Shim for imaging magnets

1. An arrangement for producing a substantially toroidal imaging region in a magnetic resonance imaging (MRI) system, comprising:

a solenoidal magnet arrangement ( 30 ) comprising primary magnet coils ( 32 ) arranged symmetrically about an axis (A-A);

characterized in that a shim arrangement ( 42 ) is provided, such that the MRI system provides a first imaging region ( 12 ) of first maximum radial (r) diameter with the shim arrangement inoperative, and provides a second imaging region ( 14 ) of substantially toroidal shape and of a second maximum radial (r) diameter, greater than the first, with the shim arrangement operative in a forward direction, such that magnet field homogeneity within the second imaging region is improved by operation of the shim arrangement, while field homogeneity at the centre ( 18 ) of the second imaging region is degraded by operation of the shim arrangement.

2. An arrangement according to claim 1 wherein the first imaging region ( 12 ) has a first maximum axial (z) length with the shim arrangement inoperative, and provides a third imaging region ( 15 ) of a second axial (z) length, greater than the first, with the shim arrangement operative in a reverse direction, such that magnet field homogeneity within the third imaging region is improved by operation of the shim arrangement in the reverse direction.

3. An arrangement according to claim 1 wherein the first imaging region is of substantially cylindrical shape.

4. An arrangement according to claim 1 wherein the shim arrangement comprises an arrangement for providing additional current flow through a coil provided within the MRI system for other reasons.

5. An arrangement according to claim 1 wherein the shim arrangement comprises a shim coil set, comprising a number of shim coils ( 42 ) arranged symmetrically about the axis.

6. An arrangement according to claim 5 wherein, in operation, at least one coil of the shim coil set carries electric current in a direction opposite to that of an electric current carried by at least one other coil of the shim coil set.

7. An arrangement according to claim 5 , wherein the shim coil set comprises coils ( 42 ) of at least two differing internal radii (r 1 , r 2 ).

8. An arrangement according to claim 7 , further comprising a cylindrical gradient coil assembly ( 38 ), and wherein the differing internal radii (r 1 , r 2 ) are provided by arranging respective shim coils on radially inner and outer surfaces of the gradient coil assembly.

9. An arrangement according to claim 5 wherein the shim coil set comprises at least one saddle coil.

10. An arrangement according to claim 5 wherein the primary magnet coils ( 32 ) are located within a hollow cylindrical cryostat and wherein the shim coil set is located within the bore of the cryostat ( 36 ).

11. An arrangement according to claim 10 wherein active shielding coils ( 34 ) are provided, arranged on a former within the cryostat ( 36 ).