Design of a Magnet and Gradient Coils for a Tabletop Liquid-Helium-Free, Persistent-Mode 1.5-T $\text{MgB}_2$ Osteoporosis MRI

We have finalized the design of a full-scale tabletop 1.5-T/90-mm MgB $_2$ finger MRI magnet system for osteoporosis screening based on our preliminary test results of small coils and superconducting joints. The magnet will operate in persistent mode at 10 K with an additional 5 K temperature margin. The magnet design which includes six main coils and an iron shield satisfies the required specification of a field intensity of 1.5 T, homogeneity of $\leq$ 5 ppm over a 20-mm diameter of spherical volume, and a fringe field of $\leq$ 5 gauss at 0.5 m in radius from the magnet center. An active protection method using external heaters will be applied to prevent a local hot spot in the MgB $_2$ windings from being overheated when quench occurs. Active shield transverse and axial gradient coils for this tabletop osteoporosis MRI, having primary and shield coil pairs, are designed to minimize stray fields that can induce eddy currents on nearby metal surface and thus imaging artifacts. This paper covers design and analysis of: 1) the main coils and iron shield; 2) coil former; 3) quench protection; and 4) active shield gradient coils. We also discuss design changes of the cryostat and equipment plan for the overall system. The magnet system will be completed and then, equipped with other MRI hardware components including an in-house-made gradient coil assembly and RF coils for demonstration of 1.5-T finger MRI in 2020.