Microwave Ablation: Comparison of Simultaneous and Sequential Activation of Multiple Antennas in Liver Model Systems

Abstract

Purpose: To compare microwave ablation zones created by using sequential or simultaneous power delivery in ex vivo and in vivo liver tissue.

Materials and methods: All procedures were approved by the institutional animal care and use committee. Microwave ablations were performed in both ex vivo and in vivo liver models with a 2.45-GHz system capable of powering up to three antennas simultaneously. Two- and three-antenna arrays were evaluated in each model. Sequential and simultaneous ablations were created by delivering power (50 W ex vivo, 65 W in vivo) for 5 minutes per antenna (10 and 15 minutes total ablation time for sequential ablations, 5 minutes for simultaneous ablations). Thirty-two ablations were performed in ex vivo bovine livers (eight per group) and 28 in the livers of eight swine in vivo (seven per group). Ablation zone size and circularity metrics were determined from ablations excised postmortem. Mixed effects modeling was used to evaluate the influence of power delivery, number of antennas, and tissue type.

Results: On average, ablations created by using the simultaneous power delivery technique were larger than those with the sequential technique (P < .05). Simultaneous ablations were also more circular than sequential ablations (P = .0001). Larger and more circular ablations were achieved with three antennas compared with two antennas (P < .05). Ablations were generally smaller in vivo compared with ex vivo.

Conclusion: The use of multiple antennas and simultaneous power delivery creates larger, more confluent ablations with greater temperatures than those created with sequential power delivery.

© RSNA, 2015.

Figures

Figure 1:

Experimental design. Ex vivo and in vivo tissue models and two- or three-antenna arrays were used to assess differences in ablations created with sequential ablation or simultaneous ablation with multiple antennas.

Figure 2:

Placement of temperature sensors (+) for two- (2) or three (3)–antenna arrays (◯). Temperature sensors were placed in ex vivo tissue such that sensor A was at center of array, whereas sensors B–D were separated by 1 cm along a radial line.

Figure 3:

Representative images of two- and three-antenna ablation zones illustrate differences between energy delivery and number of antennas. Sequential ablation zones are markedly less confluent, with clefts noted between each ablation (arrows). Scale is in centimeters.

Figure 4:

Box-and-whisker plot of maximum inscribed circle diameter. Simultaneous power delivery and the use of three antennas produced larger ablation zones. Ablations were slightly larger in ex vivo model. Seq = sequential, Sim = simultaneous. ◯ = outlier.

Figure 5:

Box-and-whisker plot of circularity. Simultaneous power delivery and use of three antennas produced significantly greater ablation circularity. Unlike ablation size metrics, circularity was not influenced significantly by tissue model. Seq = sequential, Sim = simultaneous. ◯ = outliers.

Figure 6:

Graphs show mean temperatures recorded during simultaneous (black) and sequential (red) power delivery at points A (solid), B (long dashes), C (short dashes), and D (dots) in Figure 2. Two-antenna (left) and three-antenna (right) ablations are shown. Heating was faster when using simultaneous power delivery compared with sequential energy delivery (16°C/sec vs 10°C/sec for two antennas; 14°C/sec and 8.7°C/sec for three antennas; P < .001).

Figure 7:

Box-and-whisker plot of volume with side-by-side comparison of tissue type. Ablations were slightly larger in ex vivo model, but the effect of tissue model was reduced by using simultaneous energy delivery or three antennas.