My understanding is that even when they are not used there is a field of between 1 and 3 teslas, depending on the scanner. (A quick google search for things like "mri magnet is always on" and "mri safety" and such seems to confirm this.)
Edit: Also, you can get closish to the scanner with a ferromagnetic object without a problem. The field doesn't encompass the whole room or anything. 10 or 15 feet away will be fine.
They're not magnetic enough to pull a chair off the floor or things across the room (as the story indicates) when they are not in operation. You can walk around the room with your watch on (for example), but if they fire it up and you're in there with it, it's destroyed.
Just look how close they have to get the steel oxygen bottle to the center (in the video linked above) before it moves. It's almost inside it already.
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.
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.
Well, technically it encompasses the entire universe - or least the part that is in the light cone since the field was energized. :)
But magnetic fields drop off by distance^3 or ^5, so the strength drops rapidly with distance.
It also depends on if you are interacting with another magnet, or with unmagnetized iron. ^3 or ^5 is just an approximation - it can go to ^7, and it's not a definite number, it varies.
See: http://www.exo.net/~pauld/activities/magnetism/forcebetweenm...
>Well, technically it encompasses the entire universe - or least the part that is in the light cone since the field was energized. :)
As permittivity is generally a function of frequency doesn't this mean that the progress of a magnetic field through space can differ to that of a [theoretical] light cone (which bounds the volume of causal connectedness)?
Magnetism gives me headaches.
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.
You can use a fiberglass, or aluminum stretcher.