It's a certification requirement that the magnet can quench with a patient inside and the patient is not affected – the cryogens are supposed to vent outside th

It's a certification requirement that the magnet can quench with a patient inside and the patient is not affected – the cryogens are supposed to vent outside the room, or, in the even that does not happen, at least safely within the room. A blocked quench pipe and a stainless steel (typically A4 stainless) cryostat becomes a bomb: cryogens expand in volume by about a factor of ~700 when boiling and so the volume they occupy at 1 bar and 300 K is, well, far far larger than the volume of the room the scanner is in. This is the sort of thing that is a (remote) risk to life. I've only ever seen one (preclinical, not-for-human-use) magnet unexpectedly quench; copying and pasting a paragraph from my PhD thesis: *The overpressure created by the rapid boiling of the large volume of liquid helium surrounding the superconducting magnet was sufficient to burst the bursting disc (as designed), which allowed the venting of helium gas from the inner cryostat to the emergency quench pipe (the bursting disc was temporarily replaced by clingfilm in this photograph). Additionally, it was also sufficient to rip and blow off the quench pipe, break two windows, damage the floor, and relocate several light items within the magnet room (which, presumably, was not as designed)*