B0: The Magnetic Bore
A typical MRI bore, or the ring which the patient is moved into, houses the magnet for the MRI. For clinical scanners, the strength of the magnet are usually at 0.5, 1.5 or 3 Tesla (T). For comparison, a regular refrigerator magnet is about 0.005 T while a buckyballs are about 0.48 T.
The external magnetic field (aka "B0 field") generated from within the bore causes the protons in the body to align themselves to the external magnetic field and the protons begin precessing at the Larmor frequency. This is the equilibrium state for the hydrogen atoms.
As noted before, the Larmor frequency is calculated by:
w = y * B
In this case, y is the gyromagnetic ratio of the hydrogen atom ( 42.58 MHz / T ) and B is the strength of the B0 field.
Since the external magnetic field is always on, if protons are disturbed and knocked out of equilibrium, the protons will naturally return to their aligned state. This can be easily demonstrated on the larger scale by placing buckyballs near each other and manually moving some of them out of alignment - they will naturally return to their aligned orientation along the magnetic field (left). On the atomic level, you can imagine the orientation of the magnetic vector for a proton to be similar to the roly-poly Russian dolls in the sense that both return to their equilibrium state after being knocked over (right).
For future reference, the B0 field will be in the +z-direction.