A pre-polarisation magnet arrangement for generating a pre-polarisation field for use in a low field magnetic resonance imaging process, the pre-polarisation magnet arrangement including a pre-polarisation field array including a plurality of permanent pre-polarisation magnets mounted in a support and provided in a circumferentially spaced arrangement surrounding an field of view, a number of the pre-polarisation magnets being movable between respective first and second positions, wherein in the first position the pre-polarisation magnets are configured as a cylindrical Halbach array to generate a pre-polarisation field in the field of view and in the second position the pre-polarisation magnets are configured to minimize the pre-polarisation field in the field of view.