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| Result: Searchterm 'B0'
found in 5 messages |
| Result Pages: [1] |
More Results:  Database (41) News Service (1) |
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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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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
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| 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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Manuel Franco
Sat. 22 Aug.09,
12:41
[Reply (1 of 21) to:
'MRI registry equations'
started by: 'Donna Nusser'
on Tue. 18 Aug.09]
Category:
Basics and Physics
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| MRI registry equations |
Subtract the B0 field from the 1.006T and then divide the resulting 6mT by 0.5m.
Hope this helps to understand.
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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
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| 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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