An apparatus for detecting a potentially dangerous ferromagnetic object carried inadvertently by a person approaching the magnet of a magnetic resonance imaging system (14) uses the fringe field (16) of the magnet and provides guide members (11) defining a path along which the person is prescribed to pass. The path (11) is generally or approximately parallel to the field the path. At least one sense coil and generally two sets of sense coils (12, 13) are located on respective sides of the path (11) s the movement of the ferromagnetic object in the field of the magnet causes a voltage to be generated in the sense coil.