MRI image acquisition aligns hydrogen atoms with a magnetic field, uses a temporary field to pull them, and measures emitted frequency as they reorient.

“and during the period of time of that acquisition what it's doing is everything that's inside the center of that doughnut is being pulled in a certain direction all the hydrogen's on your water and fat and then the loud part of the MRI is actually a temporary magnetic field which is countering that static field and so it's actually now pulling all the hydrogen's in the opposite direction and then in that opposite direction it actually turns off and then the hydrogen's reorient to where they were in that static field and as they reorient they actually give off a different frequency”