Patent ID: 7375528

Claim:
A magnetic resonance system for producing MR images comprising a magnet which has a longitudinal axis and produces a longitudinal magnetic field over a predetermined region (the “dsv”), said magnet comprising a plurality of current-carrying coils which surround the axis, are distributed along the axis, and define a turn distribution function T(z) for the magnet which varies with distance z along the axis and is equal to the sum of the number of turns in all coils at longitudinal position z, wherein: (A) the longitudinal extent of the plurality of coils defines first and second ends for the magnet; (B) the dsv defines a midpoint M which is closer to the first end than to the second end; (C) the turn distribution function T(z) is asymmetric with respect to distance z along the longitudinal axis, with more than 50 percent of the turns being located closer to the first end than to the second end; (D) the plurality of current-carrying coils comprises: (i). a first coil which has an inner radius R in , an outer radius R out , and which: (a) provides a contribution to the magnetic field at the center of the dsv that is larger in magnitude than the magnitude of the contribution of any other of the plurality of current-carrying coils, (b) is located closer to the first end than to the second end, and (c) defines an internal envelope; and (ii) second and third coils each of which: (a) has an outer radius which is less than R in ; (b) lies at least in part within the internal envelope of the first coil; and (c) carries current in a direction opposite to the direction of current in the first coil; and (E) to reduce stray magnetic fields external to the magnet: (i) the plurality of coils comprises a fourth coil which: (a) has an inner radius which is greater than R in ; and (b) carries current in a direction opposite to the direction of current in the first coil; or (ii) the magnet comprises a ferromagnetic structure which is rotationally-symmetric about the longitudinal axis and whose minimum inner radius is greater than R in ; or (iii) the magnet comprises a ferromagnetic structure which is rotationally-symmetric about the longitudinal axis and whose minimum inner radius is greater than R in and the plurality of coils comprises a fourth coil which: (a) has an inner radius which is greater than R in ; and (b) carries current in a direction opposite to the direction of current in the first coil.