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 +/- 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.

Figure 1

Schematic of the dual-slot antenna design. Two slots of width G₁ and G₂ are separated by a length, L. The distal slot abuts a cap at the end of the antenna.

Figure 2

Cost function terms for reflection coefficient (black line, upper x-axis) and heating aspect ratio (gray line, lower x-axis) reflect the desire to maximize antenna efficiency and heating sphericity with using smooth functions and practical limits.

Figure 3

Three-dimensional scatterplot of objective function values for all input permutations. More desirable (lower) values are noted by large dark dots. This plot allows visualization of the influence of each geometric parameter, with the best objective values clustered around the G₁ = 1 mm, L = 8 mm, G₂ = 4 mm region.

Figure 4

Scatterplot of reflection coefficients and aspect ratios generated by all permutations simulated illustrates the large number of possibilities for low reflection coefficient or high SAR aspect ratio. Few points, noted in the upper left corner of this plot, exhibit low reflection coefficients and high SAR aspect ratios.

Figure 5

Reflection coefficient of the most optimal dual-slot coaxial antenna (black) and coaxial monopole antenna (red) from 0.5–6.0 GHz in normal liver tissue from simulation (dashed line) and experimental measurements (solid line). Note that the most optimal design produces a double resonance: one at 2.45 GHz and one at 1.25 GHz.

Figure 6

Normalized SAR map of the dual-slot coaxial antenna for G₁ = 1 mm, L = 8 mm, and G₂ = 4 mm at 2.45 GHz in normal liver tissue demonstrates that heating is relatively confined to the distal end with the most optimal design. Slots are located at 63 and 72 mm. Isocontours are provided from 0 dB to −60 dB below the peak SAR around the tip of the antenna.

Figure 7

Comparison of heating patterns created by a dual-slot antenna at 1.25 and 2.45 GHz. Despite low reflections of −20 dB at each frequency, the antenna only produces the desired heating pattern at 2.45 GHz.

Figure 8

Comparison of reflection coefficients and normalized SAR for the dual-slot antenna at three depths of insertion: 20, 40, and 60 mm. The air-tissue interface lies at z = 10 mm. Reflections were relatively unaffected by insertion depth at 2.45 GHz due to the truncated heating pattern, but more substantial variations were noted at 0.5–1.5 GHz due to the elongated heating at those frequencies.

Figure 9

Simulated reflection coefficient of the dual-slot antenna for baseline liver tissue (0%) and changes in relative permittivity of ±10, ±20, and ±30%. At 2.45 GHz the reflection coefficient ranges from −24 dB to −13 dB. There was virtually no change in reflection coefficient at 1.6 GHz.

Figure 10

Mean diameters and lengths of ablations created by the optimal dual-slot (black∕square) and monopole (gray∕diamond) antennas. Simulations are represented by dashed lines, experiments by symbols. The 50 °C isotherm defined the ablation boundary in simulations. Bars represent the standard error of the mean in experimental data.

Figure 11

Ablations created by the optimal dual-slot (left) and monopole (right) antennas in ex vivo bovine liver. Both images presented at equal scale. A total of 50 W at 2.45 GHz was delivered for 10 min in each case. Overall, dual-slot ablations were shorter in length and larger in diameter than monopole ablations.