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| Result: Searchterm 'Excitation'
found in 8 messages |
| Result Pages: 1 [2] |
More Results:  Database (108) News Service (1) |
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Reader Mail
Wed. 30 Dec.09,
01:26
[Reply (6 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: would be probably convenient to reformulate the original question, or at least to clarify a little.... Why are the "spins" "brought into coherence" by the 90 excitation pulse?. For instance, a hard 90 ex pulse itself does not affect the coherence of spins in normal conditions, nor brough them into coherence. However, it may add an additional decoherence if the RF field is not homogeneous or, in case it would be applied under the presence of a strong static gradient. Same thing for the 180 deg pulse.
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hithesh n
Fri. 11 Sep.09,
08:33
[Reply (2 of 12) to:
'90 excitation pulse vs 180 inversion pulse'
started by: 'Bjorn Redfors'
on Sat. 27 Jun.09]
Category:
Basics and Physics
|
| 90 excitation pulse vs 180 inversion pulse |
Hi Bjorn,
I might be able to explain this even though its too late.
Initially a 90 excitation pulse is applied, the Hydrogen protons precess in the XY plane. Now they are spinning in sync in the XY or transverse plane. This is where they emit the RF signal.
But pretty soon, the neighboring hydrogen protons go out of sync, ie one is going faster and the other is going slower. This is similar to runners running a race in a track, they all start at the same time(assume) but after a couple of secs, some run faster than the other. The faster ones are in the front and the slower ones are in the back.
How do you bring them back into sync?
This is where the 180 excitation comes into play.
Now you apply a 180 pulse, this is equivalent to making the runners run in opposite direction. Now suddenly, the slower runners are gonna be in the front and faster ones in the back. Eventually the faster ones catchup and all of them are gonna be in sync. They go out of sync again.
They go out of sync bcoz the magnetic field applied is not uniform and due to material (tissues, bones etc). Local variations in the field causes the protons to go out of sync.
The 180 brings them in to coherence, not instantly but they do catch up and become coherent.
The 90, brings them into coherence almost instantly.
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Reader Mail
Sun. 6 May.07,
17:28
[Start of:
'OVS & phase refocussing'
0 Reply]
Category:
Sequences and Imaging Parameters
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| OVS & phase refocussing |
Why do we need to rephase spins in outer-volume regions after their excitation, still applying a crusher gradient afterward? Is this not too much work, rephasing and then dephasing them?
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