Patent ID: 7714580

Claim:
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.