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| Result: Searchterm 'Frequency'
found in 14 messages |
| Result Pages: 1 2 [3] |
More Results:  Database (251) News Service (11) Resources (6) |
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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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crystal f
Sun. 18 Oct.09,
21:34
[Start of:
'Math equation'
3 Replies]
Category:
Basics and Physics
|
| Math equation |
Can some one help me w/this question.
From the following parameters what will the scan time be, in minutes, to acquire data?
TR 2000msec FREQUENCY STEPS 256
TE 60msec NEX 2
T1 800msec FLIP ANGLE 90
pixel .95x.95mm PHASE STEPS 192
THANKS
mr registry review
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Reader Mail
Mon. 14 Jan.08,
22:49
[Reply (2 of 4) to:
'Mechanical Ventilator outside of Scan room'
started by: 'Dan Barrieau'
on Wed. 20 Jun.07]
Category:
Site Planning and Specifications
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| Mechanical Ventilator outside of Scan room |
Just to add to that. The rooms are lined with copper mesh so that Radio Frequency (RF) outside the room will not affect the image quality (and vise versa so your monitors won't freak out). The tubes through the wall are long enough to absorb and basically prevent any RF from getting in or out. The place in question probably couldn't justify the money to buy an MR safe vent. And I can't think of any pump that would be safe inside the room; although, some sites will chance taking pumps in and keeping them at a distance from the magnetic field.
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Angel Lara
Thu. 5 Jan.06,
18:03
[Start of:
'siemens impact expert'
2 Replies]
Category:
Sequences and Imaging Parameters
|
| siemens impact expert |
Does anyone remember how to shim for silicon on numaris?
I am having trouble with breast implants, I do not know how to adjust center frequency for proper shimming of water and silicon.
Some images look like saline and some look like silicon.
Old machine and do not have accsess to applications. Any help will be greatly appreciated.
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