Herbert Müller
Thu. 5 May.22,
01:33
[Reply (11 of 12) to:
'90 excitation pulse vs 180 inversion pulse'
started by: 'Bjorn Redfors'
on Sat. 27 Jun.09]
 Category:
Basics and Physics
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| 90 excitation pulse vs 180 inversion pulse |
Hello,
I dont know if anyone has, for him/herself, answered this question. I know it is an old thread. But this is the question which also botheres me the most in NMR/MRI.
I refer now to a picture, which is illustrated in the book from Tim Claridge (High resolution NMR techniques).
Here, a RF pulse (lets say a 90 degree pulse) is said to have two effects on the initial z-magnetisation. First, it allows energy transfer between the two Zeeman-states in a way, that i equalizes the population difference between them. So there is no z- magnetisation anymore after 90 degree pulse. Th esecond effect is, that it induces phase-coherence. And this phase coherence is then responsible for the x-magnetisation, which is measured.
And now I have the same question. Why is this phase coherence lost (or is it not?), when applying an initial 180-degree pulse. Understandable for me is, why the population difference in z is reversed, but I dont understand, why there is no xy magnetisation anymore.
Maybe some of you have figured out the answer in the meantime?
Greetings
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