Exactly what is meant by signal

[aelster]

Dear Krimsonking,

The vertical (y-) axis in most of these diagrams is typically used to represent either the evolving MR signal intensity (S) or the longitudinal magnetization (Mz) during the course of an MR experiment. These values are closely related.

The signal (S) depends most directly on the transverse magnetization, Mxy. Mxy, in turn, represents the fraction of Mz tipped into the transverse plane by each RF-pulse. Thus Mz and S are proportional which is why they are often used interchangeably in the diagrams you have mentioned.

The decay of the MR signal is a decrease in Mxy dictated by loss of transverse coherence and T2 relaxation. During this relaxation period, Mxy is getting smaller but Mz is getting larger, regrowing by T1 mechanisms. The longer you wait until the next RF-pulse, the bigger Mz will be and the stronger the signal. If you wait a very long time (>5xT1), Mz will have returned to its maximum possible value (Mo) and produce the strongest signal possible.


Allen Elster