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| Result: Searchterm 'Magnetization'
found in 5 messages |
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More Results:  Database (155) News Service (1) |
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Math G
Fri. 30 Jun.17,
21:02
[Reply (10 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 |
I will try an answer to this rather old tread, in case someone stumble upon this like me.
The phenomenon of "coherence" that produce transverse magnetization after a 90 RF pulse cannot be answered by classical mechanics, or any simple model that represents individual protons as precessing magnets in either the parallel/antiparallel direction with regards to the MRI magnetic field.
Rather, it is a phenomenon related to quantum mechanics and the effect of a RF field on a interacting group of particles with spins (not necessarily oriented as parallel/antiparallel, I might add, even under the effect of a magnetic field).
The simplest depiction, as I understand, would be to imagine a group of spins as literally rotating as a whole under the effect of the RF. After a certain time (corresponding to a 90 degree pulse), the net magnetization that was oriented parallel to the MRI magnetic field, is now oriented in the transverse plane, causing transverse magnetization and signal. If you further apply RF, the system will continue to rotate, shifting gradually toward an antiparralel orientation, losing transverse magnetization in the process.
Hope its clearer!
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John Smith
Wed. 11 Nov.15,
22:14
[Start of:
'Faster pulse sequences'
0 Reply]
Category:
General
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| Faster pulse sequences |
Hi,
I have been learning about faster MRI sequences and have two questions
1) With "Fast (Turbo) gradient echo", in which we apply a spoiler gradient, do we not eventually end up with no longitudinal magnetization because TR is always shorter than T1? Hence shouldn't we eventually get no signal at all?
2) in SSFP (Steady-state free precession) we can apply an RF pulse of 90 degrees (in which T1>>T2) to get heart-blood contrast. How is this any different to a standard spin-echo sequence in terms of timing?
Thank you
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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
|
| 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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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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Sam Shelly
Sun. 2 Nov.08,
23:02
[Reply (2 of 3) to:
'Regarding recovery and decay [basic phycisc]'
started by: 'harry sanders'
on Sat. 7 Jun.08]
Category:
Basics and Physics
|
| Regarding recovery and decay [basic phycisc] |
Right on.
Recovery and delay are two separate physical processes like the last guy said. It had me stumped for a while when I was studying as well. But read carefully and think hard about it...
T1 recovery is the return of net magnetization into alignment with b0.
T2 decay is a totally different process and is, as the previous person said, simply loss of net magnetization in the transverse plain due to dephasing. The more electrons that are precessing in phase in the transverse plane, the stronger the NMV is in that plane. And, hence forth, when the RF pulse terminates, the influence of b0 gradually dephases the electrons in the transverse plane. As they dephase, the transverse NMV decreases accordingly, until the next pulse hits, rephasing the electrons and starting the process all over.
Hope that helps. MRI physics is hairy and boring at times. Load up on expressos to stay awake and stick with it, because studying MRI physics can be like taking a bottle of sleeping pills.
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