Key designs of a short-bore and cryogen-free high temperature superconducting magnet system for 14 T whole-body MRI

Abstract

A 14 T whole-body magnetic resonance imaging (MRI) magnet is designed with Bi2223 high temperature superconducting (HTS) wire. Owing to the large critical current and large critical tensile strength of the wire type HT-NX at ultra-high field, the magnet is optimized in a compact shape with length of 1.9 m and outer diameter of 1.3 m. It is in the form of stacked double pancakes (DPs) with total wire consumption of 455 km. The DPs with various inner and outer radii enable the electromagnetic design to obtain highly homogeneous field of 1.5 ppm over 400 mm diameter of spherical volume (DSV). Hoopstress by winding process, thermal contraction and Lorenz force is calculated and below the wire strength specification. Considerable reduction of screening current effect in MRI magnet application is proved by a calculation example. Field homogeneity as it is influenced by various systematic tolerances and random geometric tolerances is numerically estimated, for further design optimization and design of shimming system. Iron room for passive shielding with dimension of 5 × 5 × 10 m is found to be with wall thickness of around 30 cm to effectively contain the 5-Gauss field to the wall surface. Cooling strategy for cryogen-free technique is proposed and the magnet coils temperature is calculated as below 8 K based on the estimation of joint Ohmic heating power of 1.0 W. Quench protection adopts circuit subdivision and dumping resistors to restrict the maximum voltage below 1.9 kV. No-insulation technique for the quench protection is also discussed. Some other wire candidates like REBCO and Nb3Sn for development of such ultra-high field MRI magnets are commented.