Superconducting Shim Coils Design for the 9.4 T Whole-Body MRI Magnet

This article focuses on the design and implementation of superconducting shim coils for the 9.4 T whole-body magnetic resonance imaging (MRI) superconducting magnet developed at the Institute of Electrical Engineering, Chinese Academy of Sciences. Through conducting a comparative analysis of the harmonic components of the magnetic field, we observed that the long solenoid structure of the ultrahigh field magnet often has a better homogeneity and smaller high-order harmonics compared with lower field split-design magnets. Thus, the third-order shim coils were able to be eliminated, leading to cost reduction and a simplified magnet structure. The implementation of this active shimming strategy resulted in significant improvements in field homogeneity in an efficient manner. Compared with the bare magnetic field over a 30 cm diameter spherical volume, the peak-to-peak and root-mean-square homogeneity of the shimmed field was 15.82 ppm and 4.11 ppm, which achieved 78.9% and 83.2% improvement, respectively. These enhancements will effectively facilitate subsequent passive shimming procedures. The research findings presented in this work provide valuable insights into the practical implementation of shimming methods for the ultrahigh field whole-body MRI superconducting magnet.