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There are two sorts of MRI scanner - permanent and superconducting. The computing components of the scanners (software etc.) can be turned off simply as you would any computer. However, if you're referring to the magnetic field, there are different answers for each scanner.
A permanent magnet (often called an open magnet) cannot be switched off. It is made of an alloy which has been exposed to a magnetic field and is thus made magnetic itself. You can't switch it off any more than you can switch off a fridge magnet.
A superconducting electromagnet more commonly found in hospitals can go through a process called "quenching". This can happen accidentally very occasionally or deliberately in an emergency. If, for example, someone was trapped between a large magnetic object and the scanner, the operator can hit the quench button. This blows valves leading from the scanner to the outside of the building, venting the liquid helium used to supercool the magnet in a gaseous form, and over several seconds the magnetic field will dissipate. This is to be avoided and only done in an absolute emergency.
The engineer can also perform a controlled quench for certain types of work to be done, but again, this is rarely done as it can take some time to get the scanner up and running again.
So, with just one or two exceptions, the magnetic field to an MRI scanner is never switched off, even if the computer side of it is shut down e.g. at the end of the working day.
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