Abstract: Proposed is an improvement in an opposed-magnet magnetic circuit assembly with permanent magnets suitable for use, for example, in an MRI instrument comprising a pair of upper and lower permanent magnets to form a magnetic-field gap space therebetween, a pair of magnetic-field adjustment plates each on the surface of the permanent magnet to face the gap space and a pair of back yokes each on the back surface of the permanent magnet. According to the first aspect of the invention, a combination of a gradient coil and a shimming plate is mounted on the magnetic-field adjustment plate with intervention of several pieces of shim members and, further, another set of second shim members are bonded to the surface of the shimming plate facing the gap space with an object to improve the uniformity of the magnetic field in the gap space.

Abstract: Proposed is an improvement in an opposed-magnet magnetic circuit assembly with permanent magnets suitable for use, for example, in an MRI instrument comprising a pair of upper and lower permanent magnets to form a magnetic-field gap space therebetween, a pair of magnetic-field adjustment plates each on the surface of the permanent magnet to face the gap space and a pair of back yokes each on the back surface of the permanent magnet. According to the first aspect of the invention, a combination of a gradient coil and a shimming plate is mounted on the magnetic-field adjustment plate with intervention of several pieces of shim members and, further, another set of second shim members are bonded to the surface of the shimming plate facing the gap space with an object to improve the uniformity of the magnetic field in the gap space.

Abstract: The present invention provides is an improvement in a magnetic circuit system with a pair of opposite permanent magnets to construct an MRI instrument comprising a pair of upper and lower permanent magnets connected with yokes to form an air gap space therebetween, in which the patient under MRI inspection is kept lying, a pair of shimmed pole pieces each mounted on the permanent magnet to face the air gap and a pair of gradient coils each mounted on the shimmed pole piece to face the air gap. The improvement comprises mounting a cancellation magnetic plate, which is a thin plate of a magnetic material having a coercive force of 0.1 to 500 Oe, on the gradient coil to face the air gap with an object to cancel the residual magnetization of the shimmed pole piece by the gradient magnetic field. The cancellation magnetic plate has a thickness not exceeding 0.5 mm and has a base area which is 5 to 75% relative to the base area of the gradient coil.

Abstract: Proposed is an improvement in a magnet assembly of an MRI instrument of the permanent magnet type comprising a pair of permanent magnets facing each other and forming a magnet gap therebetween. A pair of pole pieces each of which are mounted on one of the permanent magnets and a pair of gradient-field coils each of which are mounted on one of the pole pieces where both the pole pieces and coils face the gap. With an object of the invention to cancel the adverse influences caused by the residual magnetization of the pole pieces by the gradient magnetic field generated in the gradient-field coils. The invention proposes providing a pair of magnetic-field compensation members each on one of the gradient-field coils so that the residual magnetization in the compensation members generates a magnetic field which is compensatory for the magnetic field due to the residual magnetization in the pole pieces.