The electric dipole excitation is more advantageous compared to the loop excitation at 7T to yield higher signal efficiency [1]. Although the dipole antenna excites the Poynting vector outside the near-field region [2], its field excitation pattern is omnidirectional and linearly-polarized. At 7T human brain imaging, typically volume excitation to image larger field-of-view (FOV) or a loop coil was used. While the volume coil enhances the field at the center of the human brain, the surface coils enable high field efficiency just under the coil with limited field-of-view [3]. The aim of this study was to investigate the crossed-dipole antenna by means of electromagnetic simulations in which the dipoles placed in a crossed form mounted upon/in a dielectric with a conducting shield to create the circularly-polarized field by enhancing the transmit efficiency in larger FOV for 7 T human brain imaging, and compare it with the surface quadrature head loop and volume head coils in terms of B₁⁺ efficiency.

Electromagnetic field simulations: Finite difference time-domain (FDTD) simulations were performed on Sim4Life 2.0 (ZMT, Zurich MedTech AG, Zurich, Switzerland) on a Virtual Family human model [5] at 1 mm iso-gridded. The exact design of the circularly-polarized single-channel transmit/ 8-channel receive volume (Rapid Biomedical, Rimpar, Germany) and in-house built quadrature double loop coils (diameter:10 cm) used for 7T human brain imaging were modeled, and their simulated in-vivo B₁⁺ maps were similar to the measured in-vivo B₁⁺ maps (figures not shown).

Crossed-dipole antenna: Its design was studied with FDTD simulations at 297.2 MHz. Two dipole antenna (5×1 cm² each conductor) placed perpendicular to each other (Fig.1a). Rear dipole (Fig.1b) mounted in the dielectric and the front dipole (Fig.1c) mounted upon directly on the dielectric perpendicular to the rear dipole. The front dipole was placed directly on the head with 5 mm air gap to prevent the high local SAR deposition [4]. A conducting shield was mounted upon the dielectric at the opposite side of the front dipole (Fig.1a) to ensure the field direction towards the head instead of air. A dielectric substrate (surface area: 11×15 cm²) was placed between the front dipole and conducting shield with the thicknesses ranging from 1 to 3 cm. The crossed-dipole was placed behind the occipital lobe.

Fig.2 shows the circularly-polarized magnetic field created in the occipital lobe of the human brain with the crossed-dipole mounted upon/in a 3-cm thick dielectric (ε_r=200). Fig.3 shows the B₁⁺ map comparison of the crossed dipole, quadrature head loop and volume head coils. The crossed-dipole shows an increased mean B₁⁺ value by 32% compared to the loop coil and by %47 compared to the volume coil in the occipital lobe. The mean B₁⁺ value in the occipital lobe and cerebellum was evaluated for various dielectric constants ranging from 50 to 300 and for the dielectric thicknesses of 1, 2 and 3 cm (Fig.4). The surface area of the dielectric was kept same. The 3-cm thick dielectric with a dielectric constant of 300 enhances the B₁⁺ distribution in the occipital lobe (Fig4a,c). For the cerebellum, similar results were observed for the 3cm-thick dielectric with a dielectric constant of 150 (Fig.4b,d).The crossed-dipole enables higher transmit efficiency in the occipital lobe with a larger FOV compared to the conventional head loop and volume head coil. With the crossed-dipole design, the circularly-polarized magnetic field distribution is achieved under the dielectric in the occipital lobe and compared to the previous studies [6-7], the other novelty is adding the conducting layer behind the dielectric, which enhances the transmit field in the brain by partly shielding the field from the air and directing it towards to the head. In practice to build the prototype, the dielectric must be chosen in powder and the rear dipole can be embedded in the powder dielectric while its lumped elements and connection to the coaxial cable can be realized with the extension cables behind the ground plane. Dimension, dielectric constant of the dielectric, placement and the size of the crossed-dipole may be further optimized to be a suitable element for the whole-brain array element. We concluded that based on the electromagnetic field simulations it should be feasible to build a cross-dipole mounted upon/in a high-permittivity dielectrics with an one-side shielded conductor for 7 T human brain imaging.