A yoked magnetic structure for use in an MRI device to produce a predetermined volume of substantially uniform magnetic field in a region, said volume having an axis of symmetry parallel to the direction of said magnetic field, said magnetic structure comprising:
- a first permanent magnet assembly having a first and a second surface thereof;
- a second permanent magnet assembly having a first and a second surface thereof;
- and
- an open ferromagnetic yoke having a first yoke surface and a second yoke surface, said second yoke surface is shaped as a mirror image of said first yoke surface and is spaced apart from said first yoke surface along said axis of symmetry,
- said first permanent magnet assembly is attached to said ferromagnetic yoke at a first location with said first surface of said first permanent magnet assembly facing one side of said region and said second surface thereof facing said first yoke surface,
- said second permanent magnet assembly is attached to said ferromagnetic yoke at a second location with said first surface of said second permanent magnet assembly facing said first surface of said first permanent magnet assembly on an opposite side of said region, and said second surface of said second permanent magnet assembly facing said second yoke surface such that said region is between said first surface of said first permanent magnet assembly and said first surface of said second permanent magnet assembly,
- said first permanent magnet assembly having a first annular permanent magnet with a first and a second surface thereof, said first surface of said first annular permanent magnet being of a first magnetic polarity and said second surface of said first annular permanent magnet being of a second magnetic polarity, said first annular permanent magnet having an inside diameter, said first annular permanent magnet having at least a portion of said first surface of said first annular magnet lying in a first plane to provide a first magnetic field in said region, said first magnetic field having a zero rate of change in a first direction at a first point in said region,
- said first permanent magnet assembly also having at least a second annular or disc like magnet with a first and a second surface thereof, said first surface of said second annular magnet being of said first magnetic polarity and said second surface of said second annular permanent magnet being of said second magnetic polarity, said second annular permanent magnet having an outside diameter which is smaller than said inside diameter of said first annular permanent magnet, with at least a portion of said first surface of said second annular magnet lying in a second plane spaced from said first plane to provide a second magnetic field whereby said second magnetic field is superimposed upon said first magnetic field in said region, having a zero rate of change in said first direction at a second point different from said first point,
- said second permanent magnet assembly having a third annular permanent magnet with a first and a second surface thereof, said first surface of said third annular permanent magnet being of said second magnetic polarity and said second surface of said third annular permanent magnet being of said first magnetic polarity, said third annular permanent magnet having an inside diameter, said third annular permanent magnet having at least a portion of said first surface of said third annular magnet lying in a third plane to provide a third magnetic field, such that said third magnetic field is superimposed on said first and second magnetic fields in said region, having a zero rate of change in said first direction at a third point different from said first and second points,
- said second permanent magnet assembly also having at least a fourth annular permanent magnet having a first and a second surface thereof, said first surface of said fourth annular magnet being of said second magnetic polarity and said second surface of said fourth annular permanent magnet being of said first magnetic polarity, said fourth annular permanent magnet having an outside diameter which is smaller than said inside diameter of said third annular permanent magnet, with at least a portion of said first surface of said fourth annular permanent magnet lying in a fourth plane spaced from said third plane to provide a fourth magnetic field, whereby said fourth magnetic field is superimposed upon said first, second and third magnetic fields, in said region, having a zero rate of change in said first direction at a fourth point different from said first, second and third points.