(K-416) Experimental Evaluation of Microwave Ablation Antennas in Ex Vivo Tissue

Microwave ablation (MWA) is a minimally invasive procedure in clinical use for the treatment of unresectable tumors in the liver, lung, kidney, and other sites. MWA uses image-guidance to deliver microwave power into tissue where it is absorbed, leading to heating and coagulative necrosis of tissue at temperatures exceeding ~55 °C. MWA procedures would benefit from methods to assess the transient growth of the treatment zone in real-time, thus providing a means to limit thermal damage to non-targeted tissue, while ensuring adequate treatment of the targeted tumors. Current approaches for assessing treatment technical success involve use of contrast-enhanced imaging to identify ablated regions which appear as non-perfused areas. Our long-term goal objective is to develop an electromagnetic transmission-coefficient based approach to conduct real-time monitoring of ablation zone growth, which we have so far investigated with MWA devices having directional radiation patterns. In the present study we conducted comparative experimental assessment of candidate omni-directional MWA devices to support future investigations of transmission-coefficient based monitoring of MWA using omni-directional devices.

We comparatively assessed three MWA device designs: a water-cooled monopole antenna (monopole length = 8.5 mm) with a polyimide outer sheath (o.d. = 2.8 mm); a water-cooled monopole antenna (monopole length = 8.5 mm) with a fiberglass outer sheath (o.d. = 2.2 mm); and a water-cooled monopole antenna (monopole length = 11 mm) with a choke and enclosed in a polyimide outer sheath (o.d. = 2.8 mm). Custom devices for each design were fabricated in our laboratory using UT-34 coaxial cable, hemostatic valves, and y-Lauer valves. Experiments were conducted in fresh ex-vivo bovine liver tissue to determine the size (long and short axis diameters) and shape (axial ratio = long axis / short axis) of the ablation zones for each antenna type when using applied power levels of 30 W or 65 W for 5 or 10 minutes. All experiments were performed in triplicate, for a total of 36 ablation experiments.

The table in Figure 1 lists the mean +/- standard deviation of the long and short axis dimensions of the ablation zone and the axial ratio, observed in ex vivo liver tissue for the three antenna designs investigated, across all four applied power/time combinations. Also in Figure 1 is an image of each device as well as an example of ablation for the three devices at each power/time combination. Choke antennas are anticipated to assist with constraining long-axis of ablation zones for high applied powers and/or ablation times. Consistent with this expectation, we observed for the higher power/time setting (65 W, 600 s), the polyimide monopole antenna with choke yielded the most spherical ablation zone, compared to the other two antennas. At lower power/time combinations, there was not a substantial difference in ablation zone sphericity between monopole and choke designs for polyimide outer sheath. However, the fiberglass monopole was consistently less spherical across all power/time combinations. We decided to select the polyimide monopole antenna with choke for future transmission coefficient-based studies, as experiments demonstrate this antenna yields the most spherical ablation zones, of comparable size to the other antennas at higher powers, which are most likely to be used in vivo.

The polyimide monopole with a choke design performed best for high power and longer duration ablations. Based on these findings, we plan to use the polyimide monopole with a choke device to test transmission-coefficient based monitoring to conduct real time monitoring of ablation zone.