In magnetic resonance imaging (MRI) devices with superconducting magnets, the superconductive coil is “submerged” in a cryogenic fluid (liquid helium) able to keep the temperature constant to a value of around -269°C. This temperature is close to the absolute zero, 0 K. To work at this temperature allows the establishment of a permanent static inductive magnetic field. In this case, MRI device is both a medical apparatus for medical investigations (according to Regulation (EU) 2017/745 on medical devices) and a under pressure equipment/assemblies, or rather, a dewar containing liquid helium in thermodynamic equilibrium with its gaseous phase (according to Directive 2014/68/EU).
A vessel with these characteristics (toroidal shape) has to satisfy specific architectural requirements of thermic insulation and mechanical strength and the presence of safety equipment. For example, overpressure valves allow to partially discharge the vapor phase in case of an increase of the concentration of vapor phase into the head of the dewar, restoring the normal operating conditions. Additionally, in case of excessive overpressure, the presence of bursting disc on the head of the dewar guarantees the safe exit of the cryogenic fluid, avoiding explosions. In this specific case, the liquid helium at -269°C is subjected to a change in the temperature of the surrounding environment (ambient temperature) and this causes a sudden change of state (liquid -> vapor) of almost all the present helium, resulting in the leak of vapor at a pressure that can reach 5 bar, according to the type of device. Aim of this paper is to propose the main fundamentals for a safe management of pressure dewar in magnetic resonance, from the essential safety requirements prescribed in the Directive 2014/68/EU to the optimal operating conditions. Finally, it is relevant to stress that the improvements in performance of the MRI with superconductive magnets used in the medical field and the resulting use of higher static magnetic field and cryogenic fluid content, will increase in the upcoming years the risks related to pressure control.