Improved B1 distribution of an MRI RF coil element using a high-impedance-surface shield

In this paper we propose an approach to improve the B₁ distribution in terms of homogeneity and penetration depth of a coil element by utilizing a high impedance surface (HIS) as the RF shield for 7 T magnetic resonance imaging (MRI). The transverse magnetic field distribution in the case of a HIS and a perfect electrical conductor (PEC) being the shielding plate are compared for different separation distances from the dipole coil to the shielding plate. As the PEC shield is adjacent to the dipole coil, an undesired surface current is induced on the PEC shielding plate by the dipole coil, whereas the induced surface current on the HIS shield is sufficiently suppressed due to the high surface impedance. As a result, the dipole coil with a HIS shield exhibits a broader and stronger field distribution, and thus achieves an improvement on the transverse B₁ homogeneity as well as the penetration depth. As the separation distance increases, the impact of the induced current is weakened and thus variations on the field distribution with different shielding scenarios (HIS and PEC) are reduced. The proposed approach has been validated by numerical simulations and experimental measurements, which show a good agreement.