fMRIs use a crazy strong magnetic field—typically >= 3 Tesla. The signal that is measured is not electrical but rather the blood oxygen level. Effectively, this is a correlate of the metabolic expenditure of nearby neurons. Wass it is also like a low-pass filter except here it’s more like 0.5 Hz or slower. On the plus, you get much improved spatial resolution

MRIs use two magnetic fields, a static one and a dynamic one. And they use a radio signal according to the larmor frequency to excite the spin of the atoms. I was wondering about that radio signal component.