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| Result: Searchterm 'signal'
found in 52 messages |
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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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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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marta lola
Wed. 15 Apr.09,
12:11
[Start of:
'Feridex/endorem and bone MRI'
1 Reply]
Category:
Contrast Agents
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| Feridex/endorem and bone MRI |
I would like to label cells with a paramagnetic particle to see it in vivo by MRI after implantation inside a bone defect.
I am thinking on feridex/endoderm, it has contrast effect T2, predominantly negative enhacement.
Does anybody know something about this?
Will I be able to see this kind of signal by MRI? White or blak?
Thanks a lot.
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Pim vanOoij
Fri. 27 Feb.09,
15:27
[Start of:
'PCA aliasing problem'
2 Replies]
Category:
Artifacts
|
| PCA aliasing problem |
Hi everyone,
Has anybody who reads this ever encountered aliasing in phase contrast measurements in pixels where velocity did not exceed Venc? If so, could this be due to signal leakage of surrounding vessels? Thanks for your help!
Pim
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Maricella Sauceda
Thu. 16 Oct.08,
22:33
[Start of:
'Translation of MRI Result - Help'
0 Reply]
Category:
Applications and Examinations
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| Translation of MRI Result - Help |
Okay... So below is what the MRI breakdown tells me, can someone tell me if I should definitely be worried... I think it says I have brain lessions associated to sinus problems, but then they said it might be a sign of MS? HELP... Any additional clarity will be greatly appreciated...
Technique: Sequences spin - echo-enhanced in T1, T2 protons and density, flair at the axial, sagittal and coronal, 5 mm thick.
Report:
Small punctate signals focal hypo-intense in T1 and hypertensive in the other sequences and without apparent mass effect in the lobar white matter and subcortical frontal, bilateral parietal subcortical and peri-ventricular the atriums of both lateral ventricles.
Others are not displayed disturbances of encephalic parenchyma or blood collections or intra extraxiales. The medulla oblongata, pons and the cerebral peduncles with normal morphology and signal. Tanks of the base, the subarachnoid space and the cortical ventricular system above and infratentorial not dilated, without compression or movement.
Thickening of the lining which is the frontal sinuses, ceidillas etmoidales and maxillary sinuses, in the latter left with the presence of liquid level.
Comments
Many small pictures that compromise the lobar white matter and subcortical frontoparietal and peri-ventricular atrial bilateral and that although might be nonspecific, it is not possible to rule out entirely demyelinating etiology, so it must be correlated with the clinic's patient, findings of physical examination and clinical-. A new study of control in timely fashion or according to clinical course can be useful.
Pan-chronic sinusitis with signs of acute in the left maxillary sinus.
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