Interesting. When people think "metal near magnets", they're usually thinking of ferromagnetic stuff (iron and iron alloys like steel being the most common example), which has a very strong response to magnetic fields. Bismuth is diamagnetic, which means that it's repelled by magnets rather than attracted to them, and that this response is, relatively speaking, very weak (for reference, water is diamagnetic. Not as much as bismuth, but still).
My point being that, metallic or not, bismuth is not going to react to the MRI like something made of steel would. Maybe its high diamagnetism means you get bad results or something.