Introduction

The half-wave dipole antennas provide high penetration depth and relatively uniform transmit fields and hence, are one of the favorites in Ultrahigh field MRI. The half-wave dipole antenna consists of two conductors of the same lengths on both sides. The half-wave dipole antenna is fed in the center and produces higher electric field strength on the ends of the half-wave dipole antennas, and the magnetic field is more robust in the center. The electric length of the dipole primarily determines the resonant frequency of the dipole antenna. Therefore, it is challenging to change the dipole length or coil size at a given frequency to meet the need of a specific application. The high-permittivity dielectric materials are known to affect the frequency and electric/magnetic fields of various RF coil elements. In practice, to tune the half-wave dipole antennas at specific frequencies, we can only change the conductors' length and thickness. Hence, for 300MHz/7T applications, the total dipole length can be as long as 50cm. The larger length introduces an issue when the region of interest is too small compared to the dipole length. Hence according to their MR application, it is crucial to resize them based on the region of interest. The Meander elements are one of the approaches to tune small-sized half-wave dipole antennas at specific frequencies. Our study presents a new method that utilizes high dielectric material to tune the smaller half-wave dipole antennas at 300MHz and also enhance the B1 field of the dipole.

Method

We modeled three half-wave dipole antenna designs: a conventional half-wave dipole antenna resonating at 300MHz, a conventional half-wave dipole antenna of target length, and a dielectric material-coated half-wave dipole antenna of target length. The conventional half-wave dipole antenna for 300MHz is as shown in Fig 1a. The dimensions for a Conventional half-wave Dipole antenna are as follows: one arm length: 23.2cm, total length:46.4cm, the diameter of the conductor:4mm, feed gap between two arms:1mm. Our goal was to tune a shorter half-wave dipole antenna at 300MHz; hence, we specified our target length as 25 cm. For comparison, we built a conventional half-wave dipole antenna of 25 cm and evaluated the field distributions and the resonant frequency. The dimensions for a 25 cm Conventional half-wave dipole antenna are as follows: one arm length: 12.5cm, total length:25cm, the diameter of the conductor:4mm, feed gap between two arms:1mm. The resonant frequency of the 25 cm antenna was found out to be 548 MHz. Finally, to tune this shorter dipole antenna at 300MHz, we added a dielectric material coating across the length of each conductor. We computed parametric sweep, tried different thicknesses and relative permittivity for the high dielectric material, and chose the best parameters at which the antenna was tuned at 300MHz. The 25 cm dielectric material-coated half-wave dipole antenna can be seen in Fig 3 a, b. The dielectric material-coated half-wave dipole antenna dimensions are as follows: one arm length: 12.5cm, total length:25cm, the diameter of the conductor:4mm, feed gap between two arms:1mm, the thickness of the dielectric material: 5.4mm. The dielectric material used in the "dielectric material-coated half-wave dipole antenna" has the following properties: ε_r: 1000, conductivity: 0 s/m, relative permeability: 1.

Results

The simulation results of the Conventional half-wave dipole antenna tuned at 300MHzHH, a 25 cm Conventional half-wave dipole antenna, and the Dielectric coated half-wave dipole antenna operated at 300MHz are obtained as shown in fig.1,2 &3. The conventional half-wave dipole antenna tuned at 300MHz produced the highest electric fields of 1.54×10⁴ V/m. The 25 cm conventional half-wave dipole antenna produced the highest electric fields of 1.39×10⁴ V/m while the "dielectric material-coated half-wave dipole antenna" produced the highest electric fields of 1.25×10⁴ V/m. Additionally, the "dielectric material-coated half-wave dipole antenna" produced more penetrating and more robust B1 field distribution when compared with the conventional half-wave dipole antenna (135µT vs. 16.6µT)

Discussion/Conclusion

In this study, we proposed a new approach that includes using dielectric material-coated dipoles, aiming to control the dipole length at a given frequency and enhance the B1 field of a dipole. The Field distributions and the S-parameter plots were evaluated by Electromagnetic simulation using COMSOL Multiphysics Software. We conclude that the new approach using dielectric material coating effectively tuned the shorter half-wave dipole antennas at specific frequencies, reduced the electric fields, and enhanced the B1 field distribution.

Acknowledgements

No acknowledgement found.

References

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