A lightweight cylindrical Frequency Selective Surface (FSS) using high-pass inductive mesh grid is proposed as a radiofrequency (RF) shield for solenoid coils at 2.84 MHz working in a portable MRI system with a transversal B0-field averaged at 67 mT. Using a solenoid coil of 60 mm diameter as an example, the proposed FSS shield shows good shielding effectiveness at different wall-to-wall distances between the coil and the shield (5-25 mm). It also shows less compromise in B1-sensitivity when compared to the coil with a copper shield. The effects of the FSS shield were examined numerically and validated experimentally. When the wall-to-wall distance is 5 mm, the solenoid coil with the proposed FSS shield is compact, lightweight, and shows B1-sensitivity of more than 50% higher than the copper counterparts with comparable shieding effectiveness. Compared to an unshielded coil, it shows a wider bandwidth which benefits the excitation of an MRI system that has less homogeneous fields and a smaller inductance which indicates less trapping of energy and thus can enable different pulse sequence programming by taking this approach. This study shows that an FSS shield with inductive mesh grids around a solenoid coil is a promising approach for compact shielding without a big shielding box. It will contribute to the compactness of a portable MRI system.

Halbach array is a popular permanent magnet array used in head dedicated portable MRI. It supplies transversal main magnetic field (B0), and it works well with solenoid RF coils when B0 inhomogeniety is small. The B0 a Halbach array supplies is low. To compensate this, it is desired to have a high coil sensitivity of the solenoid coil that works in such a system. Hence in this paper, an asymmetrical tapered solenoid was proposed and an optimization was carried out for both the coil profile and pitches between turns for high coil sensitivity and homogeneity. Multi-objective genetic algorithm was used. The optimized solenoid coils are constructed and compared with a reference coil of comparable dimension. The optimal design shows B1 field increase of ∼33% and minimal tradeoff in homogeneity of ∼10 %. The measured results are in good agreement with simulated and calculated results.