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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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Bjorn Redfors
Sat. 27 Jun.09,
12:31
[Start of:
'90 excitation pulse vs 180 inversion pulse'
11 Replies]
 Category:
Basics and Physics
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| 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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Brenna Bray
Tue. 10 Mar.20,
15:59
[Start of:
'Proton excitation from B0 to B1'
0 Reply]
Category:
General
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| Proton excitation from B0 to B1 |
I'm new to MRI imaging and currently taking an introductory course online. I understand that the introduction of a small magnetic field/RF pulses on top of the static magnetic field (B0) excites protons into a high energy state (B1). Does this mean that the protons are excited from a spin that is parallel to the longitudinal B0 plane to one that is antiparallel to the longitudinal B0 plane, or that the proton spin is excited from one that is aligned/polarized with B0 to one that is either parallel or antiparallel to B0 in the B1/xy plane?
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shahrokh Rad
Sat. 4 Oct.14,
13:44
[Reply (9 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 |
All of the responds are misleading.
what is the correct answer??????
why does the 90 excitation pulse result in phase coherence while the 180 pulse won't.
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Reader Mail
Wed. 1 Jun.11,
14:14
[Start of:
'Dixon technique'
1 Reply]
Category:
Sequences and Imaging Parameters
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| Dixon technique |
I have been searching the web for information regarding a "double-echo two-excitation pulse sequence encoding fat and water signals for a phase-sensitive three-point Dixon type analysis". Does anyone know what pulse sequence(s) this is referring to, or a publication that might be helpful???
Thanks
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Oliver Lyttelton
Mon. 1 Mar.10,
13:39
[Reply (8 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 |
Okay, so this thread is answering close to a question I had, which is how to conceptually understand what happens with alpha>90 degrees excitation pulses.
I can imagine spinning tops, precessing at the Larmor frequency, I can imagine that as you apply the excitation pulse which is always in the transverse plane to the main magnet, you start to pull the tops further away from the B0 axis and bring them into coherence so like lots of little lighthouses they are all bright/dark in phase with each other. I can imagine a 90 degree pulse bring the spins completely into the transverse plane. I can imagine them relaxing, dephasing quickly and then slowly reducing their angle of precession back up towards initial state close to direction B0.
But what I can't understand in my (rather newtonian) model, is what happens as you continue to excite beyond the 90 degree transverse plane. I sort of get that somehow the spins continue to rotate in some (weird) dimension, and that they have to come back through that (weird) dimension first before returning from 90 degrees back to the relaxed state. But what happens in "weird" dimension is beyond my conceptual model. Can someone extend my model for me, preferrably without signal equations?
tar muchly,
Oliver
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