Quenching refers to the events that occur when the liquid cryogens that cool the magnet coils boil off rapidly, which results in helium escaping very rapidly from the cryogen bath. This means that the coils cease to be superconducting and become resistive. A quench will in general be accompanied by a loud bang or thundering or hissing or rushing sound with the cold gas expulsion.
Quenching may occur by activation of the magnet STOP button, or spontaneously, caused by a fault in the magnet itself. The magnet emergency stop button should only be used in the event of the magnetic field causing patient or personnel injury, and a shutdown of the static field is necessary, or if fire or some other unforeseen occurrence requires the quick access of emergency personnel to the examination room. Note however, that initiating a quench may not result in total removal of the magnetic field, and a danger may still exist. Some magnets only partially quench and so the field is not completely removed. In such cases care should still be taken when handling ferromagnetic objects near the magnet.
One of our magnet manuals suggests that the field strength is likely to be reduced to ~20 mT after about 20 seconds of a quench.