There are low field scanners that use conventional electromagnets, which aren't always on. However, the scanner shown is a 1.5 Tesla (T) or 3T scanner (difficult to say which from just the housing), and the main magnetic field is always on. The main field is supplied by a superconducting ring immersed in liquid helium. As a previous poster noted, the only way to shut one down is to dump the helium, and the cost to bring the scanner back up is in the tens of thousands of dollars. There are smaller electromagnets involved to apply field gradients across the bore of the scanner (on the order of 50 mT/m) which are off when the scanner is not operating, but these are quite small compared to the main field.

My Ph.D. dissertation was on data acquisition and reconstruction techniques for MRI. I've logged hundreds of hours operating high field scanners like the one shown, and dozens of hours being scanned for various research studies. I've also (carefully) hauled a variety of strange things in and out of scanner rooms. Most of the tools we used were non-ferrous, and we had to be extremely careful with the few ferrous pieces of equipment we had to use. The magnetic field does drop off rapidly as you move away from the scanner, so objects more than 10 feet or so away are unlikely to be pulled in.