I think it could be due to a large body of slow-flowing-deoxy blood doing
something to the phase maps and causing errors in shimming. But it is a far
reaching statement and might not be right.
One way to check is do a manual 3D shim with a slab over the brain. We
always do it for DTI/EPI studies and never had a funky looking artifact like
yours.
Or just run a balanced sequence like truefisp and see if you have any
artifacts within the volume. With good shimming you should be able to get an
artifact-free brain image.
pk
----- Original Message -----
From: "Mikolaj Pawlak" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, November 21, 2007 9:43 AM
Subject: Re: [FSL] DTI: Vibration Artefacts in Siemens 3T
> Hi Peter,
>
> Do you think that this artifact is driven by blood velocity in the
> saggital sinus? Why cant we see artifacts around other major vessels?
>
> Thanks,
>
> Mikolaj
>
> On Nov 21, 2007 9:39 AM, Peter Kochunov <[log in to unmask]> wrote:
>> Markus,
>> The fact that you see this artifact localized around saggital sinus makes
>> me
>> suspect things other than simple vibrations. One way to check this would
>> be
>> to run a head phantom with a DTI sequence and see if anything like it
>> appears. If it doesn't, I would focus my attention on shimming. DTI
>> sequence
>> does the 3D phase map shipping on the slab. Depending on how the slap is
>> defined for your sequence you might be introducing areas on inhomogeneity
>> into the volume.
>> pk
>>
>> ----- Original Message -----
>> From: "Markus Gschwind" <[log in to unmask]>
>> To: <[log in to unmask]>
>>
>> Sent: Wednesday, November 21, 2007 2:56 AM
>> Subject: Re: [FSL] DTI: Vibration Artefacts in Siemens 3T
>>
>>
>> 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
>> --
>>
>> 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
>>
>
>
>
> --
> ****************************************
> Mikolaj A. Pawlak, MD PhD
> Neuroradiology Research Division
> Department of Radiology
> University of Pennsylvania
> 3600 Market Street Suite 370
> Philadelphia, PA 19104
> Phone: 215-662-3688
> Fax: 215-615-3681
> mail: [log in to unmask]
>
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