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| Result: Searchterm 'HIS'
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Sherri Simpson
Fri. 9 Oct.09,
21:56
[Start of:
'GE HDe 1.5 scanner'
1 Reply]
 Category:
Devices, Scanner, Machines
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| GE HDe 1.5 scanner |
How well does this scanner perform with 50 gradients vs 120 gradients most scanners have?
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Lyle Downing
Sat. 26 Sep.09,
20:27
[Reply (4 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 |
Perhaps this will help shed some light on this.
Keep in mind that before the initial 90 pulse all protons contributing to the MR signal are in a relaxed state completely in alignment with the static magnetic field. Flipping them 90 degrees into the transverse plane does align them up initially and yes they do relax at different rates as they give up their energy. The 180 pulse takes whatever state they are in at the time and flips them in order to not make them all 180, but to quickly get a cleaner non contaminated representation of the tissues in question. So for example after the initial 90 and after letting the protons relax for a bit you might see water at say 50 degrees and fat at say 70 degrees flipping them 180 keeps whatever energy state they are in the time.
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James R
Sun. 13 Sep.09,
04:24
[Reply (5 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 |
i don't know if it's just me but i feel the questions on the MIC course are way off topic sometimes. when i answer these questions myself, i feel like i know nothing cause the questions IMO are being asked in the "beating around the bush method"
i've been told that the registry is more basic than the questions they ask. pus they cover way too much than the ARRT exam question breakdown.
i'm going to use the black and yellow & lippincott book plus the answered question MIC from here on in.
can anyone who knows of or has taken the registry recently than can clarify this.
any advise or info will be greatly appreciated cause i'm pretty intimitaded by this test and i want it done by the end of this year
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hithesh n
Fri. 11 Sep.09,
08:33
[Reply (2 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 |
Hi Bjorn,
I might be able to explain this even though its too late.
Initially a 90 excitation pulse is applied, the Hydrogen protons precess in the XY plane. Now they are spinning in sync in the XY or transverse plane. This is where they emit the RF signal.
But pretty soon, the neighboring hydrogen protons go out of sync, ie one is going faster and the other is going slower. This is similar to runners running a race in a track, they all start at the same time(assume) but after a couple of secs, some run faster than the other. The faster ones are in the front and the slower ones are in the back.
How do you bring them back into sync?
This is where the 180 excitation comes into play.
Now you apply a 180 pulse, this is equivalent to making the runners run in opposite direction. Now suddenly, the slower runners are gonna be in the front and faster ones in the back. Eventually the faster ones catchup and all of them are gonna be in sync. They go out of sync again.
They go out of sync bcoz the magnetic field applied is not uniform and due to material (tissues, bones etc). Local variations in the field causes the protons to go out of sync.
The 180 brings them in to coherence, not instantly but they do catch up and become coherent.
The 90, brings them into coherence almost instantly.
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Reader Mail
Mon. 7 Sep.09,
01:04
[Reply (1 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 |
Yes, I have been searching for the answer to this question all summer. I never find the answer, and then keep coming back to it. You are right, I can find no literature that explains this. If anyone could answer it would be appreciated.
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