Dual-slot antennas for microwave tissue heating: parametric design analysis and experimental validation.

PURPOSE: Design and validate an efficient dual-slot coaxial microwave ablation antenna that produces an approximately spherical heating pattern to match the shape of most abdominal and pulmonary tumor targets.
METHODS: A dual-slot antenna geometry was utilized for this study. Permutations of the antenna geometry using proximal and distal slot widths from 1 to 10 mm separated by 1-20 mm were analyzed using finite-element electromagnetic simulations. From this series, the most optimal antenna geometry was selected using a two-term sigmoidal objective function to minimize antenna reflection coefficient and maximize the diameter-to-length aspect ratio of heat generation. Sensitivities to variations in tissue properties and insertion depth were also evaluated in numerical models. The most optimal dual-slot geometry of the parametric analysis was then fabricated from semirigid coaxial cable. Antenna reflection coefficients at various insertion depths were recorded in ex vivo bovine livers and compared to numerical results. Ablation zones were then created by applying 50 W for 2-10 min in simulations and ex vivo livers. Mean zone diameter, length, aspect ratio, and reflection coefficients before and after heating were then compared to a conventional monopole antenna using ANOVA with post-hoc t-tests. Statistical significance was indicated for P <0.05.
RESULTS: Antenna performance was highly sensitive to dual-slot geometry. The best-performing designs utilized a proximal slot width of 1 mm, distal slot width of 4 mm +/- 1 mm and separation of 8 mm +/- 1 mm. These designs were characterized by an active choking mechanism that focused heating to the distal tip of the antenna. A dual-band resonance was observed in the most optimal design, with a minimum reflection coefficient of -20.9 dB at 2.45 and 1.25 GHz. Total operating bandwidth was greater than 1 GHz, but the desired heating pattern was achieved only near 2.45 GHz. As a result, antenna performance was robust to changes in insertion depth and variations in relative permittivity of the surrounding tissue medium. In both simulations and ex vivo liver, the dual-slot antenna created ablations greater in diameter than a coaxial monopole (35 mm +/- 2 mm versus 31 mm +/- 2 mm; P<0.05), while also shorter in length (49 mm +/- 2 mm versus 60 mm +/- 6 mm; P < 0.001) after 10 min. Similar results were obtained after 2 and 5 min as well.
CONCLUSIONS: Dual-slot antennas can produce more spherical ablation zones while retaining low reflection coefficients. These benefits are obtained without adding to the antenna diameter. Further evaluation for clinical microwave ablation appears warranted.