Original Research
Vascular and Interventional Radiology
November 23, 2012

Comparison of Microwave Ablation and Multipolar Radiofrequency Ablation In Vivo Using Two Internally Cooled Probes

Abstract

OBJECTIVE. The objective of our study was to compare the effectiveness of microwave ablation (MWA) and multipolar radiofrequency ablation (RFA) in vivo using two internally cooled probes.
MATERIALS AND METHODS. MWA (n = 24) was performed by simultaneous application of double internally cooled-shaft antennae. Three power settings (60, 70, and 80 W) were used. Multipolar RFA (n = 16) was also performed by simultaneously using two internally cooled bipolar applicators (lengths: 3-cm T30 and 4-cm T40) at 60 and 80 W. Probe spacing was 2 cm. Each power setting was applied for eight ablations with a 10-minute duration for each. The cooled-shaft probes were inserted approximately 7 cm into the liver parenchyma of seven adult Wuzhishan pigs under ultrasound guidance, and ablations were performed in various segments of porcine liver. The long-axis diameter (Dl), short-axis diameter (Ds) and the ratio Ds/Dl for each ablation was measured. Temperature curves at 0, 2, and 3 cm from the middle of the two probes and the time to reach 60°C at 0 cm from the parallel central line between the two probes were recorded.
RESULTS. The long-axis diameter and short-axis diameter for all the power settings of MWA were significantly larger than that of both kinds of multipolar RFA (p < 0.05). The rates of temperature rise to 60°C at 0 cm from the parallel central line between the two probes for all MWA power settings were significantly faster compared with RFA.
CONCLUSION. MWA, by the simultaneous application of double antennae, can generate a larger ablation zone, in vivo, compared with multipolar RFA.
Imaging-guided thermal ablation using different energy sources, such as radiofrequency and microwaves, has become increasingly attractive in treating malignant hepatic tumors [13]. Regardless of the primary energy source, all of these modalities induce cellular destruction by means of the direct effects of heat, with irreversible cellular damage occurring at temperatures above 50°C. This cellular damage is most effective when applied for approximately 4–6 minutes and occurs almost instantaneously at temperatures above 60°C [4].
However, compared with radiofrequency ablation (RFA), microwave ablation (MWA) has significant advantages, including consistently higher intratumoral temperatures, larger tumor ablation volumes, and faster ablation times. In addition, along with the ability to use multiple applicators, MWA can also provide improved convection profiles, optimal heating of cystic masses, and less procedural pain [57].
In recent years, RFA has achieved a larger coagulation zone using bipolar and multipolar arrays [810]. The combination of multiple bipolar applicators using the multipolar mode has led to more focal and more efficient deposition of radiofrequency energy in the target tissue, encouraging larger coagulation zones [8]. Yu et al. [9] compared MWA with bipolar RFA, both ex vivo and in vivo, using porcine livers and concluded that MWA may have greater potential for complete destruction of liver tumors compared with RFA. However, they raise the limitation of their study that the synergistic effect of using multiple MW antennae simultaneously and multipolar radiofrequency mode was not observed.
With this in mind, we designed an experiment to compare, simultaneously, the effectiveness of MWA versus multipolar RFA using two internally cooled probes in porcine liver.
Fig. 1 Microwave and radiofrequency applicators.
A, Photograph shows prototype microwave applicators. Microwave antennae are embedded in slit-radiating segments (arrow).
B, Photograph shows two internally cooled bipolar radiofrequency probes T30 (top) and T40 (bottom) with two electrodes (arrows) on single shaft separated by insulators (arrowheads).

Materials and Methods

Microwave System

A microwave delivery system (FORSEA, Qinghai Microwave Electronic Institute) was used. This system consisted of an MTC-3 microwave generator (FORSEA) with a frequency of 2450 MHz, power output of 10–150 W, flexible low-loss cable, and 14-gauge cooled-shaft antenna. The generator was equipped with two output sources, which allowed simultaneous application of microwave energy through two antennae. The cooled-shaft antenna, which consisted of a 10-cm-long cable connection portion, a 16.5-cm-long shaft coated with fluoropolymer (Teflon, Dupont) and a 1.5-cm-long active tip coated with polytetrafluoroethylene, was used to deliver energy to the liver tissue (Fig. 1A). The antenna shaft contained two lumina that enabled the delivery of saline solution (cooled to 4°C) to the tip of the shaft and the return of the warmed solution to a 500-mL plastic bag outside the body. A steady-flow pump (BT01–100 LanGe-Pump, LanGe Steady Flow Pump) was used to push the chilled saline solution circulating within the lumina of the antenna shaft at 50–60 mL/min. The amount of circulating chilled solution could be adjusted to maintain a mean shaft temperature of 10°C ± 2 SD.
Fig. 2 Diagram shows two electrodes (A) and (B) separated by insulator (black). Two bipolar electrodes in multipolar mode show possible electrode couples and electrical current paths (arrows).

Radiofrequency System

All radiofrequency procedures were performed using a 470 ± 10 kHz multipolar radiofrequency generator (CelonLab POWER, Celon Medical Instruments) that provided a maximum power output of 250 W. Radiofrequency energy was deposited by using an internally cooled applicator with two electrodes (CelonPro Surge, Celon). The electrode shaft (shaft diameter, 1.8 mm; length, 15 cm) contained two lumina that enabled internal fluid circulation. Saline solution was delivered at a rate of 30 mL/min, at room temperature by using a peristaltic pump (CelonAquaflow III, Celon). The exposure tip of the bipolar applicator had lengths of 3 cm (T30) and 4 cm (T40), respectively, and consisted of two uninsulated electrodes that were separated by 3 cm and 4 mm of insulation, respectively (Fig. 1B). When one bipolar applicator was connected to the radiofrequency generator, the energy was applied in bipolar mode. When two or three bipolar applicators were connected, the radiofrequency system operated in multipolar mode (Fig. 2).

Temperature Measuring System

To investigate the temperature change within the tissue, three 20-gauge thermistor probes (FORSEA, Qinghai Microwave Electronic Institute) with a response time of < 1 second and accuracy within 0.1°C were placed at t0, t2, and t3 locations at 0, 2, and 3 cm, respectively, from the parallel central line between the two probes (Fig. 3). Temperatures were recorded at 1-minute intervals for each parameter setting. A thin insulation–epoxy resin was used to insulate the metallic shaft of the thermistor probes (but the tip of the thermocouple for temperature monitoring was exposed) to avoid any interference from metal in the electric field.
Fig. 3 Positions of temperature sensors. T0, T2, and T3 denote temperature at location t0, t2, and t3, respectively.
A and B, Schematics show two microwave (M) antennae (A) and two radiofrequency (R) applicators (B).

In Vivo Study

ProcedureApproval from the university subcommittee on animal research experiments was obtained before the initiation of these studies. After being fasted overnight, seven adult Wuzhishan pigs, weighing 50–65 kg, were anesthetized by intramuscular injection of ketamine hydrochloride (10–15 mg/kg of body weight) and maintained by means of intramuscular injection of pentobarbital sodium (3 g/100 mL, 0.25 mL/kg) and IV injection of diazepam (5–10 mg). Cardiac and respiratory parameters were monitored throughout the procedure. Laparotomy was performed, and the liver was exposed.
Under normal hepatic blood flow, cooled-shaft probes were inserted approximately 7 cm into the liver parenchyma at different segments of porcine liver. Ultrasound guidance (15L8W-s linear transducer, Sequoia 512, Acuson) was used to avoid inserting the probes into the intrahepatic vessels and ensure that the probes were placed at least 3 cm from a previous ablation site. Immediately after performing the ablation, we excised the livers and then sacrificed the animals by incising the inferior vena cava. All ablation procedures were performed by three of the authors who have 10, 3, and 8 years of experience with thermal ablation procedures.
Ablation parameter settings—Under ultrasound guidance, two probes were simultaneously inserted into the liver tissue using the same implantation depth and power output. MWA (n = 24) was induced by using three different power outputs (60, 70, and 80 W) (Table 1). Multipolar RFA (n = 16) was performed using pulsed-energy mode with a rated power of 60 W (probes with a 3-cm (T30) active tip) and 80 W (probes with a 4-cm (T40) active tip) (Table 1) to facilitate the most ideal ablation zone according to the manufacturers' recommendations. Every power setting was applied for eight ablations and 10-minute heating duration. All spacing between the two probes was set at 2.0 cm.

Evaluation of Lesion Diameter, Shape, and Temperature Data

After excision, the liver specimens were immediately sectioned along the two parallel needles within each lesion. We assessed whether coagulation was confluent, partially confluent, or not confluent. The long-axis diameters (Dl, along the needle insertion axis) and short-axis diameters (Ds, perpendicular to the longitudinal plane) of the coagulation zone were assessed macroscopically using calipers. The sphericity index was simplified to the ratio of short/long axis (Ds/Dl). Temperature curves at locations t0, t2, and t3 were recorded. The time to rise to 60°C was recorded at location t0. The results of all diameter measurements were made by consensus between two investigators.

Statistical Analysis

Results of lesion diameter and temperature data were reported as mean ± SD. Differences in Dl, Ds and Ds/Dl ratios between the two groups were evaluated by independent-samples Student t test. Differences in the temperature rise to 60°C at location t1 were also evaluated by independent-samples Student t test. All statistical analyses were performed by using SPSS 16.0 statistical software (SPSS), and p values less than 0.05 were considered statistically significant.

Results

Evaluation of Lesion Size and Shape

T30 multipolar RFA produced very small and partially confluent coagulation zones in the first experiment, thus we only performed two ablations for T30 multipolar RFA and ignored this group. All coagulation zones were ellipsoidal using MWA and T40 multipolar RFA. As the microwave power output increased, the short- and long-axis diameters increased, accordingly. MWA achieved a larger coagulation zone and a wider and deeper color-charring zone around the shaft compared with T40 multipolar RFA (Fig. 4). The Dl and Ds for all the power settings of MWA were significantly larger compared with T40 multipolar RFA (p < 0.05). The ratios of Ds/Dl were not significantly different between MWA and T40 multipolar RFA.
TABLE 1: Technical Parameters of Microwave Ablation (MWA) Versus Multipolar Radiofrequency Ablation (RFA)

Evaluation of Temperature Effect

For all ablations, the temperature recorded at locations t1 and t2 at approximately 7 minutes of heating time reached its relative plateau. However, at location t3, because of limited temperature rise amplitude, the temperature curve approximated the horizontal line (Fig. 4.). The maximum temperatures at locations t0, t2, and t3, for all MWAs at all power settings were higher compared with those recorded using T40 multipolar RFA (p < 0.05) (Table 2). Figure 5B shows that at location t2 the temperature of T40 multipolar RFA was the lowest compared with all other temperatures recorded using MWA. The rates of temperature rise to 60°C at location t1 for all MWA power settings were significantly faster compared with those recorded during both multipolar RFA techniques.
TABLE 2: Coagulation Necrosis Parameters of Microwave Ablation (MWA) Versus Multipolar Radiofrequency Ablation (RFA)
Fig. 4 Gross specimens of porcine liver tissue treated with microwave ablation (MWA) and radiofrequency ablation (RFA) in vivo. Each ablation was produced using two internally cooled probes at intervals of 2.0 cm for 10 minutes. Scale in cm.
A–C, Photographs show MWA with two probes at 80 W (A), MWA with two probes at 70 W (B), and MWA with two probes at 60 W (C). Dl = long-axis diameter, Ds = short-axis diameter.
D, Multipolar RFA using two T40 probes rated at 80 W.
Fig. 5A Graphs show different temperature rise curves at all ablation settings.
A, Temperature rise curve t0 (A), t2 (B), and t3 (C) for all ablations. T40 = T40 probe.
Fig. 5B Graphs show different temperature rise curves at all ablation settings.
B, Temperature rise curve t0 (A), t2 (B), and t3 (C) for all ablations. T40 = T40 probe.
Fig. 5C Graphs show different temperature rise curves at all ablation settings.
C, Temperature rise curve t0 (A), t2 (B), and t3 (C) for all ablations. T40 = T40 probe.

Discussion

RFA and MWA are both widely used in the management of nonresectable malignant liver tumors [10, 11]. A crucial issue, however, in local thermoablative treatment of hepatic malignancies is the limited coagulation zone size, particularly when the treated tumor is larger than 5 cm [12, 13]. Thus, technical advances that allow a larger ablation zone are needed. One innovation recently developed involves multipolar RFA. This technique combines the benefits of multiprobe insertions, internally cooled probes, and resistance-controlled pulsed-energy deposition to provide larger ablation zones because the energy is focused on the target zone [68, 14, 15].
Although multipolar RFA has many advantages, we found that MWA generated significantly larger ablation zones compared with both kinds of multipolar RFA (p < 0.05) in our study. Compared with multipolar RFA, MWA had better thermal efficiency when using two probes simultaneously. Several reasons may explain this finding. First, RFA is fundamentally restricted by the need to conduct electrical energy into the body. As the temperature rises and charring occurs, increased impedance limits further deposition of electricity into tissue [16]. However, MWA does not rely on conduction of electricity into tissue and is not limited by charring. Second, in RFA, heating is due to thermal conduction, which decreases exponentially away from the radiofrequency emission source. Finite-element computer modeling suggests that this results in an inefficient transformation of electrical energy into heat [17]. Third, MWA is less affected by the heat-sink effect of local blood vessels in vivo, which is thought to contribute to local recurrence after RFA [1]. In addition, inherent theoretic advantages also affect the results. For multipolar RFA to achieve a larger volume of coagulation, a prolonged RFA is required at a lower power output [6].
In comparing the Ds/Dl ratios, there were no significant differences for power settings between MWA and multipolar RFA (p > 0.05). Simultaneously using two probes generated longer Ds, and all ratios of microwave ablation were > 0.8. This, in turn, produced ablation shape that approximated a sphere, which is characteristic of most tumors and thus would facilitate the treatment of larger liver tumors in one ablation session.
All power settings in MWA produced higher temperatures compared with multipolar RFA (Table 2). Especially at location t2, the temperature of T40 multipolar RFA had a greater disparity compared with the temperature of MWA. This finding further shows that microwaves penetrate more deeply than do radiofrequency waves, and the delivery of microwave energy depends less on tissue texture and impedance. For all MWA power settings, we found that the time for the temperature to rise to 60°C (the temperature at which irreversible cellular damage occurs almost instantaneously) at location t0 was shorter compared with T40 multipolar RFA, which indicated that MWA had a faster rate of temperature rise with less ablation time.
In this RFA system, continuous application of radiofrequency energy leads to an inverse relationship between volume of coagulation and power output [14]. Therefore, we used an 80-W setting for the T40 multipolar RFA to facilitate the best ablation zone according to commercial recommendations but avoided 120, 140, and 160 W (effectively 60, 70, and 80 W) to match the power applied using MWA. To compare both ablation techniques using the same unit-time limit, an application time of 10 minutes was used for all ablations.
Our study had several limitations. We used normal porcine liver as a proxy for human liver tumors. The size and morphologic characteristics of porcine liver cannot be extrapolated to clinical practice because liver tumors, as well as cirrhotic liver, have different tissue compositions and vascularity. In addition, because of engineering differences, the geometry of ablation zones was specific to the antenna used and cannot be applied to other probes using the same design. Finally, our study used a small sample size, making comparisons difficult. Additional studies will be needed to determine if MWA applies greater ablation zones than RFA in liver tumors compared with ablation zones obtained in the pig model with normal livers.
In conclusion, MWA, using the simultaneous application of double antennae, can generate a larger ablation zone in porcine liver compared with multipolar RFA. Evaluation of the therapeutic efficacy of these two systems in clinical trials is warranted.

Footnotes

W. Fan and X. Li contributed equally to this work.
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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: W46 - W50
PubMed: 22194514

History

Submitted: February 18, 2011
Accepted: May 18, 2011

Keywords

  1. ablation
  2. microwave ablation
  3. radiofrequency ablation

Authors

Affiliations

WeiJun Fan
All authors: Department of Medical Imaging and Interventional Radiology, Cancer Center and State Key Laboratory of Oncology in South China, Sun Yat-sen University, No. 651 Dongfeng East Rd, Guangzhou, Guangdong Province, 510060, China.
Xin Li
All authors: Department of Medical Imaging and Interventional Radiology, Cancer Center and State Key Laboratory of Oncology in South China, Sun Yat-sen University, No. 651 Dongfeng East Rd, Guangzhou, Guangdong Province, 510060, China.
Liang Zhang
All authors: Department of Medical Imaging and Interventional Radiology, Cancer Center and State Key Laboratory of Oncology in South China, Sun Yat-sen University, No. 651 Dongfeng East Rd, Guangzhou, Guangdong Province, 510060, China.
Hua Jiang
All authors: Department of Medical Imaging and Interventional Radiology, Cancer Center and State Key Laboratory of Oncology in South China, Sun Yat-sen University, No. 651 Dongfeng East Rd, Guangzhou, Guangdong Province, 510060, China.
JianLei Zhang
All authors: Department of Medical Imaging and Interventional Radiology, Cancer Center and State Key Laboratory of Oncology in South China, Sun Yat-sen University, No. 651 Dongfeng East Rd, Guangzhou, Guangdong Province, 510060, China.

Notes

Address correspondence to W. Fan ([email protected]).

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