A technique is provided for improving the homogeneity of the magnetic field within a cavity disposed inside a magnetic stucture having an opening to permit convenient access to an imaging region. The technique is based upon the step of inserting layers of magnetized material between plates (20, 24) of high magnetic permeability material. The layers are inserted at the boundary of the cavity. The technique includes (1) computing the surface charges on the plates as used to derived the distribution of the magnetostatic potential, (2) analyzing the uniformity of the magnetic field within the imaging region as effected by the opening in the magnetic structure by expanding the magnetostatic potential in a spatial harmonic series, and (3) determining the geometry and the composition of the inserted layers based on the successive elimination of increasing orders of spatial harmonics in the potential expansion within the imaging region. This invention includes the structure resulting from the technique of the invention.