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Preinjected Fluids do not Benefit Microwave Ablation as Those in Radiofrequency Ablation

Posted Aug 31, 2011 Updated Apr 10, 2023
By Qiao Ji MD , Zuofeng Xu MD PhD , Guangjian Liu MD PhD , Manxia Lin MD , Ming Kuang MD PhD , Mingde Lu MD DMSc
10 min read

Rationale and Objectives

To detect whether the efficacy of microwave ablation (MWA) could be improved by preinjected fluids in an ex vivo porcine liver model.

Materials and Methods

Ablations were performed for 12 minutes using energy output of impedance-based (power output gradually rose to 200W, maintained until increases in tissue impedance of 20 Ω, reduced to 10W, and switched on again 15 seconds later) in radiofrequency ablation (RFA) or 80 W in MWA. Before ablation, 5 mL of ethanol, distilled water, 0.9% NaCl solution, or 10% NaCl solution ( n = 6 each) was injected into the targeted liver tissue. Ablations without fluid injection served as control. The ablation diameter, volume, shape index, and temperature were recorded and compared.

Results

Preinjection of 0.9% or 10% NaCl solution resulted in larger coagulation volumes than that of the control group in RFA experiments (28.1 ± 2.9 cm 3 , 45.3 ± 6.3 cm 3 , 20.0 ± 2.5 cm 3 , respectively; P < .05). Ethanol and distilled water had no impact on coagulation volumes in RFA. Preinjection of ethanol or 10% NaCl solution created smaller coagulation volumes than that of the control group in MWA experiments (34.3 ± 2.0 cm 3 , 33.9 ± 4.1 cm 3 , 58.0 ± 6.6 cm 3 , respectively; P < .001). 0.9% NaCl solution and distilled water had no impact on coagulation volumes in MWA.

Conclusion

In an ex vivo porcine liver, preinjected fluids do not benefit microwave ablation as those in radiofrequency ablation.

Image-guided microwave ablation (MWA) and radiofrequency ablation (RFA) are two widely used thermal ablation therapies to eliminate primary or metastatic liver tumors, especially for patients who are not candidates for surgical resection or liver transplantation . In RFA, a high-frequency alternating electric current (375–500 kHz) is used to create ionic flow, which produces frictional heat and heat conduction to induce tissue necrosis. Tissue electrical and thermal conductivities are important during RFA . In contrast, MWA involves the application of electromagnetic field by perturbing polar molecules (primarily H 2 O) in tissue. Tissues with high water content readily absorb microwave .

Currently, thermal ablation therapy is being developed to yield a large area of controlled coagulation necrosis with a single application of energy. Several strategies have been developed to increase the amount of coagulation necrosis, including increasing energy deposition, modulating tissue characteristics, and modifying tissue blood flow . RFA efficacy was shown to be improved when tissue was injected with ethanol or NaCl solution prior to, or during the ablation procedure . In MWA, however, the mechanisms for increasing ablative coagulation may not necessarily be the same, because heating relies on a different mechanism. To improve MWA, the question of whether preinjected liquids improve ablation should be explored independently. The purpose of this study was to detect whether the efficacy of MWA could be improved by preinjected fluids, as those in RFA.

Material and methods

Ablation Systems

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Figure 1, (a) Representative image of the thermocouple probes that were inserted 1, 2, 3, and 4 cm from the microwave antenna (shown at the far left) in microwave ablation (MWA) experiments. (b) Corresponding ultrasound image with the echogenic microwave antenna indicated with a star and the inserted thermocouple probes labeled as A-D for distances 1–4 cm to the right of the antenna, respectively.

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Ablation Experiments

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Statistical Analysis

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Results

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Figure 2, Representative images of the coagulation zones associated with radiofrequency ablation experiments using an ex vivo porcine liver. (a) Control group, Dl 3.8 cm, Ds 3.0 cm; (b) 0.9% NaCl solution group, Dl 4.0 cm, Ds 3.4 cm; (c) 10% NaCl solution group, Dl 5.0 cm, Ds 4.4 cm. The short-axis diameter of the 0.9% NaCl solution group was larger than that of the control group, yet was smaller than that of the 10% NaCl solution group. The coagulation zones of both NaCl solution groups were larger and more spherical than the control group.

Figure 3, Representative images of the coagulation zones associated with microwave ablation (MWA) experiments using an ex vivo porcine liver. (a) Control group, Dl 6.3 cm, Ds 4.6 cm; (b) ethanol group, Dl 5.0 cm, Ds 3.7 cm; (c) 10% NaCl solution group, Dl 5.1 cm, Ds 3.3 cm. The coagulation zones of the ethanol and 10% NaCl solution groups were smaller and less spherical than the control group.

Table 1

Mean Diameter, Volume, Shape Index, and Temperature of RFA Experiments

Parameters Control Preinjected Fluids Ethanol Distilled Water 0.9% NaCl Solution 10% NaCl Solution Coagulation diameter (cm) Long-axis 3.9 ± 0.2 4.1 ± 0.3 4.1 ± 0.2 4.1 ± 0.2 4.8 ± 0.1 ∗ Short-axis 3.1 ± 0.1 3.3 ± 0.2 3.3 ± 0.2 3.6 ± 0.1 ∗ 4.2 ± 0.3 ∗ † Dl/Ds ratio 1.26 ± 0.06 1.24 ± 0.07 1.26 ± 0.10 1.15 ± 0.05 ∗ 1.14 ± 0.05 ∗ Volume (cm 3 ) 20.0 ± 2.5 23.5 ± 3.3 22.9 ± 2.5 28.1 ± 2.9 ∗ 45.3 ± 6.3 ∗ † Maximum temperature (°C) Probe A 76.8 ± 8.2 81.1 ± 8.2 80.0 ± 8.5 86.7 ± 6.7 ∗ 97.3 ± 5.9 ∗ † Probe B 58.8 ± 7.4 59.5 ± 9.7 57.8 ± 7.4 59.2 ± 5.4 71.9 ± 8.5 ∗ Time to 54°C, probe A (seconds) 135.0 ± 33.9 110.0 ± 35.2 130.0 ± 39.0 88.3 ± 33.1 ∗ 61.7 ± 22.3 ∗

Six ablations were performed for each group. Data are the mean values ± standard deviation.

Dl, long-axis diameter; Ds, short-axis diameters; RFA, radiofrequency ablation.

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Table 2

Mean Diameter, Volume, Shape Index, and Temperature of MWA Experiments

Parameters Control Preinjected Fluids Ethanol Distilled Water 0.9% NaCl solution 10% NaCl Solution Coagulation diameter (cm) Long-axis 5.9 ± 0.2 5.2 ± 0.2 ∗ 6.0 ± 0.2 5.8 ± 0.2 5.3 ± 0.2 ∗ Short-axis 4.3 ± 0.2 3.6 ± 0.1 ∗ 4.3 ± 0.2 4.2 ± 0.2 3.5 ± 0.1 ∗ Dl/Ds ratio 1.36 ± 0.08 1.47 ± 0.08 ∗ 1.40 ± 0.08 1.38 ± 0.04 1.53 ± 0.03 ∗ Volume (cm 3 ) 58.0 ± 6.6 34.3 ± 2.0 ∗ 56.5 ± 5.0 54.5 ± 6.8 33.9 ± 4.1 ∗ Maximum temperature (°C) Probe A 94.2 ± 6.3 84.3 ± 9.8 ∗ 89.8 ± 6.1 95.0 ± 5.8 82.0 ± 8.6 ∗ Probe B 73.4 ± 8.6 61.2 ± 7.3 ∗ 69.2 ± 8.4 71.2 ± 6.9 60.4 ± 8.3 ∗ Time to 54°C, probe A (seconds) 98.3 ± 35.4 145.0 ± 39.9 ∗ 133.3 ± 32.0 116.7 ± 37.2 163.3 ± 38.3 ∗

Six ablations were performed for each group. Data are the mean values ± standard deviation.

Dl, long-axis diameter; Ds, short-axis diameters; MWA, microwave ablation.

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Figure 4, Temperatures detected at 1 cm (a) and 2 cm (b) from radiofrequency electrode during radiofrequency ablation experiments with duration of 12 minutes are plotted. Temperatures were recorded in presence or absence of preinjected fluids as indicated. Livers preinjected with 10% NaCl solution were associated with higher tissue temperatures that were achieved more rapidly than the other treatment groups.

Figure 5, Temperatures detected at 1 cm (a) and 2 cm (b) from microwave antenna during microwave ablation experiments with duration of 12 minutes are plotted. Temperatures were recorded in presence or absence of preinjected fluids as indicated. The rate of the increase in temperature associated with ethanol or 10% NaCl solution were lower than the control group.

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RFA

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Microwave Ablation

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Discussion

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Figure 6, Representative images of the needle tract area following the injection of methylene blue into ex vivo porcine liver tissue. (a) An injection of 2.5 mL methylene blue, resulting in a stained area of ∼2 cm × 6 cm. This volume did not cover the whole ablation area. (b) An injection of 5 mL methylene blue, resulting in a stained area of ∼4 cm × 6 cm. This volume covered the ablation area exactly. (c) An injection of 10 mL methylene blue, resulting in a stained area of ∼7 cm × 7 cm. This volume was in excess of the ablation area.

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Academic Radiology, Volume 18, Issue 9
Journals General Radiology
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