US Patent 10353030 Low-field magnetic resonance imaging methods and apparatus

An apparatus for providing a B0 magnetic field for a magnetic resonance imaging system, the apparatus comprising: at least one permanent B0 magnet to produce a magnetic field to contribute to the B0 magnetic field for the magnetic resonance imaging system, the at least one permanent B0 magnet comprising a plurality of permanent magnet rings, each of the plurality of permanent magnet rings comprising a plurality of permanent magnet segments having a respective height in a direction normal to the respective permanent magnet ring, wherein the height of at least one first permanent magnet segment is different than the height of at least one second permanent magnet segment, and wherein the at least one first permanent magnet segment and the at least one second permanent magnet are in different ones of the plurality of permanent magnet rings.