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 (D_min) 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 D_min 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.Radiofrequency (RF) tumour ablation is increasingly being utilized as an alternative option in patients with unresectable primary and secondary liver malignancies.^1,2 In the treatment of small hepatocellular carcinomas (HCCs), RF ablation (RFA) has been shown to yield satisfactory local tumour control, with one study pathologically demonstrating complete tumour necrosis in 83% of HCCs <3 cm.³ Indeed, according to the recent Barcelona Clinic Liver Cancer staging and treatment strategy guidelines for HCCs, RFA is favoured over surgical resection for very early stage HCCs (single nodule <2 cm) in patients with Child–Pugh A liver cirrhosis.⁴ Furthermore, a recent systematic review paper by Cucchetti et al⁵ reported that for very early HCCs (single nodule <2 cm) in Child–Pugh Class A patients, RFA provided similar life expectancy and quality-adjusted life expectancy at a lower cost than did surgical resection.However, for single HCCs 3–5 cm in diameter, resection was shown to provide better life expectancy and to be more cost effective than RFA owing to high local tumour progression rates after RFA.^5–12 This is in large part owing to the limited ability of currently available RFA devices in creating a sufficiently large ablation zone encompassing HCCs 3–5 cm in diameter along with a safety margin.^7,11,13,14 Therefore, an ideal RFA system would provide the capability to create coagulations >5 cm in short-axis diameter within a reasonable time frame (<30 min) for the treatment of tumours >3 cm in diameter considering a sufficient safety margin (5–10 mm in thickness). Currently, multiple overlapping ablations are often used for the treatment of liver tumours >2 cm in order to cover the complete tumour volume as well as to create a 1-cm-thick peripheral ablation margin.^15,16 However, there is considerable technical difficulty in probe repositioning during overlapping ablations, especially under ultrasound guidance, owing to gas bubble formations, ultimately resulting in incomplete ablations.^17–19Recently, multiple-electrode RFA approaches, including the switching monopolar (SM) mode, bipolar mode and multipolar mode, have been attempted with each demonstrating efficiency in creating a larger ablation zone in liver tissue than in the standard monopolar RF technique.^2,20–26 Theoretically, RFA in switching bipolar (SB) mode using multiple electrodes should further improve the thermal and electronic efficiency of RFA devices compared to conventional monopolar modes. However, until now, the efficacy of SB-RFA with internally cooled wet (ICW) electrodes, which allow simultaneous internal cooling and saline infusion, in creating 3- to 5-cm coagulation areas, have not been tested in previous in vivo studies.Therefore, the purpose of this study was to evaluate whether SB-RFA using three ICW electrodes can induce coagulations >5 cm in diameter in porcine livers with better efficiency than consecutive monopolar (CM) or SM mode.     

Multipolar radiofrequency ablation with internally cooled electrodes: experimental study in ex vivo bovine liver with mathematic modeling

PURPOSE: To evaluate the size and geometry of thermally induced coagulation by using multipolar radiofrequency (RF) ablation and to determine a mathematic model to predict coagulation volume. MATERIALS AND METHODS: Multipolar RF ablations (n = 80) were performed in ex vivo bovine livers by using three internally cooled bipolar applicators with two electrodes on the same shaft. Applicators were placed in a triangular array (spacing, 2-5 cm) and were activated in multipolar mode (power output, 75-225 W). The size and geometry of the coagulation zone, together with ablation time, were assessed. Mathematic functions were fitted, and the goodness of fit was assessed by using r(2). RESULTS: Coagulation volume, short-axis diameter, and ablation time were dependent on power output and applicator distance. The maximum zone of coagulation (volume, 324 cm(3); short-axis diameter, 8.4 cm; ablation time, 193 min) was induced with a power output of 75 W at an applicator distance of 5 cm. Coagulation volume and ablation time decreased as power output increased. Power outputs of 100-125 W at applicator distances of 2-4 cm led to a reasonable compromise between coagulation volume and ablation time. At 2 cm (100 W), coagulation volume, short-axis diameter, and ablation time were 66 cm(3), 4.5 cm, and 19 min, respectively; at 3 cm (100 W), 90 cm(3), 5.2 cm, and 22 min, respectively; at 4 cm (100 W), 132 cm(3), 6.1 cm, and 27 min, respectively; at 2 cm (125 W), 56 cm(3), 4.2 cm, and 9 min, respectively; at 3 cm (125 W), 73 cm(3), 4.9 cm, and 12 min, respectively; and at 4 cm (125 W), 103 cm(3), 5.5 cm, and 16 min, respectively. At applicator distances of 4 cm (>125 W) and 5 cm (>100 W), the zones of coagulation were not confluent. Coagulation volume (r(2) = 0.80) and RF ablation time (r(2) = 0.93) were determined by using the mathematic model. CONCLUSION: Multipolar RF ablation with three bipolar applicators may produce large volumes of confluent coagulation ex vivo. A compromise is necessary between prolonged RF ablations at lower power outputs, which produce larger volumes of coagulation, and faster RF ablations at higher power outputs, which produce smaller volumes of coagulation.     

Large-volume radiofrequency ablation of ex vivo bovine liver with multiple cooled cluster electrodes

Three methods of creating large thermal lesions with cool-tip cluster electrodes were compared. Three cluster electrodes were arranged 4 cm apart in a triangular array. Eight lesions were created ex vivo in fresh bovine liver (from a butcher) with each method: sequential ablation (three electrodes, 12 minutes each); simultaneous activation of electrodes (12 minutes); and rapid switching of power between electrodes (12 minutes), for which an electronic computer-controlled switch was developed. For sequential, rapid switching, and simultaneous methods, lesion volumes were 137.5 cm(3)+/- 22.2, 116.4 cm(3)+/- 15.2, and 22.3 cm(3)+/- 6.4 (P < .05), respectively, and final temperatures at lesion center were 80 degrees C +/- 5, 97 degrees C +/- 8, and 41 degrees C +/- 3 (P < .001), respectively. Because of electrical interference between electrodes, simultaneous method led to little heating at the center between the electrodes and created small discontinuous lesions. Rapid switching created large round lesions by employing multiple electrodes concurrently, which substantially reduced treatment time and resulted in more effective heating between electrodes.     

Bipolar radiofrequency ablation using internally cooled electrodes in ex vivo bovine liver: prediction of coagulation volume from applied energy

OBJECTIVE: We sought to evaluate the relationship between parameters of bipolar radiofrequency (RF) ablation using internally cooled electrodes. MATERIALS AND METHODS: Bipolar RF ablations (n = 24) were performed in ex vivo bovine liver using an internally cooled applicator with 2 electrodes located on the same shaft. The power-output was systematically varied (20-75 W). On the basis of our experimental data, mathematical functions were fitted and the goodness-of-fit was assessed by the parameter R. RESULTS: The duration to induce an increase of tissue resistance and the amount of applied energy increased with a decreased power-output. The maximum short-axis was 4.5 cm (20 W) and required an application of 64 kilojoules (kJ). The volume of coagulation can be determined as a function of the duration of energy application (R = 0.954) and the amount of applied energy (R = 0.945). CONCLUSION: The amount of applied energy and the duration of energy application can predict the volume of induced coagulation and may be useful to control internally cooled bipolar RF 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.     

Hepatic radiofrequency ablation: in vivo and ex vivo comparisons of 15-gauge (G) and 17-G internally cooled electrodes

Objective:

To compare the performance of the 15-G internally cooled electrode with that of the conventional 17-G internally cooled electrode.

Methods:

A total of 40 (20 for each electrode) and 20 ablation zones (10 for each electrode) were made in extracted bovine livers and in in vivo porcine livers, respectively. Technical parameters, three dimensions [long-axis diameter (Dl), vertical-axis diameter (Dv) and short-axis diameter (Ds)], volume and the circularity (Ds/Dl) of the ablation zone were compared.

Results:

The total delivered energy was higher in the 15-G group than in the 17-G group in both ex vivo and in vivo studies (8.78 ± 1.06 vs 7.70 ± 0.98 kcal, p = 0.033; 11.20 ± 1.13 vs 8.49 ± 0.35 kcal, p = 0.001, respectively). The three dimensions of the ablation zone had a tendency to be larger in the 15-G group than in the 17-G group in both studies. The ablation volume was larger in the 15-G group than in the 17-G group in both ex vivo and in vivo studies (29.61 ± 7.10 vs 23.86 ± 3.82 cm³, p = 0.015; 10.26 ± 2.28 vs 7.79 ± 1.68 cm³, p = 0.028, respectively). The circularity of ablation zone was not significantly different in both the studies.

Conclusion:

The size of ablation zone was larger in the 15-G internally cooled electrode than in the 17-G electrode in both ex vivo and in vivo studies.

Advances in knowledge:

Radiofrequency ablation of hepatic tumours using 15-G electrode is useful to create larger ablation zones.Radiofrequency ablation (RFA) is the most widely used local ablation technique for the management of primary and metastatic liver tumours. However, previous studies have reported that RFA showed a relatively higher local tumour progression rate than did hepatic resection.^1,2 One of the most important factors affecting local tumour progression was insufficient tumour-free ablation margin of hepatic parenchyma around the tumour margin.^3–6Several strategies have been developed to obtain sufficient ablation margin. In the aspect of RFA techniques, overlapping technique and combined treatment with transcatheter arterial chemoembolization can be used.^7–9 Another strategy is to use switching monopolar, bipolar or multipolar modes to deliver radiofrequency (RF) energy more efficiently.^10,11 Sufficient ablation margin can also be achieved by more efficient electrodes: internally cooled electrode increases the size of ablation zone by preventing charring around the electrode tip.^12,13 Perfusion electrodes can also enlarge the ablation zone by increasing electrical conductance and thermal conductivity.^14–16The diameter of an electrode is also known to be associated with the size of the ablation zone. Theoretically, as the diameter of an electrode becomes larger, the contact surface of the electrode with the surrounding tissue becomes bigger, thereby increasing the active electric field.^17,18 As a result, an electrode with a larger diameter is likely to create a larger ablation zone. In a previous study, Goldberg et al¹⁷ reported that the extent of coagulation necrosis by RFA increases as the diameter of an electrode increases through an in vivo experimental study. However, this study was performed with an electrode without an internal cooling system. Recently, a clinical study comparing therapeutic efficacy and safety between 15-G and 17-G internally cooled electrodes of RFA for hepatocellular carcinoma was published.¹⁹ According to that study, the 15-G internally cooled electrode created a larger ablation volume than did the 17-G electrode. However, the study was limited by selection bias owing to the retrospective study design. In addition, the ablation protocol was not exactly the same between the two groups. Therefore, the issue whether an internally cooled electrode with a larger diameter creates a larger ablation volume should be verified with ex vivo and in vivo experimental studies.The purpose of this experimental study was to compare the performance of the 15-G internally cooled RF electrode with that of the conventional 17-G electrode in both ex vivo and in vivo studies.     

Multipolar radiofrequency ablation using internally cooled electrodes in ex vivo bovine liver: correlation between volume of coagulation and amount of applied energy

Purpose

To evaluate the relationship between applied energy and volume of coagulation induced by multipolar radiofrequency (RF) ablation.

Methods and materials

Multipolar RF ablations (n = 80) were performed in ex vivo bovine liver. Three bipolar applicators with two electrodes located on each applicator shaft were placed in a triangular array. The power-output (75–225 W) and the distance between the different applicators (2, 3, 4, 5 cm) were systematically varied. The volume of confluent white coagulation and the amount of applied energy were assessed. Based on our experimental data the relationship between the volume of coagulation and applied energy was assessed by nonlinear regression analysis. The variability explained by the model was determined by the parameter r².

Results

The volume of coagulation increases with higher amounts of applied energy. The maximum amount of energy was applied at a power-output of 75 W and an applicator distance of 5 cm. The corresponding maximum volume of coagulation was 324 cm³ and required an application of 453 kJ. The relationship between amount of applied energy (E) and volume (V) of coagulation can be described by the function, V = 4.39E^0.7 (r² = 0.88). By approximation the volume of coagulation can be calculated by the linear function V = 0.61E + 40.7 (r² = 0.87).

Conclusion

Ex vivo the relationship between volume of coagulation and amount of applied energy can be described by mathematical modeling. The amount of applied energy correlates to the volume of coagulation and may be a useful parameter to monitor multipolar RF ablation.     

Radiofrequency renal ablation: in vivo comparison of internally cooled, multitined expandable and internally cooled perfusion electrodes

PURPOSE: To evaluate the in vivo efficiency of radiofrequency ablation using an internally cooled-perfusion (ICP) electrode for inducing coagulation necrosis compared with those of RFA using internally cooled or multitined expandable electrodes in porcine kidneys. MATERIALS AND METHODS: Using a 200 W generator and internally cooled and ICP electrodes or a 150 W generator and a multitined expandable electrode, a total of 15 radiofrequency ablations were performed in the kidneys of nine pigs. After placement of an electrode in the lower pole of a kidney, one ablation zone was created using one of three different regimens: group A, radiofrequency ablation using an internally cooled electrode; group B, radiofrequency ablation using an ICP electrode with 14.6% NaCl solution instillation at 1 mL/minute; group C, radiofrequency ablation using a multitined expandable electrode. Three days after the procedures, contrast-enhanced CT scans were obtained to evaluate ablation region volumes, and kidneys were harvested for gross measurements. The three groups were compared with respect to technical parameters such as changes in impedance and current during radiofrequency ablation. The dimensions of thermal ablation zones created in the three groups were compared histologically. RESULTS: In vivo study showed that ICP electrode allowed a greater energy delivery than internally cooled or multitined expandable electrode during radiofrequency ablation: 63.3 +/- 8.8 kJ in group A; 101 +/- 3.3 kJ in group B; and 61.8 +/- 12.5 kJ (P < .05). In vivo studies showed radiofrequency ablation using ICP electrode achieved larger mean coagulation volumes than radiofrequency ablation using the other electrodes: 12.0 +/- 3.9 cm(3) in group A; 30.5 +/- 7.6 cm(3) in group B; and 11.6 +/- 6.7 cm(3) in group C (P < .05). In addition, group B had a larger mean short-axis diameter of radiofrequency-induced coagulation necrosis than groups A or C: 2.6 +/- 0.5 cm in group A; 3.6 +/- 0.4 cm in group B; and 2.4 +/- 0.7 cm in group C (difference between groups B and C: P < .05). CONCLUSIONS: Radiofrequency ablation using an ICP electrode showed better performance at creating coagulation necrosis than radiofrequency ablation using internally cooled or multitined expandable electrodes in this porcine renal model.     

Hepatic bipolar radiofrequency ablation using perfused-cooled electrodes: a comparative study in the ex vivo bovine liver

The purpose of this paper was to demonstrate the efficacy of the dual probe bipolar radiofrequency (RF) system with the perfused-cooled electrodes inducing coagulation necrosis in the ex vivo bovine liver. The perfused-cooled electrode that allows simultaneous internal cooling and interstitial hypertonic saline perfusion has been developed for RF ablation (RFA). RF was applied to excised bovine liver in a bipolar mode at 150 W using a 200 W generator with two perfused-cooled electrodes for 10 min. After placing the electrodes at 3 cm spacing in the explanted liver, 45 ablation zones were created with three different regimens: Group A, using both intraelectrode cooling and interstitial perfusion; group B, using only the intraelectrode cooling; and group C, using only interstitial perfusion. In groups A and C, RFA was performed with the infusion of 6% hypertonic saline at the rate of 2 ml min(-1). During RFA, we measured the tissue temperature at the midpoint between the two electrodes. The dimensions of the ablation zones and the changes in impedance, currents and liver temperature during RFA were compared in these three groups. The mean tissue impedance during RFA in group A (56.7+/-21.7 Omega) and group C (56.9+/-20.6 Omega) was significantly lower than group B (112+/-19.7 Omega) (p<0.001). The mean current was higher in group A (1765+/-128 mA) than groups B (760+/-321 mA) and C (1298+/-349 mA) (p<0.05). In addition, the shortest vertical diameter of coagulation necrosis was greater in groups A (4.9+/-0.5 cm) and C (4.6+/-0.7 cm) than in group B (3.5+/-0.4 cm) (p<0.05). The temperature at the mid-point between the two probes was higher in group A than other groups: 99 degrees C in group A, 88.9 degrees C in group B, and 94.3 degrees C in group C (p>0.05). The ratios of the diameter of the long-axis to the diameter of the vertical-axis of groups A, B and C were 1.1+/-0.1, 1.2+/-0.1, and 1.1+/-0.2, respectively (p<0.05). Bipolar RFA using intraelectrode cooling and the interstitial saline perfusion simultaneously produced ablation zones significantly larger than the area produced by only one measure.     

A comparison of microwave ablation and bipolar radiofrequency ablation both with an internally cooled probe: Results in ex vivo and in vivo porcine livers

Purpose

The purpose of this study was to compare the effectiveness of microwave (MW) ablation and radiofrequency (RF) ablation using a single internally cooled probe in a hepatic porcine model.

Materials and methods

In the ex vivo experiment, MW ablations (n = 40) were performed with a 2450 MHz and 915 MHz needle antenna, respectively at 60 W, 70 W power settings. Bipolar RF ablations (n = 20) were performed with a 3-cm (T30) and 4-cm (T40) active tip needle electrodes, respectively at a rated power 30 W and 40 W according to automatically systematic power setting. In the in vivo experiment, the 2450 MHz and 915 MHz MW ablation both at 60 W and T30 bipolar RF ablation at 30 W were performed (n = 30). All of the application time were 10 min. Long-axis diameter (Dl), short-axis diameter (Ds), ratio of Ds/Dl, the temperature data 5 mm from the needle and the time of temperature 5 mm from the needle rising to 54 °C were measured.

Results

Both in ex vivo and in vivo models, Ds and Dl of 915 MHz MW ablations were significantly larger than all the RF ablations (P < 0.05); the Ds for all the 2450 MHz MW ablations were significantly larger than that of T30 RF ablations (P < 0.05). 2450 MHz MW and T30 RF ablation tended to produce more elliptical-shaped ablation zone. Tissue temperatures 5 mm from the needle were considerably higher with MW ablation, meanwhile MW ablation achieved significantly faster rate of temperature rising to 54 °C than RF ablation. For in vivo study after 10 min of ablation, the Ds and Dl of 2450 MHz MW, 915 MHz MW and Bipolar RF were 2.35 ± 0.75, 2.95 ± 0.32, 1.61 ± 0.33 and 3.86 ± 0.81, 5.79 ± 1.03, 3.21 ± 0.51, respectively. Highest tissue temperatures 5 mm from the needle were 80.07 ± 12.82 °C, 89.07 ± 3.52 °C and 65.56 ± 15.31 °C and the time of temperature rising to 54 °C were respectively 37.50 ± 7.62 s, 24.50 ± 4.09 s and 57.29 ± 23.24 s for three applicators.

Conclusion

MW ablation may have higher potential for complete destruction of liver tumors than RF ablation.     

High-power generator for radiofrequency ablation: larger electrodes and pulsing algorithms in bovine ex vivo and porcine in vivo settings

PURPOSE: To prospectively maximize the extent of tissue coagulation by using a high-power (1000-W, 4000-mA) radiofrequency (RF) generator to optimize pulsing algorithms. MATERIALS AND METHODS: The institutional animal care and use committee approved the use of the animal model in the in vivo portion of this study. RF ablations (n = 258) were performed in ex vivo bovine livers by using a 500-kHz high-power generator. Through internally cooled 3.0-cm single and 2.5- and 4.0-cm cluster electrodes, RF energy was applied for 12 minutes. For each electrode, simplex optimization was used to determine the pulsing algorithms to be used (ie, 5-50-second "on" [energy application] and 10-50-second "off" [cooling without RF heating] periods). Three-dimensional contour maps expressing the relationship between pulsing parameters and resultant coagulation were constructed. Then, 31 RF ablations were performed with optimal settings in vivo in porcine livers, and the results were compared with those obtained in control ablations performed by using a 2000-mA commercial generator. Finally, in 108 experiments, RF energy was applied in ex vivo livers for 6, 12, and 20 minutes with maximum current settings (1000-4000 mA) by using the optimal on and off settings for all three electrodes, and the results were analyzed with multivariate analysis of variance (MANOVA). RESULTS: For all three electrodes, a relationship between the on and off times during the pulsing cycle and the resultant coagulation was established (P < .01). With 3.0-cm single electrodes, maximum coagulation (mean, 5.2 cm +/- 0.1 [standard deviation] ex vivo and 3.6 cm +/- 0.2 in vivo) was achieved with pulse settings of 10-18 seconds on and 11-20 seconds off. With cluster electrodes, greater coagulation was achieved (mean, 6.5 cm +/- 0.6 ex vivo and 3.9 cm +/- 0.3 in vivo with 2.5-cm tip; 8.3 cm +/- 0.3 ex vivo and 5.2 cm +/- 0.8 in vivo with 4.0-cm tip) with optimal pulse settings. Thus, use of the high-power generator yielded substantially increased tissue coagulation in vivo compared with the coagulation achieved with the standard generator. MANOVA revealed that increased maximum current and RF ablation durations of up to 20 minutes were associated with greater coagulation, the size of which also varied according to electrode type (P < .01). CONCLUSION: Markedly larger coagulation zones can be achieved with optimized high-power RF ablation. This may require longer pulsing intervals compared with those previously used.     

Porcine liver: morphologic characteristics and cell viability at experimental radiofrequency ablation with internally cooled electrodes

PURPOSE: To evaluate morphologic characteristics and cell viability of radiofrequency ablation zones in porcine liver. MATERIALS AND METHODS: Approval of the study protocol was obtained from the Ethics Committee on Use of Live Animals for Teaching and Research at University of Hong Kong. Internally cooled electrodes were used to produce 120 ablated zones ex vivo and 60 ablated zones in vivo with single electrodes (1-, 2-, and 3-cm exposed lengths) or clustered electrodes (1.0-, 2.0-, and 2.5-cm exposed lengths) at 4, 8, 12, and 16 minutes of ablation (ex vivo) and 8 and 12 minutes of ablation (in vivo). Morphologic measurements of each ablated zone were performed. Cell viability in each ablated zone was assessed qualitatively with histochemical staining and quantitatively with measurement of intracellular adenosine 5'-triphosphate (ATP) concentration. RESULTS: Exposed length of electrode (coefficient = 0.79, standard error = 0.04, P < .001), duration of ablation (coefficient = 0.14, standard error = 0.01, P < .001), and clustered electrode design (coefficient = 1.21, standard error = 0.05, P < .001) were independent factors that affected minimal transverse diameter and volume of ablated zone in ex vivo study. Similar morphologic characteristics existed among ablated zones in in vivo study. Mean distance of ablation beyond the electrode tip remained constant (ex vivo, 1.0 cm +/- 0.08 [standard deviation]; in vivo, 0.5 cm +/- 0.05) regardless of different ablation conditions. Histochemical staining revealed no viable hepatocytes from center to margins of white zone in each ablated area. Mean intracellular ATP concentration in margins of white zone (9.5 x 10(-12) mol/microg DNA +/- 1.43) was lower than that in red zone (4088 x 10(-12) mol/microg DNA +/- 65.97, P < .001) and in adjacent normal liver (4528 x 10(-12) mol/microg DNA +/- 52.74, P < .001). CONCLUSION: Distance of ablation beyond the tip of the electrode remained constant (ex vivo, 1.0 cm; in vivo, 0.5 cm) with different conditions of ablation. Complete and uniform cellular destruction was achieved in the white zone of ablated area.     

Radiofrequency ablation in pig lungs: in vivo comparison of internally cooled, perfusion and multitined expandable electrodes

The purpose of this study was to compare the amounts of in vivo coagulation obtained by radiofrequency (RF) ablation in porcine lung, using three types of electrodes. 15 in vivo ablation procedures were performed in the lungs of five pigs using three kinds of currently available RF devices under CT guidance. After placing an electrode in the lung, three ablation zones were created at each of three different regimens: Group A: RF ablation with an internally cooled electrode; Group B: RF ablation with a perfusion electrode, with instillation of 0.9% NaCl solution at a rate of 1.5 ml min(-1); Group C: RF ablation with a multitined expandable electrode. According to the manufacturer's recommendations, RF application times were 12 min in group A and 20 min in group B. In group C, RF energy was delivered for 7 min after a mean temperature of 110 degrees C was reached at 5 cm deployment. 36 min after the procedures, contrast-enhanced CT scans were obtained to evaluate the volume of zone of coagulation, and lungs were harvested for gross measurements. After macroscopic and histopathological analyses of 5 mm-thick lung sections, diameters, volumes and variation coefficients of regions of central coagulation were assessed. During RF ablation, the perfusion electrode allowed a larger energy delivery than the internally cooled or the multitined expandable electrodes, i.e. 33.6+/-4.7 kJ in group A, 40.0+/-8.2 kJ in group B and 23.5+/-6.1 kJ in group C (p<0.05). On gross observation, the cut surface of the gross specimen containing RF-induced coagulation showed that the ablated tissue appeared to be a central, firm, dark-brown area surrounded by an irregular outer margin (approximately 3-10 mm thick) of bright red tissue. In vivo studies showed that RF ablation using the perfusion electrode achieved larger coagulation volume than RF ablation using the other electrodes (p<0.05): 7.2+/-4.1 cm3 in group A; 16.9+/-5.5 cm3 in group B; 7.5+/-3.3 cm3 in group C. The corresponding variation coefficients were 0.55, 0.31, and 0.45, respectively. Our study shows that RF ablation using a perfusion electrode achieves a larger coagulation volume with an irregular margin than RF ablation using internally cooled or multitined expandable electrodes in the porcine lung.     

Multipolar radiofrequency ablation using 4-6 applicators simultaneously: a study in the ex vivo bovine liver

In this study the volume and shape of coagulation zones after multipolar radiofrequency ablation (RFA) with simultaneous use of 4-6 applicators in the ex vivo bovine liver were investigated. The RF-applicators were positioned in 13 different configurations to simulate ablation of large solitary tumors and simultaneous ablation of multiple lesions with 120 kJ of applied energy/session. In total, 110 coagulation zones were induced. Standardized measurements of the volume and shape of the coagulation zones were carried out on magnetic resonance images and statistically analyzed. The coagulation zones induced with solitary applicators and with 2 applicators were imperceptibly small and incomplete, respectively. At 20mm applicator distance, the total ablated volume was significantly larger if all applicators were arranged in a single group compared to placement in 2 distant applicator groups, each consisting of 3 applicators (p=.001). The mean total coagulated volume ranged from immeasurably small (if 6 solitary applicators were applied simultaneously) to 74.7 cc (if 6 applicators at 30 mm distance between neighboring applicators were combined to a single group). Applicator distance, number and positioning array impacted time and shape. The coagulation zones surrounding groups with 4-6 applicators were regularly shaped, homogeneous and completely fused, and the axial diameters were almost constant. In conclusion, multipolar RFA with 4-6 applicators is feasible. The multipolar simultaneous mode should be applied for large and solitary lesions only, small and multiple tumors should be ablated consecutively in standard multipolar mode with up to 3 applicators.     

Wet radio-frequency ablation using multiple electrodes: comparative study of bipolar versus monopolar modes in the bovine liver

OBJECTIVE: To determine whether hypertonic saline (HS)-mediated bipolar radio-frequency (rf) application as advantages over monopolar simultaneous and alternating rf applications for creating larger areas of coagulation necrosis. MATERIALS AND METHODS: A total of 60 rf ablations using double perfused-cooled electrodes and a 200 W generator (CC-3 model, Radionics) were performed in three different modes in explanted bovine livers: simultaneous monopolar mode (groups A and A'); alternating monopolar mode (groups B and B'); or bipolar mode (groups C and C'). Electrodes were placed at inter-electrode distances of 3 and 5 cm, and HS (6% NaCl solution) was instilled into tissue at a rate of 1 mL/min through the electrodes. rf was applied for 10 (3 cm distance) or 15 min (5 cm distance). During rf application, we measured the tissue temperature at the mid-point between the two electrodes. Dimensions of the thermal ablation zones, and temperatures were compared between the 3 groups using analysis of variance or the Kruskal-Wallis test. To compare configurations of the ablation zones in each group, the ratio of longitudinal diameter (Dl) to vertical diameter (D(v)) was calculated. RESULTS: With a 3-cm inter-electrode spacing, the D(v) between the electrodes of ablated lesions was 2.4 +/- 1.2 cm in group A, 4.5 +/- 1.0 cm in group B, and 6.1 +/- 0.9 cm in group C (P < 0.05), and at a 5-cm spacing, groups B' and C' produced a single ablation area, but group A' produced two separated ablation spheres: the D(v)s were 1.4 +/- 0.2 in group A, 2.9 +/- 1.0 mm in group B, and 6.6 +/- 0.4 cm in group C (P < 0.05). For both 5- and 3-cm spacings, the temperatures at the mid-point were higher in bipolar mode than in either monopolar simultaneous or alternating modes. The ratios of Dl/D(v) of groups A, B, and C were 2.5 +/- 0.2, 1.4 +/- 0.1, and 1.1 +/- 0.1, respectively, and the corresponding figures of groups A', B' and C' were 4.5 +/- 0.2, 2.7 +/- 0.1, and 1.1 +/- 0.1, respectively (P < 0.05). CONCLUSION: HS-enhanced bipolar rf ablation creates larger, more regular coagulation necrosis than either monopolar simultaneous or alternating rf ablation.     

In vivo efficiency of four commercial monopolar radiofrequency ablation systems: a comparative experimental study in pig liver

RATIONALE AND OBJECTIVES: To evaluate the efficiency of 4 radiofrequency (RF) systems by assessing the amount of delivered energy for each thermal induced lesion after perfusion mediated RF ablation and to compare the influence of perfusion mediation types on the energy efficiency. METHODS: A total of 43 ablations in 16 male landrace pigs with 4 RF devices were performed strictly according to the manufacturers' instructions. Total absorbed energy was computed and then related to 3D volumetry obtained after histopathological evaluation. Sixteen ablations were performed under physiological liver perfusion and 27 ablations with occlusion of portal vein, hepatic artery, or both vessels. Energy efficiency values of the RF systems for different vascular occlusion techniques were compared and analyzed by a nonparametrical rank sum test. RESULTS: Under physiological perfusion, the average energy delivered to produce 1-cm3 lesion size was calculated to 1650 +/- 929, 3097 +/- 389, 8312 +/- 2068, and 5493 +/- 2306 Watt x s/cm3 for the Berchtold, Radionics, Radiotherapeutics, and RITA system, respectively. After perfusion-mediated RF ablation, artery occlusion was not as effective as portal vein occlusion, which reduced the energy to 587 +/- 148, 869 +/- 276, and 903 +/- 394 Watt. s/cm3 for the Berchtold, Radionics, and Radiotherapeutics system, respectively. The occlusion of vessels, portal vein, and artery or portal vein alone increased the energy efficiency compared with physiological liver perfusion or occlusion of the artery (P = 0,003). CONCLUSIONS: Under physiological liver perfusion the open perfused system and the internally cooled system provided the best efficiency values with lowest standard deviations. The energy efficiency was increased markedly for all systems after occlusion of the portal vein either alone or in combination with arterial occlusion. Occlusion of the hepatic artery did not improve the efficiency.