Cryostat comprising a heat exchanger to provide cooling for a thermal shield

A cryostat comprises a cryogen vessel 3, an access neck (13, fig. 1), a refrigerator interface 4 and a thermal shield 6. The refrigerator interface includes a gas flow passageway from the cryogen vessel to atmosphere, where the thermal shield is thermally coupled 21 to the refrigerator interface. A heat exchanger 24 is provided in thermal contact with the refrigerator interface and thermal shield, so that cryogen gas flowing from the cryogen vessel via the refrigerator interface provides cooling for the thermal shield, thereby reducing the rate of loss of cryogen gas from the cryostat. Preferably, the heat exchanger is pressed in place and held in thermal contact with the refrigerator interface by struts 32 connected to a sealing plate 33. In order to reduce the room temperature radiation onto the heat exchanger, one or more radiation baffles 34 may be fitted to the struts. Preferably, the cryogen vessel houses a superconducting magnet winding of a magnetic resonance imaging (MRI) system. In use, the invention is designed to minimise the loss of a cryogen 10 during the transportation of a superconductive magnet system, where the heat exchanger is fitted in place of a refrigerator, by directing part of the boil-off gas from the cryogen to cool the thermal shield.