High-Performance Permanent Magnet Array Design by a Fast Genetic Algorithm (GA)-based Optimization for Low-Field Portable MRI

Abstract

A permanent magnet array (PMA) is a preferred source of magnetic field for body-part-dedicated low-field (< 0.5 T) portable magnetic resonance imaging (MRI) because it has a small footprint, no power consumption, and no need for a cooling system. The current popular PMA is limited by the field strength to be below 100 mT and the transversal field direction where advanced technologies developed for the long-bore MRI systems (e.g., multi-channel techniques) cannot be applied. In this paper, a sparse high-performance PMA is proposed based on inward-outward ring pairs and using magnet blocks that can be bought off the shelf, targeting on portable head imaging. Through a fast genetic algorithm (GA)-based optimization, the proposed PMA has a longitudinal magnetic field with an average field strength of 111.40 mT and a monotonic field pattern with inhomogeneity of 10.57 mT (an RF bandwidth of < 10%) within a Field of View (FoV) of 20 cm in diameter and 4.5 cm in length. The field generated by the design was validated using analytic calculations and numerical simulations. The field can be used to supply gradients in one direction working with gradient coils in the other two directions, or can be rotated to encode signals for imaging with an axial selection. The encoding capability of the designed PMA was examined through checking the quality of the simulated reconstructed images. When it is used for encoding, the field pattern favors imaging with good quality, which even outperforms a linear pattern. The magnet array is 57.91 cm wide, 38 cm long, has a 5-gauss range of 87 × 87 × 104 cm 3 , allowing an operation in a small space. It weights 126.08 kg, comprising of a stationary main array that supplies a homogeneous fields with high field strength, and a light rotatable sub-gradient-array (5.12 kg) that supplies a monotonic field for signal encoding. It has a magnetic field generation efficiency of 0.88 mT/kg which is the highest among sparse PMAs that offer a monotonic field pattern. The force each magnet experiences in the design was calculated and the feasibility of a physical implementation is examined. The design can offer an increased field strength thus an increased signal-to-noise ratio (SNR). It has a longitudinal field direction that allows the applications of technologies developed for a long-bore system, such as surface coils and multi-channel technology without compromising coil efficiency. This proposed PMA can be a promising alternative to supply the main field and gradient fields combined for dedicated portable MRI.