Multiple-Antenna Microwave Ablation: Spatially Distributing Power Improves Thermal Profiles and Reduces Invasiveness

Abstract

BACKGROUND: Microwave ablation is an emerging tumor ablation modality. To date, microwave systems have generally utilized single large-diameter antennas to deliver high input powers. OBJECTIVE: To determine whether spatially distributing power through an array of multiple smaller antennas creates a more uniform thermal profile and increases peripheral tissue temperatures when compared with microwave ablation using a single larger antenna. METHODS: Microwave ablations were performed in ex vivo bovine liver using a single 2.45-GHz magnetron generator and a constant total input power (90 W) delivered through either a single 13-gauge antenna, two 17-gauge antennas, or three 18-gauge antennas. Multiple antennas were driven coherently. Temperatures were recorded at 5-mm radial distances and the resulting thermal profiles and ablation zones were compared using analysis of variance. RESULTS: Multiple-antenna configurations were less invasive (ie, the area of tissue punctured was smaller) than the single-antenna configuration; despite this, ablation zones created using multiple smaller antennas were larger and as circular when compared with those created using a single larger antenna. Multiple-antenna configurations resulted in more uniform thermal profiles and higher peripheral tissue temperatures. CONCLUSION: Distributing power evenly among multiple smaller antennas resulted in larger ablation zones with more uniform thermal profiles than more invasive ablations with a larger single antenna.

Figures

Figure 1

Experimental setup. In this example, two 17-gauge triaxial antennas (right arrow) separated by 1.5 cm were inserted through an acrylic template (up arrow) into freshly excised bovine liver. Four fiberoptic thermosensors (Luxtron, left arrow) were placed 0.5, 1.0, 1.5, and 2.0 cm from the center of the array to a depth equal to that of the base of the antenna (the depth at which the electric field peaks).

Figure 2

Antenna and temperature sensor configurations. 17- and 18-gauge antennas were separated by 1.5 and 2.0 cm, respectively. During half of the multiple-antenna ablations, temperatures were recorded 0.5, 1.0, 1.5 and 2.0 cm from the center of the target tissue along an axis that bisected two antennas (Axis 1). During the other multiple-antenna ablations, temperatures were recorded along an axis running through the center of the target tissue and an antenna (Axis 2).

Figure 3

Cross-sections of microwave ablation zones created using a single 13-gauge antenna (A, yellow circle), two 17-gauge antennas (B, blue circle), and three 18-gauge antennas (C, green circle). Note the increased size and similar shape of multiple-antenna ablation zones despite the same total input power.

Figure 4

Ablation-to-invasiveness ratio. The ratio was calculated by dividing mean ablation zone cross-sectional area by the total cross-sectional area of the antennas used for the ablation—4.15, 3.54, and 3.39 mm2 for one 13-, two 17-, and three 18-gauge antennas, respectively.

Figure 5

Maximum tissue temperatures recorded along Axis 1 (A) and Axis 2 (B) shown in Figure 2. Temperatures near the center of the target tissue were highest during ablations performed using a single 13-gauge antenna. Ablations performed with three 18-gauge antennas resulted in the highest temperatures near the periphery (>1.5 cm from the center of the tissue) as well as the most uniform temperature distribution.

Figure 5

Maximum tissue temperatures recorded along Axis 1 (A) and Axis 2 (B) shown in Figure 2. Temperatures near the center of the target tissue were highest during ablations performed using a single 13-gauge antenna. Ablations performed with three 18-gauge antennas resulted in the highest temperatures near the periphery (>1.5 cm from the center of the tissue) as well as the most uniform temperature distribution.

Figure 6

Temperature profiles (A, B) and 50 °C isotherms (C) for single- and multiple-antenna groups. Note that the 50 °C isotherms during two-antenna ablations were greater than during single-antenna ablations along the axis of the antennas (B), but not perpendicular to it (A). In contrast, ablations with three 18-gauge antennas resulted in greater 50 °C isotherms in all directions when compared with those performed with one 13-gauge or two 17-gauge antennas.

Figure 6

Temperature profiles (A, B) and 50 °C isotherms (C) for single- and multiple-antenna groups. Note that the 50 °C isotherms during two-antenna ablations were greater than during single-antenna ablations along the axis of the antennas (B), but not perpendicular to it (A). In contrast, ablations with three 18-gauge antennas resulted in greater 50 °C isotherms in all directions when compared with those performed with one 13-gauge or two 17-gauge antennas.

Figure 6

Temperature profiles (A, B) and 50 °C isotherms (C) for single- and multiple-antenna groups. Note that the 50 °C isotherms during two-antenna ablations were greater than during single-antenna ablations along the axis of the antennas (B), but not perpendicular to it (A). In contrast, ablations with three 18-gauge antennas resulted in greater 50 °C isotherms in all directions when compared with those performed with one 13-gauge or two 17-gauge antennas.