[Niles]

Hi

I am reading about the spin-echo technique, and I have no idea of how to interpret what I am reading. As far as I have understood, by reversing the orientation of the spin, we "re-phase" the system, but does anyone know what is meant by "re-phasing" and how it can be visualized?

Any help will be greatly appreciated.

Best,
Niles.

[softmatterresources]

In a normal NMR experiment a sample is placed in a magnetic field which produces a net magnetisation in the z direction, Mz. This net magnetization is then rotated from the z plane into the xy-plane by a 90 degree r.f. pulse.

When this pulse has finished, the net xy-magnetisation created will undergo exponential free induction decay as the spins de-phase via T2* relaxation. It is this decay that is measure in an NMR experiment.

For spin echo, after the 90 degree pulse, during the decay,  a second rf pulse of 180 degrees will completely reverse the phases of the decaying spins so any non-uniformities in the local magnetic field that were acting to de-phase the spins, will now act to rephrase them and net magnetisation will be regained, producing a signal;the spin echo.

This site has some NMR animations that might help.

http://mutuslab.cs.uwindsor.ca/schurko/nmrcourse/animations.html

[Niles]

Hi

Thank you for that explanation and also for the link. There is one point that I am unsure of: After the the second rf pulse (of 180 degrees) applied at half the evolution time 2t, the spins re-phase -- but wont they only re-phase assuming that the evolution time would take a non-decaying spin from e.g. y-axis to the opposite side?

I hope you understand. Basically, in this animation http://en.wikipedia.org/wiki/Spin_echo, the re-phasing at the opposite side of the y-axis will only occur for one special 2t, right? (By the y-axis I mean the one we reach after the first rf pulse of 90 degrees).

Best,
Niles.