Magnet Quench

Quenching is the process whereby there is a sudden loss of absolute zero of temperature in the magnet coils, so that they cease to be super conducting and become resistive, thus eliminating the magnetic field. This results in helium escaping from the cryogen bath extremely rapidly. It may happen accidentally or can be manually instigated in the case of an emergency.

Quenching may cause severe and irreparable damage to the super conducting coils, and so a manual quench should only be performed in extreme cases when the physician and service engineer are involved in the decision to quench. A fire in the scan room may also be a cause to quench the magnet, so the firefighting personnel can safely enter the room (see MRI Code Red Protocol). All systems should have helium-venting equipment, which removes the helium to the outside environment in the event of a quench. However if this fails, helium will vent into the room and replace the oxygen. For this reason all scan rooms should contain an oxygen monitor that sounds an alarm if the oxygen falls below a certain level. Under these circumstances immediate evacuation of the patient and personnel is necessary.

If the scan room door is closed when a quench occurs and helium escapes into the scan room, the depletion of oxygen causes a critical increase in pressure in the room compared with the control area. This produces high pressure in the scan room, which may prevent opening of the door. If this should happen, the glass partition between the scan and control rooms should be broken to release the pressure. The scan room door can then be opened as usual and the patient evacuated. In such a case the patient should be immediately evacuated and evaluated for asphyxia, hypothermia and ruptured eardrums.