No, you were right. MRI machines that can be turned on and off are of two varieties, the regular electromagnet, and the superconducting one. The regular electromagnet versions are obsolete, they take up to an hour of being turned on before you can get accurate measurements from them. Almost all MRI machines in operation today are of the superconducting type.
Superconducting magnets take hours to turn on. They need to get a lot of electricity running through them. It needs to be kept at a superconducting temperature, and charging it heats it up. They also take quite a while to turn off, though faster than turning on. Their fast emergency shutdown involves releasing their liquid helium coolant. If you want a video of this, search "MRI quench".
These MRIs are only turned off for maintenance. MRI machine rooms are highly restricted, anyone entering must be screened for potentially dangerous objects. For more info, search for "are MRI machines always on".
The platform security processor, PSP. There were some mumblings of open sourcing the implementation details of it in 2017 or so, but that never happened, so it's still a black box that is potentially exploitable.