Exactly what is meant by signal

[hurstg@upstate.edu]

M means Magnetization. Mo means the equilibrium value magnitude of magnetization. When the spins are exposed to Bo (patient goes into the magnet), before any RF pulses happen, the magnetization aligns along the Bo direction, conventionally assigned to the z direction; so the time-series vector diagrams often start with a vector labelled "Mo" pointing along z. This Mo along z is not detected by MR rf coils, primarily because it is not oscillating (it is "static," like Bo), and also because the RF coils are designed & oriented to only/primarily observe magnetization in the transverse (x-y) plane, perpendicular to z.

Nutation ("flipping") some or all of the Mo into the x-y plane - e.g., with a 90 degree pulse that puts all of the z-direction Mo into the x-y plane - produces a vector commonly called Mxy. This vector precesses around z/Bo, at the Larmor frequency, and will induce electromagntic force (EMF) into an RF receiver coil. This force is the basis of the detected RF signal. The intensity of this signal depends on the size of Mo - which is the biggest it can be - as well as how much of Mo gets converted into Mxy (and other spin shenanagins like defocussing and refocussing by additional RF and/or gradient pulses, before the signal is "sampled").

Eventually, when the RF flipping and relaxation processes are done, the spins return to equilibrium, i.e. Mxy reverts back into Mz, with a strength Mo.

I hope this is helpful.