Movable magnetic compensating pieces are disposed on a lower magnetic path unit of an electron lens. The compensating pieces are in partial contact with the lower magnetic path unit and moved toward and away from each other on the lower magnetic path unit by manipulation form the exterior of the lens.
A magnetic focus system in which a cylindrical wire coil is enclosed by a magnetic shield in the shape of a hollow cylinder. An annular gap is provided in the interior wall of the shield. The shield is formed of a plurality of substantially parallel, magnetically conducting fibers in a matrix of dielectric material oriented with respect to the magnetic field so that the permeability of the shield is anisotropic.
A passive shimming assembly is provided which allows flexible positioning of a plurality of arcuate segments of steel in the bore of a magnet, with enough degrees of freedom to adequately shim a magnetic resonance magnet to a level of inhomogeneity required for imaging without the use of correction coils.
