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| Result: Searchterm 'Coherent'
found in 2 messages |
| Result Pages: [1] |
More Results:  Database (16) News Service (1) |
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Iosif Sogolov
Sun. 3 Jan.10,
20:49
[Reply (7 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 |
prior 90: spins precess around B0 uncoherently, there for the sum of their projections on TRANSVERSE plane is ZERO, they are "unfocused" in this plane. 90 and right after: all above mentioned spins are forced to rotate around B1, it should be stressed - in only one for ALL of them chosen direction of rotation (depends of B1 direction) to the TRANSVERSE plane, they all will come compact to this plane and now they do give here NET MAGNETIZATION, become "focused".
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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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