Comparison of Microwave Hyperthermia Applicator Designs with Fora Dipole and Connected Array

Abstract

In microwave hyperthermia tumor therapy, electromagnetic waves focus energy on the tumor to elevate the temperature above its normal levels with minimal injury to the surrounding healthy tissue. Microwave hyperthermia applicator design is important for the effectiveness of the therapy and the feasibility of real-time application. In this study, the potential of using fractal octagonal ring antenna elements as a dipole antenna array and as a connected array at 2.45 GHz for breast tumor hyperthermia application was investigated. Microwave hyperthermia treatment models consisting of different fractal octagonal ring antenna array designs and a breast phantom are simulated in COMSOL Multiphysics to obtain the field distributions. The antenna excitation phases and magnitudes are optimized using the global particle swarm algorithm to selectively increase the specific absorption rate at the target region while minimizing hot spots in other regions within the breast. Specific absorption rate distributions, obtained inside the phantom, are analyzed for each proposed microwave hyperthermia applicator design. The dipole fractal octagonal ring antenna arrays are comparatively assessed for three different designs: circular, linear, and Cross-array. The 16-antenna dipole array performance was superior for all three 1-layer applicator designs, and no distinct difference was found between 16-antenna circular, linear, or cross arrays. Two-layer dipole arrays have better performance in the deep-tissue targets than one-layer arrays. The performance of the connected array with a higher number of layers exceeds the performance of the dipole arrays in the superficial regions, while they are comparable for deep regions of the breast. The 1-layer 12-antenna circular FORA dipole array feasibility as a microwave hyperthermia applicator was experimentally shown.

Keywords: cancer therapeutics; connected array; dipole antenna; fractal octagonal ring array; microwave hyperthermia; particle swarm optimization.

Figure 1

(a) FORA dipole element. (b) FORA—connected element and the inter-digital capacitor between the two elements in consecutive layers.

Figure 2

(a) Simulated S11 parameter as a function of frequency for a single FORA Dipole antenna in a linear array configuration, (b) and for different FORA—connected array elements.

Figure 3

(a) Circular array with 16 FORA dipoles (top view). (b) Cross—array with 12 FORA dipoles (top view). (c) Linear array with eight FORA dipoles (side view). (d) Connected array with 39 FORA elements and three layers (side view).

Figure 4

Flowchart of the optimization scheme.

Figure 5

The $TBR$ values evaluated at the target locations on the x-axis for 1-layer N-antenna circular dipole array ($CA$) with: (a) $\Delta r$ = 10 mm, (b) $\Delta r$ = 2 mm.

Figure 6

The $TBR$ values evaluated at the target locations on the x-axis for 1-layer N-antenna linear dipole array ($LA$) with $dx$ = 2 mm and (a) $dy$ = 0.6 ${\lambda }_0$, (b) $dy$ = 0.9 ${\lambda }_0$, (c) $dy$ = 1.2 ${\lambda }_0$.

Figure 7

The $TBR$ values evaluated at the target locations on the x-axis for 1-layer N-antenna cross-dipole array ($XA$) with: (a) $dy$ = 0.3 ${\lambda }_0$, (b) $dy$ = 0.6 ${\lambda }_0$, (c) $dy$ = 0.9 ${\lambda }_0$.

Figure 8

The $TBR$ values evaluated at the target locations on the x-axis for: (a) 1-layer 8-antenna, (b) 1-layer 12-antenna, (c) 1-layer 16-antenna circular dipole array ($CA$), linear dipole array ($LA$) and cross-dipole array ($XA$) applicators.

Figure 9

The $TBR$ values evaluated at the target locations on the x-axis for: (a) 2-layer circular dipole array ($CA$) with $\Delta r$ = 10 mm and N = 16, N = 32 and 1-layer 16-antenna $CA$ with $\Delta r$ = 10 mm, (b) 2-layer linear dipole array ($LA$) with $dx$ = 2 mm and $dy$ = 1.2 ${\lambda }_0$ and N = 16, N = 32 and 1-layer 16-antenna $LA$ with $dx$ = 2 mm and $dy$ = 1.2 ${\lambda }_0$, and (c) 2-layer cross-dipole array ($XA$) with $dx$ = 10 mm and $dy$ = 0.6 ${\lambda }_0$ and N = 16, N = 32 and 1-layer 16-antenna $XA$ with $dx$ = 10 mm and $dy$ = 0.6 ${\lambda }_0$.

Figure 10

The $TBR$ values evaluated at the target locations on the x-axis for FORA—connected array ($ConA$) with multi-layer, 1-layer 16-antenna cross-dipole array ($XA$) with $dx$ = 10 mm and $dy$ = 0.6 ${\lambda }_0$ and 2-layer 32-antenna cross-dipole array ($XA$) with $dx$ = 10 mm, $dy$ = 0.6 ${\lambda }_0$ and $dz$ = 0.6 ${\lambda }_0$.

Figure 11

$SAR$ distributions [W/kg] focused at (10, 0, 0), (30, 0, 0), (0, 30, 0), and (20, 10, 0) mm positions, and obtained with 1-layer of FORA dipole elements. (a–d) Circular array with 16 antennas and $\Delta r$ = 10 mm. (e–h) Cross—array with 16 antennas, $dx$ = 10 mm, and $dy$ = 0.6 ${\lambda }_0$. (i–l) Linear array with 16 antennas, $dx$ = 2 mm, and $dy$ = 1.2 ${\lambda }_0$.

Figure 12

Temperature (℃) at (10, 0, 0) mm target point as a function of exposure time for different total input power levels (W).

Figure 13

(a) The block diagram of the experimental system. (b) The circular FORA dipole array prototype with the bottom half of the fat-mimicking phantom in the middle.

Figure 14

Computational results of the experimental setup. (a) Optimized SAR distribution (W/kg), (b) Temperature profile after 60 min (°C).

Figure 15

Experimental results. (a) Temperature distribution interpolated from the 13 points of the thermometer readings (°C), (b) Thermal camera image.