Comment Re:other implications (Score 1) 189
The attractive force that a magnet exerts on a ferromagnetic object is only in part a function of the depth of the hole. The more important function is the steepness of the sides of the hole. A gradual slope will attract objects to the bottom, but a steep slope will be harder to overcome.
Magnets have gotten more powerful. 1.5 Tesla magnets are run-of-the-mill today, and not the highest strength available as was the case 15 years ago. The bigger factor is that MRI systems today now have what's called 'active shielding' which essentially condenses the magnetic field emanating from the MRI into a smaller footprint.
Going back to my hole-in-the-ground analogy, the hole isn't necessarily deeper, but the sides are much, much steeper. Instead of having the gentle slope-to-the-center profile of the bottom of a pond, it's now like a sinkhole, dropping off suddenly.
You might be able to get closer to contemporary magnets before feeling the tug of the magnet, but the distance between the gentle tug and the MRI ripping whatever it is out of your grasp and stuffing it into the hole in the magnet is shorter and shorter.
For things like a backhoe, where the driver won't necessarily feel the little 'tug', it's easy to inadvertently wander into the higher strength part of the field. And the larger the object, the further from the MRI it will be subject to the magnetic attractive effects.(The bigger and more ferromagnetic the object, the bigger and steeper the hole in the analogy)
There are plenty of urban legends around MRI, but I've come to find that most are substantiated. Yes, some are puffed in the retelling, and some are patently false, but my experience leads me to believe that stories are true unless dis-proven (or clearly impossible).
http://www.ajronline.org/cgi/content/full/178/5/1092
http://www.mri-planning.com/videos/video_dl.html
http://www.ajronline.org/cgi/content/full/177/1/27
http://www.simplyphysics.com/flying_objects.html
