Hi,
I used the cushion supplied by Siemens to keep some distance between the LargeFlex and the body. It is only good for small heads but large ones or big noses are not going to fit into the head matrix coild together with the cushion. (However, in the individual I used it on it made the artefact going away almost completely.)
That also means my putting the cushion in does not only affect the vibrations, presumably, but also created a maximum distance between the sag sup sinus and the table. So maybe, you are in fact right. I do certainly agree that the artifact itself seems to have some intimate relation to the big venous drainage - although it is not an anatomically strict one (i.e. it seems to propate downwards and laterally in some individuals).
Anyway, your testing sounds much more thorough and in-depths than mine and we may eventually arrive at some common denominator.
Thanks for sharing your experiences.
Cheers-
Andreas
________________________________
Von: FSL - FMRIB's Software Library im Auftrag von Markus Gschwind
Gesendet: Fr 18.01.2008 18:41
An: [log in to unmask]
Betreff: Re: [FSL] AW: [FSL] DTI: (NOT A) Vibration Artefacts in Siemens 3T
Hi Andreas,
I now have completed a row of evaluations with our 3T Trio:
- TrueFISP seems OK
- The fieldmap seems OK
- Manual shimming did not improve much
- The siemens foam cushion makes it less likely than the vacuum
polystyreneballs cushion but not reliably (there are still some
subjects with foam cushion AND artifacts.
- Longer TR/with maximum possible TE makes it disappear (but very bad SNR)
A Phantome (two water bottles):
- doesnt show the artefact.
- x-gradient sequence (manually changed directions with just variation
of the x)
1, 0, 0
0.8, 0, 0
0.6, 0, 0
etc. until
-1, 0, 0
...gives evidence that in high x values the water movements are strong.
- applying those artificial sequences to a human subject shows the
artifacts in high x values.
And here the most interesting:
- Head orientation: 2x2x2 varaition of
1. physical Head position (SUPINE vs PRONE),
2. Box position (HFS vs HFP)
3. used CoordinateSystem (xyz vs prs)
...shows that we have artifacts ONLY when lying supine and in
xyz-coordinate system, but NOT under lying prone and not under
prs-Cordinate System.
I am not yet very clear what we learn from all that:
I think it could confirms the hypothesis, that the anatomical features
play an important role: The venous sagital sinus beeing closed to the
table, together with the x gradient caused vibrations would be my
explication so far.
Thanks for all other ideas!
Cheers,
Markus
PS: What water cushion did you use?
Quoting Andreas Bartsch <[log in to unmask]>:
> Hi,
>
> ok: placing a water cushion seems to almost completely eliminate
> this artifact. So it may in fact be vibration related!
> I would be happy to hear about the experience of others.
> Cheers-
> Andreas
>
> ________________________________
>
> Von: Bartsch, Andreas
> Gesendet: Mi 19.12.2007 16:26
> An: FSL - FMRIB's Software Library; [log in to unmask]
> Betreff: AW: [FSL] DTI: (NOT A) Vibration Artefacts in Siemens 3T
>
>
> Hi,
> just to follow up on this: The artfact does also occur without
> angulation - I have tested it.
> We have involved Siemens, and as soon as I hear from them I'll keep
> you posted. Usually, they are pretty dedicated to solve issues like
> that.
>
> Cheers-
> Andreas
>
> ________________________________
>
> Von: Bartsch, Andreas
> Gesendet: Mi 21.11.2007 10:40
> An: FSL - FMRIB's Software Library; [log in to unmask]
> Betreff: AW: [FSL] DTI: Vibration Artefacts in Siemens 3T
>
>
> Good thought! Ha someone observed the artefact without any angulation?
> Cheers-
> Andreas
>
> ________________________________
>
> Von: FSL - FMRIB's Software Library im Auftrag von Martin Kavec
> Gesendet: Mi 21.11.2007 10:39
> An: [log in to unmask]
> Betreff: Re: [FSL] DTI: Vibration Artefacts in Siemens 3T
>
>
>
> Hi all,
>
> I am guessing here if this could be a coincidence with the slice
> prescription,
> I mean oblicity. If for some cases the slice prescription leaves the TE/2
> period after the 180 pulse very packed or the gradients (particularly) the x
> is due to the slice rotation too high, the gradients could indeed shake.
> Consequently, increasing the TE could change the gradient amplitude/timing to
> less demanding mode, and hence the artifact may disappear.
>
> When you see the artifact, I would change the slice prescription to zero
> angulation in all directions and see.
>
> Martin
>
> On Wednesday 21 November 2007 09:56:35 Markus Gschwind wrote:
>> Hi all there!
>>
>> @Peter Kochunov <[log in to unmask]>:
>>
>> I call it a vibration artifact because we thought that this is one
>> possibility:
>> When I am in the scanner I feel that the vibrations are on my occiput
>> exactly at those directions that are affected (remember only x
>> gradient!).
>> It is a Magnetom Trio TIM system. And we used the 30dir_2x2x2mm
>> sequence and the wip_30dir_2x2x2mm sequence from Siemens. There is no
>> difference between both sequences. Both of them show the same artifact.
>>
>> @Andreas Bartsch <[log in to unmask]>:
>> @Mikolaj Pawlak <[log in to unmask]>:
>>
>> Thanks! At least one who saw it!
>> In our clinic routine they have it also. But they explained it with
>> patients moving...
>> I met that artifact because, coming from fMRI, we where constantly
>> using the polystyrene-balls-vacuum-cushion and in every of 7 subjects
>> I got the same artifact in the same directions (x). Thus I think I
>> reproduce it quite constantly ;-)
>> Changing the cushion to the Siemens foam cushion made the reproduction
>> less reliable, i.e. sometimes it is there, sometimes not.
>> My current explanation is that the x direction is the less stables as
>> well as for the patient supine as for the scanner bed and that the
>> x-gradient vibrations, depending on the subjects weight and size, give
>> resonance to the head which might cause this artifact.
>> We use a 12-channel receive-only coil.
>>
>> What would you suggest to do in oder "to go into"?
>> Thanks for your help
>>
>> Markus
>
>
>
> --
> **********************************
> Senior Research Associate in Neuroradiology
> MRI Unit of the Department of Radiology
> Erasme Hospital
> Lennik Street 808
> B-1070 Brussels
> BELGIUM
>
> tel: +32-2-555-4325
> fax: +32-3-555-3994
> email: [log in to unmask]
> **********************************
>
> -----------------------------------------------
> Find a way, or make one!
>
--
Dr. med. Markus Gschwind, M.D.
Laboratory for Neurology and Imaging of Cognition
Dept of Neurosciences
University Medical Center (CMU)
1 Michel-Servet - 1211 GENEVA - CH
Tel 0041 (0) 22 379 5324
Fax 0041 (0) 22 379 5402
email: [log in to unmask]
http://labnic.unige.ch <http://labnic.unige.ch/>
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