A Newly Developed Perfused Umbrella Electrode for Radiofrequency Ablation: An Ex Vivo Evaluation Study in Bovine Liver

Perfused hypertonic-saline-augmented needle enlarges ablation zones in ex vivo porcine livers

There is a great clinical requirement to improve radiofrequency ablation (RFA) efficacy and create larger coagulation necrotic areas. The aim of the present study was to assess the ability of a hypertonic-saline (HS)-enhanced multipolar RFA technique using a perfused electrode to increase RF-created coagulation necrosis, and to compare that technique with natural saline-augmented needle and conventional multipolar RFA. A total of 18 ablations were performed in explanted porcine livers. A total of 6 thermal ablation zones were created in each of 3 groups treated with the conventional multipolar mode, the multipolar mode with 0.9% NaCl and the multipolar mode with 6% NaCl, respectively. During RFA, the dimensions and volumes of the ablation zones were compared, and gross and microscopic pathological evaluations were performed. Multipolar RFA with 6% NaCl created the largest short-axis diameters and volumes of coagulation necrosis (3.89±0.09 mm and 40.01±2.86 mm³, respectively) among the three groups (conventional group: 2.31±0.04 mm and 8.99±0.52 mm³, respectively; 0.9% NaCl solution group: 3.17±0.05 mm and 21.79±1.05 mm³, respectively). Overall, multipolar RFA with the instillation of 6% NaCl solution through an open perfusion system created a larger ablation zone compared with the conventional and 0.9% NaCl modes. Therefore, HS-enhanced multipolar RFA may be a promising approach for treating large liver tumors.

Bipolar radiofrequency ablation: development of a new expandable device

PURPOSE

To test the performance of an expandable bipolar probe as a simple technical solution for extending the coagulation volume.

METHODS

On the basis of a commercially available monopolar radiofrequency (RF) probe (LeVeen), an expandable bipolar RF probe was developed by integrating a second electrode into the probe shaft. The influence of length on the second electrode, and the distance between both electrodes and generator output was investigated by performing ten ablations for each condition on a freshly excised bovine liver. Macroscopically quantified coagulation volumes, lesion shape characteristics, and procedure durations were recorded. Results of the prototype featuring the optimal configuration were compared to the original LeVeen probe and commonly used bipolar RF probe (CelonLabPower).

RESULTS

Extension of the shaft electrode length, increasing distance between the shaft electrode and the tip electrode, and reduction of generator output resulted in increasing coagulation volumes. The coagulation volumes the prototype generated were significantly smaller and more elliptically shaped than the monopolar probe (9.4 ± 1.5 cm(3) vs. 12.1 ± 1.6 cm(3)), but were larger than the commercially available bipolar RF probe (vs. 7.3 ± 0.5). The procedure duration of the prototype was comparable to the monopolar probe (467 ± 31 s vs. 464 ± 17 s) and shorter than the bipolar probe (vs. 2009 ± 444 s). In comparison to the commercially available bipolar system, the developed prototype exhibited favorable results.

CONCLUSION

The first benchmark testing of the developed bipolar prototype had promising results. However, further optimization of the applicator design and ablation protocol is needed to enlarge the achievable coagulation volume.

Bimodal electric tissue ablation (BETA)--effect of reversing the polarity of the direct current on the size of ablation

BACKGROUND

Bimodal electric tissue ablation (BETA) is a new technique that uses the direct current in electrolysis to improve the efficacy of radio frequency (RF) ablation. It was hypothesized that attaching the cathode of the electrolytic circuit to the RF electrode will increase the tissue hydration, therefore delaying tissue desiccation during ablation. Consequently, the ablation process can continue for a longer period of time and produce larger ablations. This hypothesis was tested by reversing the polarity of the electrolytic circuit, which theoretically would cause tissue desiccation and therefore produce smaller ablations. This new setup is called reversed polarity bimodal electric ablation (RP-BEA).

MATERIALS AND METHODS

Three types of ablations standard radiofrequency ablation (RFA), BETA, and RP-BEA) were tested in a pig liver model. In BETA and RP-BEA, 9 V of direct current were provided for 10 min, after which the rf generator were switched on and both electrical circuits allowed to run concurrently. In all three setups, ablations were continued until "roll-off." The size of ablation was measured and compared with each other.

RESULTS

The duration of ablation was significantly shorted in RP-BEA compared with standard RFA and BETA (48 s verus 148 s and 84 s, respectively, P = 0.004). The sizes of ablations in RP-BEA were also significantly smaller compared with standard RFA and BETA-skin.

CONCLUSION

RP-BEA caused tissue desiccation resulting in a shorter duration of ablation and smaller ablations. Therefore, the theory that BETA increases ablation size due to the effects of increased tissue hydration around the rf electrode is correct.

Direct current combined with bipolar radiofrequency ablation: an ex vivo feasibility study

The combination of radiofrequency ablation (RFA) with direct current (DC) is a promising strategy to improve the efficiency of RFA. However, DC-enhanced monopolar RFA is limited by electrolytic injury at the positive-electrode site. The aim of this study was to investigate the feasibility of the DC-enhanced bipolar RFA. To obviate the need for the subcutaneous positive electrode, the DC circuit was combined with a commercially available bipolar RFA system, in which both poles of the DC circuit are connected to a single RF probe. DC was applied for 15 min and followed by RFA in bovine livers using the following various DC currents: (1) no DC (control), (2) 3V continued until the end of RFA, (3) 5V continued until the end of RFA, (4) 10V continued until the end of RFA, (5) 5V continued in the circuit with reversed pole, (6) 3V stopped after initiation of RFA, and (7) 5V stopped. Coagulation volume, temperatures at a distance of 5, 10, and 15 mm from the RF probe, mean amperage, ablation duration, applied energy, minimum impedance, and degree of tissue charring were assessed and compared (analysis of variance, Student-Newman-Keuls test). All combined DC and RFA groups did increase coagulation volume. The 10V continued group showed significantly lower applied energy, shortest ablation duration, highest minimum impedance, and highest degree of charring with the lowest coagulation volume (p<0.05). DC-enhanced bipolar RFA with both poles of the DC circuit on a single probe appears to be ineffective.

Algorithm optimization for multitined radiofrequency ablation: comparative study in ex vivo and in vivo bovine liver

PURPOSE

To prospectively optimize multistep algorithms for largest available multitined radiofrequency (RF) electrode system in ex vivo and in vivo tissues, to determine best energy parameters to achieve large predictable target sizes of coagulation, and to compare these algorithms with manufacturer's recommended algorithms.

MATERIALS AND METHODS

Institutional animal care and use committee approval was obtained for the in vivo portion of this study. Ablation (n = 473) was performed in ex vivo bovine liver; final tine extension was 5-7 cm. Variables in stepped-deployment RF algorithm were interrogated and included initial current ramping to 105 degrees C (1 degrees C/0.5-5.0 sec), the number of sequential tine extensions (2-7 cm), and duration of application (4-12 minutes) for final two to three tine extensions. Optimal parameters to achieve 5-7 cm of coagulation were compared with recommended algorithms. Optimal settings for 5- and 6-cm final tine extensions were confirmed in in vivo perfused bovine liver (n = 14). Multivariate analysis of variance and/or paired t tests were used.

RESULTS

Mean RF ablation zones of 5.1 cm +/- 0.2 (standard deviation), 6.3 cm +/- 0.4, and 7 cm +/- 0.3 were achieved with 5-, 6-, and 7-cm final tine extensions in a mean of 19.5 min +/- 0.5, 27.9 min +/- 6, and 37.1 min +/- 2.3, respectively, at optimal settings. With these algorithms, size of ablation at 6- and 7-cm tine extension significantly increased from mean of 5.4 cm +/- 0.4 and 6.1 cm +/- 0.6 (manufacturer's algorithms) (P <.05, both comparisons); two recommended tine extensions were eliminated. In vivo confirmation produced mean diameter in specified time: 5.5 cm +/- 0.4 in 18.5 min +/- 0.5 (5-cm extensions) and 5.7 cm +/- 0.2 in 21.2 min +/- 0.6 (6-cm extensions).

CONCLUSION

Large zones of coagulation of 5-7 cm can be created with optimized RF algorithms that help reduce number of tine extensions compared with manufacturer's recommendations. Such algorithms are likely to facilitate the utility of these devices for RF ablation of focal tumors in clinical practice.

Optimization of direct current-enhanced radiofrequency ablation: an ex vivo study

The purpose of this study was to investigate the optimal setting for radiofrequency (RF) ablation combined with direct electrical current (DC) ablation in ex vivo bovine liver. An electrical circuit combining a commercially available RF ablation system with DC was developed. The negative electrode of a rectifier that provides DC was connected to a 3-cm multitined expandable RF probe. A 100-mH inductor was used to prevent electrical leakage from the RF generator. DC was applied for 15 min and followed by RF ablation in freshly excised bovine livers. Electric current was measured by an ammeter. Coagulation volume, ablation duration, and mean amperage were assessed for various DC voltages (no DC, 2.2, 4.5, and 9.0 V) and different RF ablation protocols (stepwise increase from 40 to 80 W, 40 W fixed, and 80 W fixed). Results were compared using Kruskal-Wallis and Mann-Whitney U test. Applying DC with 4.5 or 9.0 V, in combination with 40 W fixed or a stepwise increase of RF energy, resulted in significantly increased zone of ablation size compared with 2.2 V or no DC (P = 0.009). At 4.5 V DC, the stepwise increase of RF energy resulted in the same necrosis size as a 40 W fixed protocol (26.6 +/- 3.9 vs. 26.5 +/- 4.0 ml), but ablation duration was significantly decreased (296 +/- 85 s vs. 423 +/- 104 s; P = 0.028). Mean amperage was significantly lower at 4.5 V compared with 9.0 V (P = 0.028). Combining a stepwise increase of RF energy with a DC voltage of 4.5 V is most appropriate to increase coagulation volume and to minimize procedure time.

MR monitoring of NaCl-enhanced radiofrequency ablations: observations on low- and high-field-strength MR images with pathologic correlation

PURPOSE

To test the hypothesis that magnetic resonance (MR) imaging can be used to monitor both intraparenchymal injection of NaCl solution and subsequent radiofrequency ablation (RFA) within tissues pretreated with NaCl, report the low- and high-field-strength MR appearance of NaCl-enhanced RFAs, and compare MR findings with pathologic findings.

MATERIALS AND METHODS

Ten ex vivo calf liver specimens were injected with saturated NaCl (seven were mixed with methylene blue during MR fluoroscopic monitoring) and reexamined with fast imaging with steady-state progression (FISP), true FISP, reversed FISP (PSIF), and fast spin-echo T2-weighted MR sequences. The NaCl-to-liver contrast-to-noise ratio (CNR) was calculated for various sequences, and CNRs were compared with the Student t test. Distribution on MR images was compared with the results of pathologic analysis. Forty additional in vivo monopolar RFAs were performed in paraspinal muscles of seven minipigs after animal care committee approval (10 standard control ablations, 30 were preceded by direct injection of saturated NaCl at various volumes [3-9 mL] and rates [1 or 6mL/min]). Postablation low-field-strength (n = 20) and high-field-strength (n = 20) MR examinations consisted of T2-weighted imaging, short inversion time inversion-recovery (STIR) imaging, and contrast material-enhanced T1-weighted imaging. Ablation shape, conspicuity, volume, and signal intensity were compared between the two groups and with the results of pathologic analysis. The difference in volumes with and without NaCl injection was evaluated by using two-way analysis of variance.

RESULTS

Mean CNR was highest on fast spin-echo T2-weighted images and was significantly higher for PSIF than for FISP (P < .0001) or true FISP (P = .003). NaCl distribution on MR images corresponded with the results of pathologic analysis in ex vivo livers. Interactive in vivo monitoring of NaCl injection and electrode placement was feasible. NaCl-enhanced ablations had irregular shapes, a higher CNR, and significantly larger volumes (F = 22.0; df = 1, 90; P < .00001). All ablations had intermediate or low signal intensity with high-signal-intensity rims on all images. Fluid signals overlaid NaCl-enhanced ablations on fast spin-echo T2-weighted and STIR images, particularly on high-field-strength MR images.

CONCLUSION

MR imaging can be used to reliably monitor the distribution of injected NaCl solution in tissues. Interventional MR imaging techniques can be used to guide and monitor RFAs within NaCl pretreated tissues, with good correlation with pathologic results.

Radiofrequency ablation of liver metastases-software-assisted evaluation of the ablation zone in MDCT: tumor-free follow-up versus local recurrent disease

The purpose of this study was to investigate differences in change of size and CT value between local recurrences and tumor-free areas after CT-guided radiofrequency ablation (RFA) of hepatic metastases during follow-up by means of dedicated software for automatic evaluation of hepatic lesions. Thirty-two patients with 54 liver metastases from breast or colorectal cancer underwent triphasic contrast-enhanced multidetector-row computed tomography (MDCT) to evaluate hepatic metastatic spread and localization before CT-guided RFA and for follow-up after intervention. Sixteen of these patients (65.1 + or - 10.3 years) with 30 metastases stayed tumor-free (group 1), while the other group (n = 16 with 24 metastases; 62.0 + or - 13.8 years) suffered from local recurrent disease (group 2). Applying an automated software tool (SyngoCT Oncology; Siemens Healthcare, Forchheim, Germany), size parameters (volume, RECIST, WHO) and attenuation were measured within the lesions before, 1 day after, and 28 days after RFA treatment. The natural logarithm (ln) of the quotient of the volume 1 day versus 28 days after RFA treament was computed: lnQ1//28/0(volume). Analogously, ln ratios of RECIST, WHO, and attenuation were computed and statistically evaluated by repeated-measures ANOVA. One lesion in group 2 was excluded from further evaluation due to automated missegmentation. Statistically significant differences between the two groups were observed with respect to initial volume, RECIST, and WHO (p < 0.05). Furthermore, ln ratios corresponding to volume, RECIST, and WHO differed significantly between the two groups. Attenuation evaluations showed no significant differences, but there was a trend toward attenuation assessment for the parameter lnQ28/0(attenuation) (p = 0.0527), showing higher values for group 1 (-0.4 + or - 0.3) compared to group 2 (-0.2 + or - 0.2). In conclusion, hepatic metastases and their zone of coagulation necrosis after RFA differed significantly between tumor-free and local-recurrent ablation zones with respect to the corresponding size parameters. A new parameter (lnQ1//28/0(volume/RECIST/WHO/attenuation)) was introduced, which appears to be of prognostic value at early follow-up CT.

Radiofrequency ablation of colorectal liver metastases: small size favorably predicts technique effectiveness and survival

The objective of this study was to analyze long-term results of radiofrequency thermal ablation (RFA) for colorectal metastases (MTS), in order to evaluate predictors for adverse events, technique effectiveness, and survival. One hundred ninety-nine nonresectable MTS (0.5-8 cm; mean, 2.9 cm) in 122 patients underwent a total of 166 RFA sessions, percutaneously or during surgery. The technique was "simple" or "combined" with vascular occlusion. The mean follow-up time was 24.2 months. Complications, technique effectiveness, and survival rates were statistically analyzed. Adverse events occurred in 8.1% of lesions (major complication rate: 1.1%), 7.1% with simple and 16.7% with combined technique (p = 0.15). Early complete response was obtained in 151 lesions (81.2%), but 49 lesions (26.3%) recurred locally after a mean of 10.4 months. Sustained complete ablation was achieved in 66.7% of lesions < or = 3 cm versus 33.3% of lesions > 3 cm (p < 0.0001). Survival rates at 1, 3, and 5 years were 91%, 54%, and 33%, respectively, from the diagnosis of MTS and 79%, 38%, and 22%, respectively, from RFA. Mean survival time from RFA was 31.5 months, 36.2 in patients with main MTS < or = 3 cm and 23.2 in those with at least one lesion > 3 cm (p = 0.006). We conclude that "simple" RFA is safe and successful for MTS < or = 3 cm, contributing to prolong survival when patients can be completely treated.