We've had a quick scout round the data here.
We almost always acquire in pure axial, and we don't have any
examples of this artefact in such data, but we have one experiment
when images were acquired off-axial, and data in this experiment does
occasionally appear to show the artefact, although apparently less
strongly than in the examples given. One possible reason for this is
that the orientation of the gradients is x-y and not pure x (it is an
adc experiment, hence we do not have an x-only orientation).
Cheers all
T
On 21 Nov 2007, at 09:40, Andreas Bartsch wrote:
> 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
>
>
>
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