Design and Optimization of a Ring-Pair Permanent Magnet Array for Head Imaging in a Low-Field Portable MRI System

In this paper, we present the design and optimization of a ring-pair permanent magnet array for head imaging in a low-field portable magnetic resonance imaging (MRI) system. The proposed array generates a longitudinal main static magnetic field ( ${B}_{0}$ field) with an average field strength of 169.7 mT and a homogeneity of 24 786 ppm in a field of view with a diameter of 200 mm and a thickness of 50 mm. It is a significant increase in field homogeneity by 79.7% compared to a traditional ring-pair structure of the same dimension and mass while still maintaining a similar field strength. The optimization was implemented by applying a genetic algorithm and by proposing an efficient current model for the forward calculation of the magnetic field. The effectiveness of the optimization is validated by realistic simulations using COMSOL Multiphysics. Compared to a Halbach array, where the field is transversal and the existing coil designs cannot be applied directly, the proposed array generates stronger fields. Its magnetic field is longitudinal, which allows the direct application of the advancements in RF coil designs in a conventional MRI system to the imaging system using the proposed magnet array.