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| Result: Searchterm 'Spin'
found in 58 messages |
| Result Pages: 1 2 3 4 5 6 7 [8] 9 10 11 12 |
More Results:  Database (332) News Service (64) Resources (27) |
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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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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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Reader Mail
Wed. 30 Dec.09,
01:26
[Reply (6 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 |
Hello: would be probably convenient to reformulate the original question, or at least to clarify a little.... Why are the "spins" "brought into coherence" by the 90 excitation pulse?. For instance, a hard 90 ex pulse itself does not affect the coherence of spins in normal conditions, nor brough them into coherence. However, it may add an additional decoherence if the RF field is not homogeneous or, in case it would be applied under the presence of a strong static gradient. Same thing for the 180 deg pulse.
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David Brooks
Tue. 22 Dec.09,
02:35
[Start of:
'Scan equation I don't get, HELP'
2 Replies]
Category:
Basics and Physics
|
| Scan equation I don't get, HELP |
Here's the question:
The approximate scan time of a rapid spin echo sequence with a TR of 3500ms, a TE of 90ms, a 256x256 matrix, 1 exicitation, a 220mm FOV and an echo train length of 5 is______ minutes.
Formal they give is 3500msx1x256/5=?
a.3
b. 8
c. 11
d. 15
I get 179200. What am I missing in my studies and the math?
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George LoGuirato
Thu. 29 Oct.09,
19:40
[Reply (1 of 4) to:
'Math equation'
started by: 'crystal f'
on Sun. 18 Oct.09]
Category:
Basics and Physics
|
| Math equation |
Time in msec = TR * #phase_steps * NEX
Time in sec = (TR * #phase_steps * NEX) / 1000
Time in minutes = (-as above-) / 60,000
note: for FAST or TURBO spin echo divide the
#phase_steps by the ETL
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