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.

Sorry, but you're wrong. The main magnetic field is always on. It's produced by superconducting rings that are "ramped up" when the magnet is initially installed and remains on at all times, even when the operator console is powered off. Basically, the ramp up procedure sets up a current loop in the superconducting coil via induction in a controlled manner. After the initial ramp up, the field can only be shut off by bringing in an engineer and special equipment to perform a ramp down procedure, the operator executing an emergency quench, or an accidental quench occuring due to a failure of the refrigeration system. Basically, a quench happens if the temperature of the superconductive rings exceeds the temperature threshold required to maintain superconductivity, the resistivity of the ring material becomes non-zero and runaway heating occurs because of the high current in the ring (lots of amperes), the runaway heating of the ring causes the surrounding liquid helium to go supercritical and rapid (and potentially explosive) boil off occurs.

The electromagnets which are only turned on during operation (used for the gradient fields and for shimming the main field) are insignificant in magnitude compared with the main field. The main field is powerful enough to lift a ferromagnetic chair off the floor if the chair gets close enough to the bore without the gradients being active.