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| Result: Searchterm 'Coherence'
found in 9 messages |
| Result Pages: 1 [2] |
More Results:  Database (19) |
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N P
Fri. 18 Dec.09,
04:39
[Reply (5 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 |
Ok. How about reading the thread before posting. We're not asking how a 180 rephasing pulse works. INVERSION VS REPHASING CONCERNING COHERENCE.
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|  View the whole thread | | |
n p
Fri. 11 Sep.09,
11:50
[Reply (3 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 |
no hithesh n, that is a 180 rephasing pulse. thats not the question. the question is how is the net magnetization changed from the z plane to the -z plane without causing phase coherence, like the case of an inversion pulse. it is supposedly 2x as long or 2x as strong as the 90 degree pulse, but doesn't cause coherence. Why not is the question.
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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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Bjorn Redfors
Sat. 27 Jun.09,
12:31
[Start of:
'90 excitation pulse vs 180 inversion pulse'
11 Replies]
Category:
Basics and Physics
|
| 90 excitation pulse vs 180 inversion pulse |
Im a medical student interested in diagnostic medicine and aim to understand (on some level) the basics of MRI theory but lack an advanced physics background.
Why are the "spins" "brought into coherence" by the 90 excitation pulse but not by a 180 inversion RF pulse?
And how is the 180 inversion pulse explained on the nuclear level, i.e. "paralell/antiparalell" orientation of single nuclei (is it possible by such a simple model?)?
Is it possible to explain this "in layman's terms"?. I find that most texts (at least those written for physicians) omit proper explanations of this.
Thank You!
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