Magnetic media (e.g. your credit card) may be affected at field strengths of 3 millitesla (mT). For comparison, this magnetic field strength is about 100 to 200 times stronger than the normal magnetic field on the surface of the Earth (depending where you are on the planet). The 3 mT field line around a typical shielded 1.5 T MRI magnet is about 2 metres from the side of the magnet, and less than 3 metres from the ends of the magnet.
Notice that the magnetic field around an MRI magnet is 500 times smaller than the strength at the isocentre at just 2 to 3 metres distance!
I wonder how the magnetic field affects the newer chip-and-pin microchip which is embedded in credit cards. Anyone care to comment?