Recent advances in designing balun-free interstitial antennas for minimally-invasive microwave ablation

We discuss recent advances in the design of balun-free interstitial antennas for minimally-invasive microwave ablation (MWA). Specifically, two different classes of balun-free MWA antennas are discussed in this paper. The first class of antennas consists of monopole-type radiators fed at their bases using coaxial cables and operated at the second resonant frequency. Under these conditions, a compact specific absorption rate (SAR) pattern is achieved without the need to use a coaxial balun encompassing the outer periphery of the feeding coaxial cable as is done in conventional coax-fed MWA antennas. The second class of antennas consists of balanced dipole or loop antennas fed with inherently balanced, shielded transmission lines. In such a structure, the current flowing on one conductor of the feeding line (and the antenna arm) is balanced by the current flowing on the other conductor (the other antenna arm). Therefore, no RF current flows on the outer surface of the floating shield and the antenna achieves a compact SAR pattern and localized heating zone. Design considerations, principles of operation, and measurement results including the results of ex vivo ablation experiments for representative prototypes of both classes of antennas are presented and discussed in the paper.