Reduced-Diameter Designs of Coax-Fed Microwave Ablation Antennas Equipped With Baluns

We present a technique for minimizing the impact of coaxial baluns on the invasiveness of coax-fed microwave ablation (MWA) antennas. The technique uses an air-filled coax section at the end of a Teflon-filled coaxial feedline. The air-filled coax section has a smaller outer-conductor diameter compared to the Teflon-filled coax section for the same inner conductor diameter and characteristic impedance. Therefore, the coaxial balun used in a typical MWA antenna can be implemented on the air-filled coax section without exceeding the outer diameter of the Teflon-filled coaxial feedline. The efficacy of this technique is demonstrated for two different MWA antennas, a choke dipole and a floating sleeve dipole. Prototypes of these antennas as well as prototypes of their unmodified versions were designed, fabricated, and used to perform ablation experiments in ex vivo porcine muscle at 7 GHz with a power level of 30 W for 5 min. Simulation and measurement results confirmed that the modified antennas had similar responses as the unmodified antennas while offering 30% smaller diameters compared to their conventional balun-equipped coax-fed MWA antennas. Specifically, the antennas exhibited good impedance matching and produced localized ablation zones with shapes and dimensions similar to those produced by the original designs.