Exactly what is meant by signal

[Krimsonking]

Hi, I have a simple question that I haven't had answered to my complete understanding.

In graphs depicting the effect of whatever variable you want the y-axis is usually signal intensity.

Figure 1: http://www.google.com/imgres?imgurl=http...CEQMygEMAQ

But I have also seen this y axis detailed as Mo

Figure 2: https://www.google.com/search?q=Signal+i...B381%3B242

My question is: What exactly is the y axis? Time after excitation is letting the excited protons relax back into the longitudinal plane. Wouldn't this mean that signal intensity is going down? Aren't we measuring the rotation of the magnetic moments through the RF coil that is all around the transverse plane? How do I relate [/align]Mo and signal intensity?

Thank you!

[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

[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.