MRI question for our Heavy Weights

[imabug]

OK, there's something I must be fundamentally confused about here, it's been too long since I've used my quantum mechanics.

They way I recall quantum spin, the spin state could be completely described as a superposition of |up> and |down> spins. Pick an axis, any axis, and the total spin can be described as a superpositon of up & down spins along that axis. The Stern-Gerlach experiment is the cannonical example - a beam of silver ions that splits into two parts when passing through a spatially varying magnetic field.

So I always imagined the spins as either being aligned or anti-aligned, i.e. either |up> or |down>, but the descriptions I see on the WWW are of precession.

Most explanations of MRI represent spin using the spinning top model (not really accurate, but simplistic enough for radiologists to understand). MRI can only be done with atoms having a net magnetic moment. It's the magnetic moments that get 'flipped' by the RF pulse (actually it's just getting rotated, no spin flipping taking place at all). As the magnetic moments realign themselves with the main field, that RF energy is radiated out again. The common layman's representation of this is a bar magnet in an external magnetic field. If you rotate the magnet so that it isn't aligned with the external field, it will want to realign itself with the external field.

 

[pervect]

OK, I found a website that explains why precession is equivalent to a superposition of up & down states - I think I see what's going on now. The bar magnet explanation _is_ a lot easier than the formal quantum mechanical one!

http://www.phys.cwru.edu/courses/p431/notes-2003/node30.html
and node31, too.

Also, the explanation that the protons do naturally line up with the magnetic field, and you have to disturb them in order to do measurements was helpful - thanks, Tsunami.