A polarizer system for polarizing a material to be used in magnetic resonance imaging, the system comprising: a container having a liquid helium bath therein, wherein the material to be polarized is positioned in the liquid helium bath; a sorption pump operable to reduce a pressure in the container and thereby vaporize a portion of the liquid helium bath; a cooling system operable to cool the sorption pump and promote molecular adsorption therein; a thermally conductive link for selectively connecting the sorption pump and the cooling system to provide selective cooling to the sorption pump; wherein the polarizer system is arranged to operate in a closed cyclical thermal cycle alternating between a polarizing phase and a reheating phase based on the connection of the sorption pump to the cooling system; said polarizer system further comprising a thermal switch for connecting and disconnecting the thermally conductive link from the sorption pump, wherein the thermal switch is operable to connect the thermally conductive link to the sorption pump during the polarizing phase and disconnect the thermally conductive link from the sorption pump during the reheating phase; a magnetic field producing device for maintaining a selected magnetic field therein and wherein the magnetic field producing device is cooled by the cooling system, wherein the magnetic field producing device further comprises: a magnet having a bore therethrough positioned about the container; and a magnet vessel to enclose the magnet, the magnet vessel containing liquid helium therein to cool the magnet; the polarizer system further comprising a recondenser attached to the magnet vessel to recondense liquid helium boiled off therefrom; characterised in that: the magnet is a superconducting magnet, and in that the polarizer system comprises a helium condenser connected to the sorption pump by way of a pumping line, said helium condenser being cooled by said cooling system through a common buss connected to the thermally conductive link, the helium condenser being arranged to recondense helium gas released from sorbent material in the sorption pump in a liquid state, to re-enter said container. a container having a liquid helium bath therein, wherein the material to be polarized is positioned in the liquid helium bath; a sorption pump operable to reduce a pressure in the container and thereby vaporize a portion of the liquid helium bath; a cooling system operable to cool the sorption pump and promote molecular adsorption therein; a thermally conductive link for selectively connecting the sorption pump and the cooling system to provide selective cooling to the sorption pump; wherein the polarizer system is arranged to operate in a closed cyclical thermal cycle alternating between a polarizing phase and a reheating phase based on the connection of the sorption pump to the cooling system; said polarizer system further comprising a thermal switch for connecting and disconnecting the thermally conductive link from the sorption pump, wherein the thermal switch is operable to connect the thermally conductive link to the sorption pump during the polarizing phase and disconnect the thermally conductive link from the sorption pump during the reheating phase; a magnetic field producing device for maintaining a selected magnetic field therein and wherein the magnetic field producing device is cooled by the cooling system, wherein the magnetic field producing device further comprises: a magnet having a bore therethrough positioned about the container; and a magnet vessel to enclose the magnet, the magnet vessel containing liquid helium therein to cool the magnet; the polarizer system further comprising a recondenser attached to the magnet vessel to recondense liquid helium boiled off therefrom; the magnet is a superconducting magnet, and in that the polarizer system comprises a helium condenser connected to the sorption pump by way of a pumping line, said helium condenser being cooled by said cooling system through a common buss connected to the thermally conductive link, the helium condenser being arranged to recondense helium gas released from sorbent material in the sorption pump in a liquid state, to re-enter said container.