The Physics of MRI

I’ve been reading up on the physics behind MRI lately, mostly on this site I’ve found it really interesting and informative. But it doesn’t seem to say why the nuclei precess at the angle theta as they do. I understand the need to precess at such an angle, but why does it occur? Is the angle distinct for each element? Can the angle be derived as a function of the magnetic field and another value? Thank you.

The frequency at which a dipole precesses about an external magnetic field (the Larmor frequency) is independent of the angle it makes with the external field. Essentially, the farther out of alignment the dipole is with the external field, the more torque the external field will exert on it; but the farther out of alignment from the axis of precession the angular momentum is, the more work the torque will have to do to change it.