This was almost certainly an older style of MRI that didn’t use superconductors. You could turn these off and on, but the strength of the magnetic field was much lower that what can be achieved with superconductors.
I also looked up the wind down time and I was mistaken. It’s a day long process to wind down and wind back up and MRI and do all of the testing and adjustments, but the loss of the magnetic field happens in a few hours. I edited my post.
That is not what “all cause mortality” means.