None that are known or expected, although I don't believe effect of high field and high field gradients are really well understood.
For what its worth there are people who have been in for far more than 20 hours (e.g. hundreds of cumulative hours).
For what its worth there are people who have been in for far more than 20 hours (e.g. hundreds of cumulative hours).
I don't think the effect has been reseached further (such as exposing birds to a very strong magnetic field and see if their perception is altered afterwards, not sure whether that would pass an ethics commitee..)
The floating frogs were supposedly fine afterwards. [2]
Supposedly, if any long term effects occur, they are subtle. There is a lot we do not know yet - for example, cells seem to be able to communicate through glass. (How? no clue. Photons have been proposed.)
[1] eg http://www.physicscentral.com/explore/action/pia-entanglemen...
[2] http://www.ru.nl/hfml/research/levitation/diamagnetic/#Whyfr...
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2660427/
which is a study from 2009 on the phenomenon.
That's fair though, I really should have been more specific: You can certainly harm a human being with the piece of machinery known as an MR scanner. Much more so than just PNS, you are dumping energy into the body via RF, so there is a specific absorption rate (SAR) limit applied to avoid going into ranges that will damage tissue.
What I was refering to was limited only to the effect of exposure to fixed magnetic fields in an MR, and gradients and RF coils as they are constrained for clinical use.