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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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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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Bjorn Redfors
Sat. 27 Jun.09,
12:31
[Start of:
'90 excitation pulse vs 180 inversion pulse'
11 Replies]
Category:
Basics and Physics
|
| 90 excitation pulse vs 180 inversion pulse |
Im a medical student interested in diagnostic medicine and aim to understand (on some level) the basics of MRI theory but lack an advanced physics background.
Why are the "spins" "brought into coherence" by the 90 excitation pulse but not by a 180 inversion RF pulse?
And how is the 180 inversion pulse explained on the nuclear level, i.e. "paralell/antiparalell" orientation of single nuclei (is it possible by such a simple model?)?
Is it possible to explain this "in layman's terms"?. I find that most texts (at least those written for physicians) omit proper explanations of this.
Thank You!
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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
|
| 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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John Jackson
Fri. 12 Sep.08,
08:48
[Reply (1 of 3) to:
'Regarding recovery and decay [basic phycisc]'
started by: 'harry sanders'
on Sat. 7 Jun.08]
Category:
Basics and Physics
|
| Regarding recovery and decay [basic phycisc] |
Hi,
I'm not so sure but I think because different physical process are being involved. Longitudal relaxation has got something to do with energy exchange to the environment. Transverse relation occurs because of spin dephasing (magnetic inhomogenity.)
John
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Reader Mail
Sun. 27 Jan.08,
08:47
[Start of:
'Plz Answer this ... Contrast MRI of Brain'
1 Reply]
Category:
Applications and Examinations
|
| Plz Answer this ... Contrast MRI of Brain |
Pre and Post contrast mri of the brain was performed in multiple planes using T1 & T2 W spin-echo sequence.
There is small ring enhancing lesion in the left occipitoparietal lobe which measures 1cm in diameter.It reveals isointense periphery on T1 & T2W images with hyperintense core on T2W images. On T1W images the core appears hypointense . A tiny mural nodule is seen within the lesion. focal perilesional edema is seen appearing hyperintense on FLAIR and T2W images.
The brainstem & cerebellum are normal.
The ventricular system is normal.
No abnormal meningeal enhancement is seen.
Intracranial vessels display normal flow void.
What needs to be done?? How serious the problem is??
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