As
m_turner mentioned, catastrophic quenching is an important feature of
MRI scanners. These scanners basically contain incredibly strong
superconducting
magnets (1.5
Tesla, for those of you who know
about such things). As the safety video says,
The Magnet is Always On
and it doesn't shut off when you're not actively scanning. Thus, if
you bring anything ferromagnetic into the scanning room, it's likely to
go flying into the scanner. That's a minor problem if the scanner is
empty and it's a small object like a paperclip; you can easily reach in
and pull it off (though it'll take quite a tug). If, however, you do
something even dumber--like bringing a wheelchair, oxygen bottle, or
sledgehammer into the scanning chamber while a patient is lying in
there--it too will go flying into the scanner at high speed aimed right at
the patient's head. Now remember,
The Magnet Is Always On
and human beings aren't strong enough to yank a heavy object out of the
scanner and off the patient (who is currently bleeding to death). In this
case, you push the Big Red Switch located on the scanner or the
operating console, which vents the coolant, wrecks the superconductivity,
scrams the magnet, fills the room with vapor, and allows you to
get the heavy metal object off the patient. It's a procedure about as
serious as venting the warp core, in that you only do it when you're
really screwed; it also has about a fifty-fifty chance of
trashing the multi-million-dollar MRI scanner, so you don't want to hit it
by accident (as my advisor once did before they installed the
molly-guard). Although you never want to see someone quench an MRI scanner, it's still good to know that the safety feature's there.