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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