Patent ID: 7199689

Claim:
A magnetic structure for generating a uniform magnetic field capable of implementing NMR imaging of small objects within a region of interest, the magnetic structure comprising: a) an elongated annular body having a longitudinal axis and comprising a plurality of substantially-uniformly-magnetized, prism-shaped blocks of permanent magnet material forming by its interior surfaces a cavity having the region of interest and sized to receive the object when introduced into the cavity along its longitudinal axis, the annular body having a cross-section in the X–Y plane of a rectangular coordinate system wherein the longitudinal axis is perpendicular to the X–Y plane and the outer boundaries of the annular body comprising the prism-shaped blocks of permanent magnet material form a regular polygon, the cavity being closed on the top, bottom and sides except for the ends for introducing and removing the object, b) the prism-shaped blocks including when viewed in the X–Y plane at least a first layer of blocks whose inner surfaces form the boundary of a regular polygonal cavity, the first layer of blocks comprising at least first, second, third, and fourth blocks and being symmetrically disposed with respect to the longitudinal axis, c) the intensity of the magnetic field Ho generated in the cavity by the structure being oriented along the Y-axis. d) the respective dimensions and the orientations of the remanences J of the first layer of blocks being such that the induction B generated by the permanent magnetic components of the structure is confined substantially inside the structure, thereby forming an essentially yokeless magnetic structure requiring no complete external yoke to form the return path for the flux generated by the prism-shaped blocks, e) the orientation of the magnetic field in the region of interest being perpendicular to an inner surface of at least one of the blocks of the first layer, f) the magnetic configuration of the overall structure being such that the magnetic field Ho generated in the cavity has sufficient uniformity to support an NMR examination of the object when inside the region of interest, g) the magnetic structure having a design parameter K characteristic of a structure that will generate an inner regular polygonal cavity and an outer regular polygonal boundary, h) the magnetic structure comprising at least a first layer of prism-shaped blocks adjacent the cavity and a further layer of prism-shaped blocks surrounding the first layer, the first layer of prism-shaped blocks having a triangular cross-section in the X–Y plane, the further layer of prism-shaped blocks having an essentially quadrangular cross-section in the X–Y plane.