Low-field magnetic resonance imaging methods and apparatus

1. An apparatus for providing a B 0 magnetic field for a magnetic resonance imaging system, the apparatus comprising:

at least one permanent B 0 magnet to produce a magnetic field to contribute to the B 0 magnetic field for the magnetic resonance imaging system, the at least one permanent B 0 magnet comprising a plurality of concentric 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 concentric permanent magnet rings; and

at least one permanent shim magnet that is different from the at least one permanent B 0 magnet.

2. The apparatus of claim 1 , wherein the plurality of permanent magnet segments comprise circular arc segments.

3. The apparatus of claim 1 , wherein the plurality of permanent magnet segments comprise rectangular blocks.

4. The apparatus of claim 1 , wherein each of the plurality of permanent magnet rings comprises a plurality of sub-rings that together form the respective permanent magnet ring.

5. The apparatus of claim 4 , wherein each of the plurality of permanent magnet segments in a given sub-ring have a length that, together, form a circumference of the given sub-ring.

6. The apparatus of claim 4 , wherein each of the plurality of sub-rings is concentric with each of the plurality of sub-rings in the respective same permanent magnet ring about a common center, and wherein the length of permanent magnet segments in a given sub-ring is greater than the length of permanent magnet segments in sub-rings of the respective permanent magnet ring that are closer to the common center.

7. The apparatus of claim 4 , wherein at least one of the permanent magnet rings comprises at least three sub-rings.

8. The apparatus of claim 4 , wherein at least one of the plurality of permanent magnet rings has a different number of sub-rings than at least one other of the plurality of permanent magnet rings.

9. The apparatus of claim 1 , wherein the at least one permanent B 0 magnet comprises a first permanent B 0 magnet and a second permanent B 0 magnet, each comprising a respective plurality of permanent magnet rings, and wherein the at least one first permanent B 0 magnet and the at least one second permanent B 0 magnet are arranged relative to one another so that an imaging region is provided there between.

10. The apparatus of claim 9 , further comprising a yoke to capture and direct at least some of the magnetic field generated by the at least one first permanent B 0 magnet and the at least one second permanent B 0 magnet to increase the magnetic flux density within the imaging region.

11. The apparatus of claim 10 , wherein differing heights are selected for respective ones of the permanent magnet segments to compensate for inhomogeneity in the magnetic field resulting from the yoke.

12. The apparatus of claim 10 , wherein the yoke comprises:

a first plate comprising ferromagnetic material coupled to the at least one first permanent B 0 magnet;

a second plate comprising ferromagnetic material coupled to the at least one second permanent B 0 magnet; and

a frame comprising ferromagnetic material coupled to the first plate and the second plate to provide a path between the first plate and the second plate for the at least some of the magnetic field generated by the at least one first permanent B 0 magnet and the at least one second permanent B 0 magnet, the frame comprising a plurality of supports forming at least a portion of the path, each of the plurality of supports comprising ferromagnetic material and spaced apart from an adjacent support by a gap.

13. The apparatus of claim 12 , wherein the frame comprises a first arm portion comprising ferromagnetic material coupled to the first plate and a second arm portion comprising ferromagnetic material coupled to the second plate, and wherein the plurality of supports are coupled between the first arm portion and the second arm portion.

14. The apparatus of claim 13 , wherein the frame is substantially C-shaped.

15. The apparatus of claim 14 , wherein the first arm portion and the second arm portion each comprises a tapered portion over which a thickness of the respective arm portion varies.

16. The apparatus of claim 15 , wherein the first arm portion and the second arm portion each comprises a plurality of tapered portions over which a thickness of the respective arm portion varies.

17. The apparatus of claim 12 , wherein the gap is an air gap.

18. The apparatus of claim 12 , wherein the first plate and the second plate are substantially circular.

19. The apparatus of claim 12 , wherein the yoke comprises at least one portion made from low carbon steel, cobalt steel (CoFe) and/or silicon steel.

20. The apparatus of claim 19 , wherein the yoke comprises at least one first portion made from a different ferromagnetic material than at least one second portion.

21. The apparatus of claim 20 , wherein the first plate and the second plate comprise a different ferromagnetic material than the frame.

22. The apparatus of claim 12 , wherein the first permanent B 0 magnet, the second permanent B 0 magnet and the yoke together weigh less than 600 kilograms.

23. The apparatus of claim 12 , wherein the first permanent B 0 magnet, the second permanent B 0 magnet and the yoke together weigh less than 500 kilograms.

24. The apparatus of claim 12 , wherein the first permanent B 0 magnet, the second permanent B 0 magnet and the yoke together weigh less than or equal to approximately 600 kilograms and greater than or equal to approximately 450 kilograms.

25. The apparatus of claim 10 , wherein the at least one first permanent B 0 magnet and the at least one second permanent B 0 magnet contribute to the B 0 magnetic field for the magnetic resonance imaging system, the B 0 magnetic field having a field strength of less than or equal to approximately 0.2 T and greater than or equal to approximately 0.1 T.

26. The apparatus of claim 10 , wherein the at least one first permanent B 0 magnet and the at least one second permanent B 0 magnet contribute to the B 0 magnetic field for the magnetic resonance imaging system, the B 0 magnetic field having a field strength of less than or equal to approximately 0.1 T and greater than or equal to approximately 50 mT.

27. The apparatus of claim 10 , wherein the at least one first permanent B 0 magnet and the at least one second permanent B 0 magnet contribute to the B 0 magnetic field for the magnetic resonance imaging system, the B 0 magnetic field having a field strength of less than or equal to approximately 50 mT and greater than or equal to approximately 20 mT.