A Short and Light, Sparse Dipolar Halbach Magnet for MRI

Recently designed dipolar Halbach magnets used in portable MRI systems are much lighter and more compact than standard permanent or superconductive magnets. However, improved designs and manufacturing techniques aiming at lower weight and smaller external size are an area of continual interest especially for application to space flight. Most Halbach magnet design techniques aim to optimize homogeneity suitable for MRI over a diameter-spherical volume (DSV) that requires the aspect ratio (length/inner diameter) to be larger than 1.5:1. Furthermore, current magnet construction techniques often use low-coercivity magnetic pieces and imperfect formers that produce a mismatch in the intended designs. As a result, Halbach magnets require complex shimming methods to improve the magnetic field homogeneity, causing further size and weight increase. Here, we propose to reduce the weight and the aspect ratio of the Halbach magnet by optimizing homogeneity over a cylindrical region of interest (ROI) rather than a DSV, applying a genetic algorithm, high-coercivity ferromagnets (N40UH) and a robust construction technique. The assembled 67 mT magnet, with aspect ratio ~ 1:1, produces almost identical homogeneity (11152 ppm) as simulations (11451 ppm) within a 12.7 cm diameter, 1 cm long cylinder ROI. The magnet structure was 3D printed ring-by-ring and assembled coaxially. The magnet can be disassembled for transportation.