"Strong" is en entirely relative term, but given the number of MRI's performed, its pretty clear that fields several orders of magnitude larger than the Earth's background 0.5 gauss are at least "mostly harmless" with an exposure of limited duration.
From an FDA publication regarding MRI scanners:
There are no known harmful side-effects associated with temporary exposure to the strong magnetic field used by MRI scanners. However, there are important safety concerns to consider before performing or undergoing an MRI scan.
This article describes human brain imaging with a 9.4T instrument, although this is about 10x larger than the strength of most MRIs. Note that "no detrimental effects" does not mean zero effects, but this is starting to get into the range of the 25T described above. There are 20T+ MRI machines avaialble, but I could not find an article describing use on live organisms. There is just this one on dead mice parts.