Arrangements and method for shimming a magnetic field

A shim arrangement for increasing the homogeneity of a magnetic field within a homogeneous field region, comprising: shim channels extending within a volume between a magnetic field generator and the homogeneous field region; at least one piece of shim material located within each shim channel; an arrangement for moving each shim piece along the corresponding shim channel; and retaining means for retaining each shim piece in position. Shimming is performed by moving shim pieces within the shim channels, with the magnet at field. No shim pieces are added to, or removed from, the shim channels during the shimming step.