Sorry, but you're (mostly) incorrect. The field for all modern MRI machines is provided by a superconducting magnet, this magnet is "on" all the time, by virtue of the superconducting ring current. The additional fields applied during analysis are inconsequential.

Where you are right is that the fields decay quite quickly (exponentially, in fact) as you move away from the magnet. For the metal chair in the picture to be "sucked in" to the field, I would assume that someone would have had to basically insert it into the bore.