Shimming a superconducting nuclear-magnetic-resonance imaging magnet with steel

Abstract

Using a recently published paper as a basis, the magnetic field produced by steel bars inserted in a superconducting NMR imaging magnet is analyzed in a spherically harmonic basis set. A description is then given of how such bars were used to improve the homogeneity of the field within the magnet’s imaging volume from 1.2 parts per thousand to about 10 ppm. The poor homogeneity was caused by the magnet’s being placed in a steel-laden environment, a situation normally shunned by investigators, and it is the author’s contention that the results obtained abrogate the main objection to NMR equipment’s being installed in an ordinary hospital building. To facilitate the latter, the equations developed may also be used to estimate, prior to installation, the effects of the proposed environment on field homogeneity.