B

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

at least one first B 0 magnet configured to produce a first magnetic field to contribute to the B 0 magnetic field for the MRI system, the at least one first B 0 magnet comprising a first plurality of permanent magnet rings including three neighboring concentric rings concentric about a common center, separated from one another by gaps, and having different heights, wherein the heights of the rings successively increase with increased distance of the respective rings away from the common center; and

a yoke configured to capture and direct at least some magnetic flux generated by the at least one first B 0 magnet to increase a magnetic flux density within an imaging region of the MRI system, the yoke comprising:

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

a frame comprising ferromagnetic material and coupled to the first plate.

2. The apparatus of claim 1 , further comprising at least one second B 0 magnet configured to produce a second magnetic field to contribute to the B 0 magnetic field for the MRI system, the at least one first B 0 magnet comprising a second plurality of permanent magnet rings including at least two rings with respective different heights.

3. The apparatus of claim 2 , wherein each of the second plurality of permanent magnet rings has a respective different height.

4. The apparatus of claim 2 , wherein the at least one first B 0 magnet and the at least one second B 0 magnet are arranged relative to one another so that the imaging region is provided therebetween, and wherein the different heights of the first plurality of permanent magnet rings and the different heights of the second plurality of permanent magnet rings are selected to obtain a first level of magnetic field homogeneity within a first field of view in the imaging region that is smaller than a second level of magnetic field homogeneity in the first field of view that would be obtained with an arrangement of the first plurality of permanent magnet rings having equal heights and the second plurality of permanent magnet rings having equal heights.

5. The apparatus of claim 2 ,

wherein the at least one first B 0 magnet and the at least one second B 0 magnet are arranged relative to one another so that the imaging region is provided therebetween,

wherein the different heights of the first plurality of permanent magnet rings and the different heights of the second plurality of permanent magnet rings are selected to obtain, in the imaging region, a first field of view having a first volume and a first magnetic field homogeneity, and

wherein the first volume is greater than a second volume of a second field of view having the first magnetic field homogeneity that would be obtained if heights of the first plurality of permanent magnet rings were equal and heights of the second plurality of permanent magnet rings were equal.

6. The apparatus of claim 2 , wherein the second plurality of permanent magnet rings is concentric about a common center.

7. The apparatus of claim 6 , wherein the first plurality of permanent magnet rings and the second plurality of permanent magnet rings have a same number of rings such that each permanent magnet ring in the first plurality of permanent magnet rings has a corresponding permanent magnet ring in the second plurality of permanent magnet rings, and wherein each pair of corresponding permanent magnet rings have permanent magnet segments of a same height.

8. The apparatus of claim 2 , wherein the at least one first B 0 magnet and the at least one second B 0 magnet contribute to the B 0 magnetic field for the MRI system, the B 0 magnetic field having a field strength of less than or equal to 0.1 T and greater than or equal to 50 mT.

9. The apparatus of claim 2 , wherein the yoke further comprises a second plate comprising ferromagnetic material, wherein the second plate is coupled to the at least one second B 0 magnet and to the frame, and

wherein the frame comprises a plurality of supports, each of the plurality of supports being spaced apart from an adjacent support by a gap.

10. The apparatus of claim 9 , wherein the frame further comprises a first arm portion coupled to the first plate and a second arm portion coupled to the second plate, and wherein supports of the plurality of supports are coupled between the first arm portion and the second arm portion.

11. The apparatus of claim 1 , wherein the first plurality of permanent magnet rings comprises a fourth permanent magnet ring farther from the common center than the three neighboring concentric rings, and wherein a fourth height of permanent magnet segments in the fourth permanent magnet ring is greater than the heights of each of the three neighboring concentric rings.

12. The apparatus of claim 1 , wherein a first permanent magnet ring of the three neighboring concentric rings closest to the common center comprises multiple permanent magnet segments and wherein the multiple permanent magnet segments in the first permanent magnet ring comprise circular arc segments.

13. The apparatus of claim 1 , wherein a first permanent magnet ring of the three neighboring concentric rings closest to the common center comprises multiple permanent magnet segments and wherein the multiple permanent magnet segments in the first permanent magnet ring comprise rectangular blocks.

14. The apparatus of claim 1 , wherein a first permanent magnet ring of the three neighboring concentric rings closest to the common center comprises multiple permanent magnet segments and wherein the multiple permanent magnet segments in the first permanent magnet ring comprise trapezoidal blocks.

15. The apparatus of claim 1 , wherein each of the three neighboring concentric rings comprise multiple permanent magnet segments.