Consideration on Current and Coil Block Placements With Good Homogeneity for MRI Magnets Using Truncated SVD

This paper describes a new method to determine the current and coil block placements with good homogeneity for magnetic resonance imaging (MRI) magnets, and discusses the relationships between the placements and homogeneity, using regularization of the truncated singular value decomposition (SVD). In the first step, the main coil (MC) is modeled as filament loop currents (FLCs) on a solenoid and the shield coil (SC) is modeled by coil blocks. The FLCs are determined so as to get a homogeneous magnetic field using a superposition of eigenmodes obtained by the SVD. In the second step, the FLCs are replaced by MC blocks. The findings are as follows. 1) The obtained FLC distribution in the axial direction has several peaks, which can be suitably replaced by MC blocks and the number of the peaks is equal to the number of MC blocks. 2) There is an optimum length to minimize the absolutely summed current and the 1542-mm length is the optimum for a 525-mm radius with six MC blocks. 3) A one-eigenmode (one MC block) increase or decrease changes the length by $+$ 87 or $-$ 105 mm, respectively. 4) Six MC blocks can provide 1-ppm peak-to-peak homogeneity in the 400-mm diameter spherical imaging volume and this is adequate for MRI magnets.