B0 magnet methods and apparatus for a magnetic resonance imaging system

1. An apparatus for providing a B 0 magnetic field for a magnetic resonance imaging 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 magnetic resonance imaging system, the at least one first B 0 magnet comprising a first plurality of permanent magnet rings including three concentric rings at least two of which have respective different heights, wherein neighboring rings among the three concentric rings are separated from each other by a respective gap;

at least one second B 0 magnet configured to produce a second magnetic field to contribute to the B 0 magnetic field for the magnetic resonance imaging system, the at least one first B 0 magnet comprising a second plurality of permanent magnet rings including three concentric rings at least two of which have respective different heights; 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, 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

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 an imaging region is provided there between,

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,

wherein the first field of view includes a sphere having a diameter in a range of 17-23 cm,

wherein the first plurality of permanent magnet rings is concentric about a common center,

wherein the first plurality of permanent magnet rings comprises a first permanent magnet ring closer to the common center than a second permanent magnet ring, and wherein a first height of permanent magnet segments in the first permanent magnet ring is smaller than a second height of permanent magnet segments in the second permanent magnet ring, and

wherein the first plurality of permanent magnet rings comprises a third permanent magnet ring farther from the common center than the second permanent magnet ring, and wherein a third height of permanent magnet segments in the third permanent magnet ring is greater than the second height of the permanent magnet segments in the second permanent magnet ring.

2. The apparatus of claim 1 , wherein each of the first plurality of permanent magnet rings has a respective different height.

3. The apparatus of claim 1 , wherein each of the second plurality of permanent magnet rings has a respective different height.

4. The apparatus of claim 1 , wherein the first level of magnetic field of homogeneity is at least a factor of two smaller than the second level of magnetic homogeneity.

5. The apparatus of claim 1 ,

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, the first field of view having a first volume and a first level of magnetic field homogeneity, and

wherein the first volume is greater than a second volume of a second field of view having the first level of 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 1 , wherein the first plurality of permanent magnet rings comprises a fourth permanent magnet ring farther from the common center than the third permanent magnet ring, and wherein a fourth height of permanent magnet segments in the fourth permanent magnet ring is greater than the third height of permanent magnet segments in the third permanent magnet ring.

7. The apparatus of claim 1 , wherein multiple permanent magnet segments in the first permanent magnet ring have the first height, and wherein multiple permanent magnet segments in the second permanent magnet ring have the second height.

8. The apparatus of claim 1 , wherein each of the permanent magnet segments in the first permanent magnet ring have the first height, and wherein each of the permanent magnet segments in the second permanent magnet ring have the second height.

9. The apparatus of claim 1 , wherein the permanent magnet segments in the first permanent magnet ring comprise circular arc segments.

10. The apparatus of claim 1 , wherein the permanent magnet segments in the first permanent magnet ring comprise rectangular blocks.

11. The apparatus of claim 1 , wherein the permanent magnet segments in the first permanent magnet ring comprise trapezoidal blocks.

12. The apparatus of claim 1 , wherein the second plurality of permanent magnet rings is concentric about a common center.

13. The apparatus of claim 12 , 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.

14. The apparatus of claim 1 , 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 magnetic resonance imaging 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.

15. An apparatus for providing a B 0 magnetic field for a magnetic resonance imaging 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 magnetic resonance imaging system, the at least one first B 0 magnet comprising a first plurality of permanent magnet rings including three concentric rings at least two of which have respective different heights, wherein neighboring rings among the three concentric rings are separated from each other by a respective gap;

at least one second B 0 magnet configured to produce a second magnetic field to contribute to the B 0 magnetic field for the magnetic resonance imaging system, the at least one first B 0 magnet comprising a second plurality of permanent magnet rings including three concentric rings at least two of which have respective different heights; 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, 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,

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 an imaging region is provided there between,

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,

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,

wherein the first volume is at least 10 percent larger than the second volume,

wherein the first plurality of permanent magnet rings is concentric about a common center,

wherein the first plurality of permanent magnet rings comprises a first permanent magnet ring closer to the common center than a second permanent magnet ring, and wherein a first height of permanent magnet segments in the first permanent magnet ring is smaller than a second height of permanent magnet segments in the second permanent magnet ring, and

wherein the first plurality of permanent magnet rings comprises a third permanent magnet ring farther from the common center than the second permanent magnet ring, and wherein a third height of permanent magnet segments in the third permanent magnet ring is greater than the second height of the permanent magnet segments in the second permanent magnet ring.

16. An apparatus for providing a B 0 magnetic field for a magnetic resonance imaging 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 magnetic resonance imaging system, the at least one first B 0 magnet comprising a first plurality of permanent magnet rings including three concentric rings at least two of which have respective different heights, wherein neighboring rings among the three concentric rings are separated from each other by a respective gap; 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, the yoke comprising:

a first plate comprising ferromagnetic material and coupled to the at least one first B0 magnet;

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

first additional ferromagnetic material coupled to the first plate to compensate for magnetic saturation induced in the first plate.

17. An apparatus for providing a B 0 magnetic field for a magnetic resonance imaging 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 magnetic resonance imaging system, the at least one first B 0 magnet comprising a first plurality of permanent magnet rings including three concentric rings at least two of which have respective different heights, wherein neighboring rings among the three concentric rings are separated from each other by a respective gap; 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, 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, wherein:

the first plate comprises a first set of one or more holes to compensate for magnetic saturation induced in the first plate.