Dual‐slot antennas for microwave tissue heating: Parametric design analysis and experimental validation

Abstract

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 6 1 mm and separation of 8 mm 6 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 À20.9 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 6 2 mm versus 31 mm 6 2 mm; P < 0.05), while also shorter in length (49 mm 6 2 mm versus 60 mm 6 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.

Percutaneous ablation of pancreatic cancer

“…Some antennae create an elongated ablation area of up to 6 cm called triaxial antennae, which can burn skin or other structures adjacent to the treated organ; newer antennae, such as modified triaxial antennae or dual slot antennae, therefore create more rounded and forward-weighted heating to treat smaller tumors[22,36,37]. Multiple antennae can be used together to create a larger and more confluent ablation area inside the tumor[22,38,39].…”

A Dual-Slot Microwave Antenna for More Spherical Ablation Zones: Ex Vivo and in Vivo Validation

“…A dual-slot antenna comprising two coaxial, annular slots at the distal tip was recently shown to inhibit proximal electric field propagation and, therefore, to shorten the length of ex vivo ablations (20). However, the antenna in that study was created from relatively malleable coaxial cables without active shaft cooling, making it unsuitable for clinical use.…”

“…The optimal cooled design provided a heating pattern and power delivery efficiency similar to those of the uncooled design previously described (20). A cooled monopole antenna served as a control P ercutaneous tumor ablation is a minimally invasive procedure that destroys cancerous tissue in situ with cytotoxic thermal energy.…”

Percutaneous Tumor Ablation Tools: Microwave, Radiofrequency, or Cryoablation—What Should You Use and Why?

“…(24,25). Many antennae produce an elongated ablation zone (as long as 6 cm) that may increase the risk for burns to the body wall and other structures, so newer designs aim for more rounded and forward-weighted heating that is amenable to treating smaller tumors (Fig 3) (26,27). Some systems also include antenna designs to transfer energy more optimally to different organ types, such as the lungs (28).…”