Methods and apparatus for magnetic field shimming

1. A method of producing a permanent magnet shim configured to improve a profile of a B 0 magnetic field produced by a B 0 magnet, the method comprising:

determining deviation of the B 0 magnetic field from a desired B 0 magnetic field;

determining a magnetic pattern that, when applied to magnetic material, produces a corrective magnetic field that corrects for at least some of the determined deviation; and

applying the magnetic pattern to the magnetic material to produce the permanent magnet shim.

2. The method of claim 1 , wherein determining the magnetic pattern comprises logically partitioning the magnetic material into a plurality of regions.

3. The method of claim 2 , wherein determining the magnetic pattern comprises determining a magnetic pattern that indicates which of the plurality of regions contribute to the corrective magnetic field.

4. The method of claim 3 , wherein determining the magnetic pattern comprises determining a magnetic pattern that indicates a magnetization polarity of regions indicated as contributing to the corrective magnetic field.

5. The method of claim 3 , wherein determining the magnetic pattern comprises determining a magnetic pattern that indicates a magnetization strength of regions indicated as contributing to the corrective magnetic field.

6. The method of claim 5 , wherein applying the magnetic pattern comprises at least one additive process that provides magnetic material to regions indicated as contributing to the corrective magnetic field in an amount corresponding to the respective indicated magnetization strength.

7. The method of claim 3 , wherein determining the magnetic pattern comprises determining a magnetic pattern that indicates which of the plurality of regions do not contribute to the corrective magnetic field.

8. The method of claim 7 , wherein applying the magnetic pattern comprises performing at least one subtractive process that removes magnetic material corresponding to regions indicated as not contributing to the corrective magnetic field.

9. The method of claim 8 , wherein applying the magnetic pattern comprising cutting the magnetic material to remove regions indicated as not contributing to the corrective magnetic field.

10. The method of claim 3 , wherein applying the magnetic pattern comprises performing at least one additive process that provides magnetic material to regions indicated as contributing to the corrective magnetic field.

11. The method of claim 3 , wherein applying the magnetic pattern comprises traversing the magnetic material with a magnetizing head that magnetizes each region indicated as contributing to the corrective magnetic field.

12. The method of claim 3 , wherein determining the magnetic pattern comprises obtaining a model of the magnetic material, and wherein logically partitioning the magnetic material comprises logically partitioning the model into the plurality of regions.

13. The method of claim 12 , wherein determining the magnetic pattern comprises performing an optimization using the model that determines the magnetic pattern.

14. The method of claim 13 , wherein logically partitioning the model comprises tessellating the model.

15. The method of claim 14 , wherein determining the magnetic pattern comprises repeating the optimization over a plurality of iterations, and wherein for each iteration at least a portion of the model is tessellated at a higher resolution than a previous iteration.

16. The method of claim 2 , wherein applying the magnetic pattern to the magnetic material results in at least one of the plurality of regions having a magnetization of a first polarity and at least one of the plurality of regions having a magnetization of a second polarity.

17. The method of claim 16 , wherein the first polarity is opposite the second polarity.

18. The method of claim 2 , wherein applying the magnetic pattern to the magnetic material results in at least one of the plurality of regions being magnetized in a direction substantially parallel to the B 0 magnetic field.

19. The method of claim 2 , wherein applying the magnetic pattern to the magnetic material results in at least one of the plurality of regions being magnetized in a direction substantially aligned with the B 0 magnetic field.

20. The method of claim 2 , wherein applying the magnetic pattern to the magnetic material results in at least one of the plurality of regions being magnetized in a direction substantially opposite to the B 0 magnetic field.

21. The method of claim 2 , wherein applying the magnetic pattern to the magnetic material results in at least one of the plurality of regions being magnetized in a direction substantially opposite to the B 0 magnetic field and at least one of the plurality of regions being magnetized in a direction substantially aligned with the B 0 magnetic field.

22. The method of claim 2 , wherein the magnetic material comprises a substantially planar surface that is logically partitioned into the plurality of regions, and wherein applying the magnetic pattern to the magnetic material results in at least one of the plurality of regions being magnetized in a direction substantially perpendicular to the planar surface.

23. The method of claim 2 , wherein the magnetic material comprises a substantially planar surface that is logically partitioned into the plurality of regions, and wherein applying the magnetic pattern to the magnetic material results in at least one of the plurality of regions being magnetized in a direction substantially parallel to the planar surface.

24. The method of claim 1 , wherein the corrective magnetic field corrects for at least some non-uniformities resulting from variability in manufacturing the B 0 magnet.

25. The method of claim 1 , wherein the corrective magnetic field corrects for at least some non-uniformities inherent in and/or introduced by a design of the B0 magnet.

26. The method of claim 25 , wherein the corrective magnetic field corrects for at least some non-uniformities resulting from a presence of a ferromagnetic yoke.

27. The method of claim 1 , wherein the corrective magnetic field corrects, at least in part, a B 0 offset.

28. The method of claim 1 , wherein determining the deviation of the B 0 magnetic field comprises measuring the B 0 magnetic field produced by the B 0 magnetic field.

29. The method of claim 1 , wherein the magnetic pattern applied to the magnetic material includes a first plurality of regions having a first magnetization and a second plurality of regions having a second magnetization, different from the first magnetization.

30. The method of claim 29 , wherein at least some regions of the first plurality of regions are adjacent to regions of the second plurality of regions.

31. A permanent magnet shim for improving a profile of a B 0 magnetic field produced by a B 0 magnet, the permanent magnet shim comprising:

a magnetic material having a predetermined magnetic pattern applied thereto that produces a corrective magnetic field to improve the profile of the B 0 magnetic field.

32. The permanent magnet shim of claim 31 , wherein the corrective magnetic field corrects for at least some non-uniformities resulting from variability in manufacturing the B 0 magnet.

33. The permanent magnet shim of claim 31 , wherein the corrective magnetic field corrects for at least some non-uniformities inherent in and/or introduced by a design of the B 0 magnet.

34. The permanent magnet shim of claim 33 , wherein the corrective magnetic field corrects for at least some non-uniformities resulting from a presence of a ferromagnetic yoke.

35. The permanent magnet shim of claim 31 , wherein the corrective magnetic field corrects, at least in part, a B 0 offset.

36. The permanent magnet shim of claim 31 , wherein the magnetic material is logically partitioned into a plurality of regions.

37. The permanent magnet shim of claim 36 , wherein at least one of the plurality of regions has a magnetization of a first polarity and at least one of the plurality of regions has a magnetization of a second polarity.

38. The permanent magnet shim of claim 37 , wherein the first polarity is opposite the second polarity.

39. The permanent magnet shim of claim 36 , wherein at least one of the plurality of regions is magnetized to produce a different field strength than at least one other of the plurality of regions.

40. The permanent magnet shim of claim 36 , wherein at least one of the plurality of regions is magnetized in a direction substantially parallel to the B 0 magnetic field.

41. The permanent magnet shim of claim 36 , wherein at least one of the plurality of regions is magnetized in a direction substantially aligned with the B 0 magnetic field.

42. The permanent magnet shim of claim 36 , wherein at least one of the plurality of regions is magnetized in a direction substantially opposite to the B 0 magnetic field.

43. The permanent magnet shim of claim 36 , wherein at least one of the plurality of regions is magnetized in a direction substantially aligned with the B 0 magnetic field and at least one of the plurality of regions is magnetized in a direction substantially opposite to the B 0 magnetic field.

44. The permanent magnetic shim of claim 36 , wherein the magnetic material comprises a substantially planar surface that is logically partitioned into the plurality of regions, and wherein at least one of the plurality of regions is magnetized in a direction substantially perpendicular to the planar surface.

45. The permanent magnet shim of claim 36 , wherein the magnetic material comprises a substantially planar surface that is logically partitioned into the plurality of regions, and wherein at least one of the plurality of regions is magnetized in a direction substantially parallel to the planar surface.

46. The permanent magnet shim of claim 36 , wherein the plurality of regions are not uniform in size.

47. The permanent magnet shim of claim 36 , wherein the plurality of regions are uniform in size.

48. The permanent magnet shim of claim 31 in combination with the B 0 magnet, wherein the B 0 magnet produces a B 0 magnetic field at a B 0 magnetic field strength equal to or less than approximately 0.2 T and greater than or equal to approximately 0.1 T.

49. The permanent magnet shim of claim 31 in combination with the B 0 magnet, wherein the B 0 magnet produces a B 0 magnetic field at a B 0 magnetic field strength equal to or less than approximately 0.1 T and greater than or equal to approximately 50 mT.

50. The permanent magnet shim of claim 31 in combination with the B 0 magnet, wherein the B 0 magnet produces a B 0 magnetic field at a B 0 magnetic field strength equal to or less than approximately 50 mT and greater than or equal to approximately 20 mT.

51. The permanent magnet shim of claim 31 in combination with the B 0 magnet, wherein the B 0 magnet produces a B 0 magnetic field at a B 0 magnetic field strength equal to or less than approximately 20 mT and greater than or equal to approximately 10 mT.

52. The permanent magnet shim of claim 31 , wherein the permanent magnet shim produces a corrective magnetic field to improve the profile of the B 0 magnetic field produced by a bi-planar B 0 magnet of a low-field magnetic resonance imaging system.

53. The permanent magnet shim of claim 31 , wherein the predetermined magnetic pattern applied to the magnetic material includes a first plurality of regions having a first magnetization and a second plurality of regions having a second magnetization, different from the first magnetization.

54. The permanent magnet shim of claim 53 , wherein at least some regions of the first plurality of regions are adjacent to regions of the second plurality of regions.

55. A low-field magnetic resonance imaging system comprising:

a B 0 magnet configured to produce a B 0 magnetic field at a field strength of less than or equal to approximately 0.1 T; and

at least one permanent magnet shim comprising magnetic material having a predetermined magnetic pattern applied thereto that produces a corrective magnetic field to improve a profile of the B 0 magnetic field.

56. The low-field magnetic resonance imaging system of claim 55 , wherein the predetermined magnetic pattern applied to the magnetic material includes a first plurality of regions having a first magnetization and a second plurality of regions having a second magnetization, different from the first magnetization.

57. The low-field magnetic resonance imaging system of claim 56 , wherein at least some regions of the first plurality of regions are adjacent to regions of the second plurality of regions.