My understanding is that this is largely urban legend. Though there have been a few anecdotal reports of people with swelling or minor irritation around their tattoos, experiments have suggested there isn't a problem. The show MythBusters on Discovery Channel tested this and found no backing for the rumor, too.

I am not as sure what having a replacement hip or other joint or large metal pins or screws in one's body will do in an MRI scan; there could possibly be some concern there that would limit its usefulness. Of course, we who have had dental cavities have plenty of metal in our teeth and are able to go through MRIs without a problem.

I suspect though that other issues will make MRIs prohibitive in use in a security setting. They tend to be fairly large and very expensive, for one ... and there are always problems with the intense magnetic field generated. I bet it would wreck your laptop and might even cause problems with pacemakers, etc.