| 摘要 |
A magnet for use in a magnetic resonant imaging device comprising pole pieces and a return magnetic path constructed of magnetic field conductive material and comprising activating coils, one located around each pole piece which, when activated, generate a magnetic field between the exposed faces of the pole pieces. To improve uniformity of the resultant magnetic field, the surface of the exposed face of each pole piece comprises a toroidal portion which may extend beyond the side surface of the cylinder of each pole piece and a concave portion surrounded by that toroidal portion. To further improve field uniformity, a plurality of projections extend from the exposed surface of each pole piece toward the exposed surface of the opposite pole piece with the distance each projection extends from the exposed surface of the respective pole piece being hydraulically adjustable. To still further improve uniformity, a plurality of shim coils are arranged on the exposed surface of the first and second pole pieces, some of which shim coils are arranged with circular symmetry with regard to the exposed surface and some of which are arranged without circular symmetry.
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