>>11359835This is gonna be a 2 parter, so get ready:
MRI machines operate on a phenomenon known as nuclear magnetic resonance (NMR). To put it VERY simply, any atom with an odd number of nucleons has a non-zero quantum magnetic spin. When under the influence of a very strong uniform magnetic field the atoms magnetic "pole" will align with it, but the atoms also have intrinsic angular momentum and they will precess around the direction the field at a very specific frequency known as the Larmor frequency (imagine a top precessing about its axis of rotation). When the atom encounters a photon that matches that frequency it can absorb it and be put out of alignment into an a higher energy unstable state. After a certain characteristic time the atom will realign and release the energy in the form of another photon. For most atoms this frequency is in the radio wave range.
Protons have a unique Lamor frequency from other atoms depending on the strength of the magnetic field they're in. So for MRI's it's more important that the RF pulse matches that frequency, though the power of the RF field is very high to create a clearer image (can peak at 35 kW). This phenomenon only occurs in very strong magnetic fields, so it won't apply to cellphones or radio towers, since the Earth's field is very weak, those radio emitters are comparatively weak, and their frequency wouldn't match the Lamor frequency, so they couldn't be adsorbed.
The reason only the extras are pushed out of alignment is because the rest have their magnetic spins cancelled out by the others spinning in the opposite directions. Alignment with the magnetic field is a slightly lower energy state, so it's somewhat more preferred by the protons. So a few more protons align with the field than against. They have no opposite spin partners to cancel out with so they have a net alignment that can be affected.
