A design method for minimum-inductance planar magnetic-resonance-imaging gradient coils considering the pole-piece effect

A design method for a planar gradient coil, which has minimum inductance with the presence of a pole piece around the gradient coil, is proposed. In magnetic-resonance-imaging systems using permanent magnets, magnetic materials of high permeability are used for the pole piece so that the magnetic field can be as uniform as possible. Owing to the high permeability of the pole piece, the gradient field produced by the conventional planar gradient coil deviates from the desired one. We have modelled the pole-piece effects as a mirror current for design of a minimum-inductance gradient coil. A finite-element method is applied to find the optimum amplitude of the mirror current that makes the magnetic field most similar to the real field. With the mirror-current model, we have derived the current-density function that gives the minimum inductance with the presence of the pole piece.