Nuclear Spin and Magnetic Resonance Imaging

[Jimmy87]

Hi pf,

Please could someone kindly answer a few questions about proton spin in the context of MRI. As I understand it when an external static magnetic field is applied to a patient the magnetic dipole moments of the protons in the hydrogen nuclei will experience a torque causing them to align with this external field. Some will align parallel and some will align antiparallel. Aswell as the alignment, the other effect this external field has is to cause the protons to precess around an axis which is aligned with the external field. I have two questions:

1) Why do some protons align anti-parallel with the external field? I get that they do this when a radio frequency pulse is sent in but before that why do some align anti-parallel when the lowest energy state is parallel?

2) What causes the protons to precess (i.e. why can't they just line up with no precession) and why do the magnetic dipole moments never fully align with the external magnetic field (there is always some angle)?

Thank you for anybody who can spare time to help answer these questions

 

[DrDu]

1) Temperature. The energy difference between aligned and anti-aligned spins is diminute, and you get a Maxwell Boltzmann distribution over these two states.
2) Uncertainty principle. If spins were to align perfectly with the magnetic field, e.g. along z-axis, the component of the spin along the x and y-axis would have to vanish. But this is not possible according to the uncertainty principle which states that not both s_x and s_y can be measured without uncertainty.