and enlargement in thermal coagulation can occur. This threshold was determined to be at 15% NaCl. Analysis of the Joule heating distribution revealed the presence of a secondary Joule heating site located along the interface between wet and dry tissue. This secondary Joule heating was responsible for the enlargement in coagulation volume and its rapid growth phase during ablation.

How large is the periablational zone after radiofrequency and microwave ablation? Computer-based comparative study of two currently used clinical devices

Purpose:

To compare the size of the coagulation (CZ) and periablational (PZ) zones created with two commercially available devices in clinical use for radiofrequency (RFA) and microwave ablation (MWA), respectively.

Methods:

Computer models were used to simulate RFA with a 3-cm Cool-tip applicator and MWA with an Amica-Gen applicator. The Arrhenius model was used to compute the damage index (Ω). CZ was considered when Ω> 4.6 (>99% of damaged cells). Regions with 0.6<Ω< 2.1 were considered as the PZ (tissue that has undergone moderate sub-ablative hyperthermia). The ratio of PZ volume to CZ volume (PZ/CZ) was regarded as a measure of performance, since a low value implies achieving a large CZ while keeping the PZ small.

Results:

Ten-min RFA (51 W) created smaller periablational zones than 10-min MWA (11.3 cm³ vs. 17.2 22.9 cm³, for 60 100 W MWA, respectively). Prolonging duration from 5 to 10 min increased the PZ in MWA more than in RFA (2.7 cm³ for RFA vs. 8.3–11.9 cm³ for 60–100 W MWA, respectively). PZ/CZ for RFA were relatively high (65–69%), regardless of ablation time, while those for MWA were highly dependent on the duration (increase of up to 25% between 5 and 10 min) and on the applied power (smaller values as power was raised, 102% for 60 W vs. 81% for 100 W, both for 10 min). The lowest PZ/CZ across all settings was 56%, obtained with 100 W-5 min MWA.

Conclusions:

Although RFA creates smaller periablational zones than MWA, 100 W-5 min MWA provides the lowest PZ/CZ.

Increasing radiofrequency ablation volumes with the use of internally cooled electrodes and injected hydrochloric acid in ex vivo bovine livers

PURPOSE

We used an impedance-controlled generator with an internally cooled electrode to perform radiofrequency ablation (RFA) in ex vivo bovine livers, with a single injection of either 38.5% sodium chloride (NaCl) or 10% hydrochloric acid (HCl), to determine the relative effects of these two solutions on tissue impedance, temperature and ablation volume.

MATERIALS AND METHODS

We performed 10 ablations each with injections of NaCl (NaCl-RFA), HCl (HCl-RFA) or nothing (RFA-alone), with a power setting of 200 W for 15 minutes. We recorded tissue impedance before and after injection. We logged temperatures obtained from thermocouple probes positioned 5, 10, 15 and 20 mm from the internally cooled RF electrode. After ablation, we measured ablation zone longitudinal and transverse diameters, and we calculated a spherical ratio (SR) for each ablation.

RESULTS

Mean post-injection impedance of 30.3 (standard deviation [SD] 2.5) ohms for HCl was significantly lower than that of 55.4 (SD 3.5) ohms for NaCl (p < .001). Mean maximum temperatures recorded at each respective distance from the RFA electrode were all highest for HCl-RFA and lowest for RFA-alone (p < .001). Mean longitudinal and transverse diameters after HCl-RFA (5.50 [SD 0.25] cm and 5.28 [SD 0.22] cm, respectively) were significantly larger than those after NaCl-RFA (4.24 [SD 0.35] cm and 3.55 [SD 0.43] cm, respectively) and after RFA-alone (3.60 [SD 0.10] cm and 2.70 [SD 0.13] cm, respectively) (p < .001). Mean SR after HCl-RFA (0.93, SD 0.02) was significantly higher than mean SR after NaCl-RFA (0.76, SD 0.06) and RFA-alone (0.72, SD 0.04) (p < .001).

CONCLUSION

Monopolar, impedance-controlled RFA, with an internally cooled electrode and a single 10% HCl injection may allow larger tumors to be treated, potentially resulting in improved patient outcomes.

Comparison of switching bipolar ablation with multiple cooled wet electrodes and switching monopolar ablation with separable clustered electrode in treatment of small hepatocellular carcinoma: A randomized controlled trial

OBJECTIVE

A randomized controlled trial was conducted to prospectively compare the therapeutic effectiveness of switching bipolar (SB) radiofrequency ablation (RFA) using cooled-wet electrodes and switching monopolar (SM) RFA using separable clustered (SC) electrodes in patients with hepatocellular carcinomas (HCCs).

MATERIALS AND METHODS

This prospective study was approved by our Institutional Review Board. Between April 2014 and January 2015, sixty-nine patients with 74 HCCs were randomly treated with RFA using either internally cooled-wet (ICW) electrodes in SB mode (SB-RFA, n = 36) or SC electrodes in SM mode (SM-RFA, n = 38). Technical parameters including the number of ablations, ablation time, volume, energy delivery, and complications were evaluated. Thereafter, 1-year and 2-year local tumor progression (LTP) free survival rates were compared between the two groups using the Kaplan-Meier method.

RESULTS

In the SB-RFA group, less number of ablations were required (1.72±0.70 vs. 2.31±1.37, P = 0.039), the ablation time was shorter (10.9±3.9 vs.14.3±5.0 min, p = 0.004), and energy delivery was smaller (13.1±6.3 vs.23.4±12.8 kcal, p<0.001) compared to SM-RFA. Ablation volume was not significantly different between SB-RFA and SM-RFA groups (61.8±24.3 vs.54.9±23.7 cm3, p = 0.229). Technical failure occurred in one patient in the SM-RFA group, and major complications occurred in one patient in each group. The 1-year and 2-year LTP free survival rates were 93.9% and 84.3% in the SB-RFA group and 94.4% and 88.4% in the SM-RFA group (p = 0.687).

CONCLUSION

Both SB-RFA using ICW electrodes and SM-RFA using SC electrodes provided comparable LTP free survival rates although SB-RFA required less ablations and shorter ablation time.

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

PURPOSE

To develop computer models to mimic the impedance-controlled pulsing protocol implemented in radiofrequency (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-min RFA was modelled 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 (BP) and considering clamping) and ex vivo (at 21 °C) conditions. The computer results obtained by programming voltage pulses were compared with current pulses.

RESULTS

The differences between voltage and current pulses 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 the 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.

Switching bipolar hepatic radiofrequency ablation using internally cooled wet electrodes: comparison with consecutive monopolar and switching monopolar modes

OBJECTIVE

To evaluate whether switching bipolar radiofrequency ablation (SB-RFA) using three internally cooled wet (ICW) electrodes can induce coagulations >5 cm in porcine livers with better efficiency than consecutive monopolar (CM) or switching monopolar (SM) modes.

METHODS

A total of 60 coagulations were made in 15 in vivo porcine livers using three 17-gauge ICW electrodes and a multichannel radiofrequency (RF) generator. RF energy (approximately 200 W) was applied in CM mode (Group A, n = 20) for 24 min, SM mode for 12 min (Group B, n = 20) or switching bipolar (SB) mode for 12 min (Group C, n = 20) in in vivo porcine livers. Thereafter, the delivered RFA energy, as well as the shape and dimension of coagulations were compared among the groups.

RESULTS

Spherical- or oval-shaped ablations were created in 30% (6/20), 85% (17/20) and 90% (18/20) of coagulations in the CM, SM and SB groups, respectively (p = 0.003). SB-RFA created ablations >5 cm in minimum diameter (Dmin) in 65% (13/20) of porcine livers, whereas SM- or CM-RFA created ablations >5 cm in only 25% (5/20) and 20% (4/20) of porcine livers, respectively (p = 0.03). The mean Dmin of coagulations was significantly larger in Group C than in Groups A and B (5.1 ± 0.9, 3.9 ± 1.2 and 4.4 ± 1.0 cm, respectively, p = 0.002) at a lower delivered RF energy level (76.8 ± 14.3, 120.9 ± 24.5 and 114.2 ± 18.3 kJ, respectively, p < 0.001).

CONCLUSION

SB-RFA using three ICW electrodes can create coagulations >5 cm in diameter with better efficiency than do SM- or CM-RFA.

ADVANCES IN KNOWLEDGE

SB-RFA can create large, regular ablation zones with better time-energy efficiency than do CM- or SM-RFA.

Monopolar radiofrequency ablation using a dual-switching system and a separable clustered electrode: evaluation of the in vivo efficiency

OBJECTIVE

To determine the in vivo efficiency of monopolar radiofrequency ablation (RFA) using a dual-switching (DS) system and a separable clustered (SC) electrode to create coagulation in swine liver.

MATERIALS AND METHODS

Thirty-three ablation zones were created in nine pigs using a DS system and an SC electrode in the switching monopolar mode. The pigs were divided into two groups for two experiments: 1) preliminary experiments (n = 3) to identify the optimal inter-electrode distances (IEDs) for dual-switching monopolar (DSM)-RFA, and 2) main experiments (n = 6) to compare the in vivo efficiency of DSM-RFA with that of a single-switching monopolar (SSM)-RFA. RF energy was alternatively applied to one of the three electrodes (SSM-RFA) or concurrently applied to a pair of electrodes (DSM-RFA) for 12 minutes in in vivo porcine livers. The delivered RFA energy and the shapes and dimensions of the coagulation areas were compared between the two groups.

RESULTS

No pig died during RFA. The ideal IEDs for creating round or oval coagulation area using the DSM-RFA were 2.0 and 2.5 cm. DSM-RFA allowed more efficient RF energy delivery than SSM-RFA at the given time (23.0 ± 4.0 kcal vs. 16.92 ± 2.0 kcal, respectively; p = 0.0005). DSM-RFA created a significantly larger coagulation volume than SSM-RFA (40.4 ± 16.4 cm(3) vs. 20.8 ± 10.7 cm(3); p < 0.001). Both groups showed similar circularity of the ablation zones (p = 0.29).

CONCLUSION

Dual-switching monopolar-radiofrequency ablation using an SC electrode is feasible and can create larger ablation zones than SSM-RFA as it allows more RF energy delivery at a given time.

Evaluation of the in vivo efficiency and safety of hepatic radiofrequency ablation using a 15-G Octopus® in pig liver

OBJECTIVE

To determine in vivo efficacy of radiofrequency ablation (RFA) in porcine liver by using 15-gauge Octopus® (15-G Octopus®) electrodes to create a large coagulation.

MATERIALS AND METHODS

A total of 18 coagulations were created by using a 180-W generator and 15-G Octopus® electrodes during laparotomy, performed in 14 pigs. Coagulation necrosis was created in the pig livers by the use of one of three RFA protocols: 1) group A, monopolar RFA using a 15-G Octopus® electrode with a 5-mm inter-electrode distance (n = 4); 2) group B, monopolar RFA using a 15-G Octopus® electrode with a 10-mm inter-electrode distance (n = 6); and 3) group C, switching monopolar RFA using two 15-G Octopus® electrodes (n = 8). The energy efficiency, shape, maximum and minimum diameters (Dmx and Dmi), and the volume of the coagulation volume were measured in each group. The Summary statistics were obtained and Mann-Whitney test was were performed.

RESULTS

The mean ablated volume of each group was 49.23 cm(3) in A, 64.11 cm(3) in B, and 72.35 cm(3) in C. The mean Dmx and Dmi values were 5.68 cm and 4.58 cm in A and 5.97 cm and 4.97 cm in B, respectively. In group C, the mean diameters of Dmx and Dmi were 6.80 cm and 5.11 cm, respectively. The mean ratios of Dmi/Dmx were 1.25, 1.20, and 1.35 in groups A, B, and C, respectively. There was one animal death during the RFA procedure, the cause of which could not be subsequently determined. However, there were no other significant, procedure-related complications during the seven-hour-delayed CT scans.

CONCLUSION

RFA procedures using 15-G Octopus® electrodes are useful and safe for creating a large ablation in a single electrode model as well as in the multiple electrodes model.

Comparison of internally cooled wet electrode and hepatic vascular inflow occlusion method for hepatic radiofrequency ablation

Background/Aims

Various strategies to expand the ablation zone have been attempted using hepatic radiofrequency ablation (RFA). The optimal strategy, however, is unknown. We compared hepatic RFA with an internally cooled wet (ICW) electrode and vascular inflow occlusion.

Methods

Eight dogs were assigned to one of three groups: only RFA using an internally cooled electrode (group A), RFA using an ICW electrode (group B), and RFA using an internally cooled electrode with the Pringle maneuver (group C). The ablation zone diameters were measured on the gross specimens, and the volume of the ablation zone was calculated.

Results

The ablation zone volume was greatest in group B (1.82±1.23 cm³), followed by group C (1.22±0.47 cm³), and then group A (0.48±0.33 cm³). The volumes for group B were significantly larger than the volumes for group A (p=0.030). There was no significant difference in the volumes between groups A and C (p=0.079) and between groups B and C (p=0.827).

Conclusions

Both the usage of an ICW electrode and hepatic vascular occlusion effectively expanded the ablation zone. The use of an ICW electrode induced a larger ablation zone with easy handling compared with using hepatic vascular occlusion, although this difference was not statistically significant.

Distant infusion of saline may enlarge coagulation volume during radiofrequency ablation of liver tissue using cool-tip electrodes without impairing predictability

OBJECTIVE

Our aim was to evaluate the capability of a Cool-tip electrode to create larger coagulation volumes combined with a low-flow (0.1 mL/min) perfusion of hypertonic saline at a distance of 2 mm (hybrid applicator) without reducing either predictability or sphericity of the coagulation zone.

MATERIALS AND METHODS

A total of 48 radiofrequency ablations were performed on a total of 12 adult pigs: 24 with the Cool-tip (group 1) and 24 with the hybrid applicator (group 2). Volumes and diameters were assessed both macroscopically and with imaging techniques (ultrasound and MRI). Digital reconstruction techniques were also used. Reproducibility of the coagulations was assessed by means of the coefficient of variation.

RESULTS

The macroscopic assessment showed a significantly larger coagulation zone in group 2 than in group 1, both with (19.40 ± 11.38 cm(3) vs 9.16 ± 5.62 cm(3); p < 0.001) and without (19.54 ± 11.39 cm(3) vs 9.21 ± 5.74 cm(3); p < 0.001) digital reconstruction. Differences were also significant in the MRI assessment. The minimum transverse diameter was also significantly (p < 0.01) larger in group 2 than group 1: 2.46 ± 0.61 versus 1.86 ± 0.55 cm for macroscopic assessment, 2.33 ± 0.96 versus 1.69 ± 0.53 cm for ultrasound, and 2.41 ± 0.58 versus 1.8 ± 0.52 cm for MRI. The coefficient of variation was similar in both groups.

CONCLUSION

The results suggest that low-flow perfusion of hypertonic saline at 2 mm from a Cool-tip electrode could increase coagulation zone volume without reducing predictability.

Radioablation settings affecting the size of lesions created ex vivo in porcine livers with monopolar perfusion electrodes

RATIONALE AND OBJECTIVES

To explore the morphological characteristics of ablated lesions and find which combination of duration, temperature, and power was preferable to create largest lesion size with monopolar perfusion electrodes.

MATERIALS AND METHODS

Using monopolar perfusion electrodes to create 72 lesions in 30 excised porcine livers with radiofrequency radiation at different durations (5, 10, 15, and 20 minutes), temperatures (83 degrees C, 93 degrees C, 103 degrees C, and 113 degrees C), and powers (20, 30, and 40 W). Lesion volumes were calculated from longitudinal diameters and transverse diameters. Morphological characteristics were assessed microscopically from slides stained with hematoxylin and eosin.

RESULTS

Positive correlations were found between duration and longitudinal diameter (r = 0.66; P < .001), transverse diameter (r = 0.66; P < .001), distance of ablation beyond the electrode tip (r = 0.56; P < .001), and volume of lesions (r = 0.66; P < .001). Temperature was also positively correlated with longitudinal diameter (r = 0.70; P < .001), transverse diameter (r = 0.72; P < .001), distance of ablation beyond the electrode tip (r = 0.61; P < .001), and lesion volume (r = 0.711; P < .001). Lesion size did not increase when duration was longer than 15 minutes and temperature was higher than 103 degrees C. Power was not correlated with lesion size. Lesion size did not increase with increasing power. Macroscopically, all lesions were elliptical in cross section and appeared three zones: a central zone (I), a coagulated necrotic zone (II), and a hemorrhagic and edematous zone (III) from inside to outside. Microscopically, cells morphology and the nucleus were irregular or even disappeared in zone I. In zone II and III, cells did not appear deformation.

CONCLUSION

Duration and temperature, not power, affected lesion size. The largest lesion size was about 3.5 cm x 2.5 cm x 2.5 cm as temperature and duration was 15 minutes/103 degrees C.

Imaging to optimize liver tumor ablation

Surgical resection remains the ideal treatment for hepatocellular carcinoma and metastasis to the liver. Many alternatives are available for treatment of nonsurgical candidates. Regardless of treatment, optimizing imaging in the pretreatment, treatment and post-treatment settings is critical in order to lower the rates of local tumor progression and maximize the effectiveness of treatment that may result in prolonged survival. This article summarizes some basic imaging techniques of primary and metastatic liver tumors with a focus on how to optimize their treatment with ablation.

CT findings after radiofrequency ablation in rabbit livers: comparison of internally cooled electrodes, perfusion electrodes, and internally cooled perfusion electrodes

PURPOSE

To compare the computed tomography (CT) findings of radiofrequency (RF) ablation of rabbit livers with the use of internally cooled electrodes, perfusion electrodes, and internally cooled perfusion (ICP) electrodes.

MATERIALS AND METHODS

RF ablation zones were created in 24 rabbit livers in vivo in three groups, each treated at 30 W for 30 minutes with an electrode with a 1-cm active portion: one group was treated with an internally cooled electrode (group A), one with a perfusion electrode (group B), and one with an ICP electrode (group C). In the latter two groups, 0.9% saline solution mixed with contrast medium (9:1 ratio) was infused at a rate of 1 mL/min. Immediate CT images were ascertained in all groups and follow-up CT was performed in the latter two groups. CT findings of the RF ablation zone and surrounding liver parenchyma were evaluated and compared among groups.

RESULTS

On immediate unenhanced CT, infused contrast medium was noted in the center of the ablation zone without extravasation. Marginal linear or tubular low densities of the RF ablation zone were noted only in groups B (n=4) and C (n=3). Liver infarction was more frequently noted in groups B (n=5) and C (n=7) than in group A (n=3). Periportal tracking was noted more frequently in groups B (n=7) and C (n=8) than in group A (n=1). Pericaval or perihepatic tracking was noted only in groups B (n=3) and C (n=3). Extensive portal venous gas was noted in one animal in group C.

CONCLUSIONS

On CT, RF ablation with the perfusion or ICP electrode seemed to show more severe surrounding parenchymal changes than RF ablation with an internally cooled electrode.

Multiple-electrode radiofrequency ablation of in vivo porcine liver: comparative studies of consecutive monopolar, switching monopolar versus multipolar modes

PURPOSE

To evaluate the in vivo efficiency of 2 multiple-electrode radiofrequency (RF) systems to create confluent areas of coagulation in porcine liver, compared with consecutive overlapping ablation.

MATERIALS AND METHODS

A total of 18 coagulations were created with 3 RF devices and 3 internally cooled electrodes at laparotomy in 6 female pigs. RF was applied to the porcine livers in a consecutive, monopolar mode (group A), in a switching monopolar mode (group B), or in a multipolar mode (group C). Energy efficiency values for the RF systems, shape and dimensions, and the coefficients of variation of the coagulation zones were compared in the 3 groups.

RESULTS

The duration of the RF ablation procedures in groups A, B, and C were 36 minutes, 18 minutes, and 21.2 +/- 1.9 minutes. The average energy delivered to produce 1 cm(3) coagulation was greater in group A (5.6 +/- 2.3 kJ/cm(3)) than in group B (1.8 +/- 0.5 kJ/cm(3)) or in group C (2.0 +/- 0.8 kJ/cm(3)) (P < 0.05). The mean volumes of the coagulations in groups A, B, and C were 28.8 +/- 13.2 cm(3) in group A, 49.1 +/- 12.3 cm(3) in group B, and 40.6 +/- 16.3 cm(3) in group C, respectively (P = 0.07). Regarding the shape of the coagulations, the coagulations of groups B (isoperimetric ratio; 0.88) and C (0.84) were more spherical than those of group A (0.69) (P < 0.05). In addition, the coefficients of variation of the volumes of the ablation zones in groups A, B, and C were 0.46, 0.25, and 0.40, respectively.

CONCLUSIONS

Multiple-electrode RF systems in switching monopolar and multipolar modes more efficiently created a larger, confluent, spherical-shaped coagulation than conventional consecutive RF ablation.

Radiofrequency hepatic ablation with internally cooled electrodes and hybrid applicators with distant saline infusion using an in vivo porcine model

AIMS

Radiofrequency ablation (RFA) of tumors by means of internally cooled (ICE) or multitined expandable electrodes combined with infusion of saline into the tissue may improve results. Our aim was to determine the efficacy of a previously optimized hybrid ICE system (ICE combined with infusion of saline into the tissue at a distance of 2mm) in comparison with a conventional ICE cluster electrode in porcine liver in vivo.

METHODS

A total of 32 RFA were performed on a total of 10 farm pigs using two RFA systems: Group A (n=16): Cluster electrode. Group B (n=16): Hybrid system (with continuous infusion of 100ml/h of 20% NaCl at 2mm distance from the electrode shaft by an independent isolated needle). Livers were removed for macroscopic and histological assessment after the procedure. Coagulation volume, coagulation diameters, coefficient of variability (CV) of coagulation volume, sphericity ratio (SR), deposited power (DP), deposited energy (DE), deposited energy per coagulation volume (DEV) and rise of animal temperature during the procedure were compared and correlated among groups. Additionally, linear regression analysis was modeled to study the relationship between deposited energy and either coagulation volume and rise of animal temperature during the procedure in both groups.

RESULTS

Both coagulation volume and short diameter of coagulation were significantly greater (p<0.05) in group B compared to group A (22.7+/-11.0 cm(3) and 3.1+/-0.7 cm vs. 13.5+/-7.7 cm(3) and 2.5+/-0.5 cm, respectively). A similar CV and SR was observed among groups (57.1% and 1.4+/-0.3 vs. 48.6% and 1.3+/-0.2 for groups B and A, respectively). In group B, DE and DP were more than double group A, but DEV was nearly twice as high (9782 J/cm(3) vs. 5342 J/cm(3), for groups B and A, respectively). No significant relationship between DE and coagulation volume was encountered.

CONCLUSION

Efficacy of a single ICE may be improved with continuous infusion of saline at around 2 mm from the electrode shaft. Coagulation volume obtained with this improved system may be even greater than that obtained with a cluster electrode.

RF tumor ablation with internally cooled electrodes and saline infusion: what is the optimal location of the saline infusion?

Background

Radiofrequency ablation (RFA) of tumors by means of internally cooled electrodes (ICE) combined with interstitial infusion of saline may improve clinical results. To date, infusion has been conducted through outlets placed on the surface of the cooled electrode. However, the effect of infusion at a distance from the electrode surface is unknown. Our aim was to assess the effect of perfusion distance (PD) on the coagulation geometry and deposited power during RFA using ICE.

Methods

Experiments were performed on excised bovine livers. Perfusion distance (PD) was defined as the shortest distance between the infusion outlet and the surface of the ICE. We considered three values of PD: 0, 2 and 4 mm. Two sets of experiments were considered: 1) 15 ablations of 10 minutes (n ≥ 4 for each PD), in order to evaluate the effect of PD on volume and diameters of coagulation; and 2) 20 additional ablations of 20 minutes. The effect of PD on deposited power and relative frequency of uncontrolled impedance rises (roll-off) was evaluated using the results from the two sets of experiments (n ≥ 7 for each PD). Comparisons between PD were performed by analysis of variance or Kruskal-Wallis test. Additionally, non-linear regression models were performed to elucidate the best PD in terms of coagulation volume and diameter, and the occurrence of uncontrolled impedance rises.

Results

The best-fit least square functions were always obtained with quadratic curves where volume and diameters of coagulation were maximum for a PD of 2 mm. A thirty per cent increase in volume coagulation was observed for this PD value compared to other values (P < 0.05). Likewise, the short coagulation diameter was nearly twenty five per cent larger for a 2 mm PD than for 0 mm. Regarding deposited power, the best-fit least square function was obtained by a quadratic curve with a 2 mm PD peak. This matched well with the higher relative frequency of uncontrolled impedance rises for PD of 0 and 4 mm.

Conclusion

Saline perfusion at around 2 mm from the electrode surface while using an ICE in RFA improves deposition of energy and enlarges coagulation volume.

Switching Monopolar Radiofrequency Ablation Technique Using Multiple, Internally Cooled Electrodes and a Multichannel Generator

OBJECTIVE

We sought to determine the optimal switching time and interprobe distance for creating a large coagulation zone in the liver by switching monopolar radiofrequency ablation (RFA) technique using a prototype multichannel radiofrequency generator and multiple electrodes.

MATERIALS AND METHODS

Using 3, 17-gauge, internally cooled electrodes and a prototype multichannel radiofrequency (RF) generator to allow automatic switching of RF energy among 3 electrodes according to their impedance changes, RF energy was applied in a switching monopolar mode to explanted bovine livers or to the livers of 7 dogs. In the ex vivo study, we evaluated the effect of the switching time (5, 15, 30, and 60 seconds) and the interprobe distance (3 cm and 4 cm) on the mean volume of the ablation zone using the switching RFA; we then compared the ablation volume of the switching RFA group (12 minutes) with that of the overlapping RFA group (12 minutes x 3 = 36 minutes). In our preliminary in vivo experiments using 2 dogs, the electrodes were placed in a triangular array (spacing 2 cm, 3 cm, 3.5 cm, or 4 cm) and were activated for 12 minutes or 20 minutes in a monopolar mode (power output 200 W). In our primary experiments using 5 dogs, RFA in a switching mode created 10 coagulation zones at a 2- to 3-cm interprobe distance. The duration of the RFA was 12 minutes. The size and geometry of the coagulation zone also were assessed.

RESULTS

In this ex vivo study, switching monopolar RFA at a 3-cm interprobe distance and at a 30-second switching time allowed the creation of a large, confluent ablation zone. There was no significant difference in the mean ablation volume between the overlapping (72.9 +/- 12.6(3) cm) and the switching RFA groups (65.7 +/- 12.6 cm(3); P>0.05). In the preliminary in vivo experiments, RFA created spherical coagulations at interprobe distances of 2 cm and 3 cm but at larger distances than 3 cm, RFA created partially confluent coagulation zones. In our principal experiments, switching RFA created areas of well-defined coagulation, ie, at a 2-cm interprobe distance, the volume and short-axis coagulation diameter were 35.5+/- 5.7(3) cm and 4.6 +/- 0.5 cm, respectively, whereas at 3 cm, they were 40.7 +/- 12.8(3) cm and 4.8 +/- 0.8 cm, respectively. The mean values of the circularity (isometric ratio) of the coagulation at the 2- and 3-cm interprobe distances were 0.95 +/- 0.02, and 0.85+/- 0.06, respectively.

CONCLUSIONS

Our study demonstrated that switching monopolar RFA using the multichannel RF system at a 2- or 3-cm interprobe distance and at a 30-second switching time can create a large, confluent coagulation zone in the liver within a clinically acceptable time frame. We believe that this technology will provide a useful tool for the treatment of large liver tumors.