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
|