Apparatus is provided for generating an accurate magnetic field in a magnetic resonance imaging device. The device comprises a number of permanent magnet assemblies (1) each of which is in thermal contact with a plate (5) having good thermal conductivity, the plate (5) is positioned between said permanent magnetic assemblies (1) and gradient coils (3). The plate (5) has connected thereto one or more temperature sensors (7) which provide a signal to control circuit (8) which is arranged to control the operation to a plurality of thermoelectric heat pumping devices (9) which are connected to a plate (5) so that the overall heat generated by the heat pumping devices (9) and the gradient coils (3) is kept constant and is dissipated by a yoke (10) of the magnetic resonance imaging apparatus.