If you don’t want the EEG to capture your brainwaves you can wear a tin foil hat to lead the magnetic field astray.
(Slightly more seriously, the diamagnetic properties of Sn would in actuality very much interfere with the B1 field modulation of the (f)MRI sequence; and disturb the local B0 homogeneity; and thus disturb the experimental results. Although that was of course not what I meant when initially responding)