The goal of this study is to reduce rotation angles for faster scans in a gradient-coil-free portable MRI system, employing rotational spatial encoding magnetic fields (rSEM) with non-linear gradients. This is achieved by utilizing a proposed evaluation metric for encoding capability, thereby making optimization feasible. The local k-space-based evaluation metric for each sub-region of image is calculated by summing the area covered by the spokes in the local k-space when the SEM rotates; each spoke corresponds to one angle. This evaluation metric is expected to correlate closely with the image quality of the reconstructed images and is applied for angle selection to accelerate scans in rSEM systems. Greedy algorithm is used for the selection. Both simulations and experiments were conducted to validate the effectiveness of the evaluation method and the selected angle set. The results demonstrate that the proposed evaluation metric for the encoding capability of rSEM has correlation coefficient greater than 0.90 with image quality metrics of the corresponding reconstructed images. Using the angle set selected by greedy algorithm with proposed method, higher image quality with reduced angles is achieved. This further validates the accuracy of the proposed evaluation metric and underscores the necessity of rapid assessment for feasible optimization. A significant reduction of rotation angles is achieved by rotation angles selection using the proposed evaluation method. This facilitates fast scans in gradient-coil-free portable MRI systems with rotating SEM while maintaining high image quality. This advancement is crucial for the practical application of this type of portable MRI.

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.