Abstract: An electromagnet having a ferromagnetic core comprising at least one substantially C-shaped yoke terminating in pole pieces (1,2) with opposed pole faces (7) separated by an air gap in which a magnetic field suitable for an NMR imaging process is, in use, produced, the core comprising stacked laminae (6) of electrical sheet steel, which are bent around the C about axes perpendicular to the plane of the C, and at least some of which are spaced apart at the pole pieces to accommodate between the spaced laminae field modifying elements (5,8) which are arranged to improve the homogeneity and/or confinement of the magnetic field in the air gap.