Back in the days of passively shielded magnets, these sort of distortions were quite common. Sequences with high gradient cycles would cause a drift in B0, as well as introduce time variable change to the eddy current. Nowadays, these magnets are all but museum pieces, and actively shielded clinical scanners have superb time stability. Thus, Matt's suggestion of inter-run recalibration of shimming/zero frequencies makes the most sense!
pk
________________________________
From: FSL - FMRIB's Software Library on behalf of Matt Glasser
Sent: Sat 11/29/2008 10:42 PM
To: [log in to unmask]
Subject: Re: [FSL] DTI acquisitions have different A-P distortions
Are you sure it is not just a position shift of the images (that simply needs a rigid body registration to correct)? I believe this can be caused by reshimming between averages. If the head does not move and the same sequence and gradients are applied, the distortion should not change.
Peace,
Matt.
________________________________
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of Markus Gschwind
Sent: Friday, November 28, 2008 6:54 AM
To: [log in to unmask]
Subject: [FSL] DTI acquisitions have different A-P distortions
Dear FSLers!
This is a question for the radiologist or physicists.
I have a DTI sequence where I am taking several acquisitions (non-stop) and I have the problem that systematically the distortion seems different.
That means, the volumes have different length in their AP direction across all gradients, but not in the LR or the IS directions (1. acquisition more anterior, 2. more posterior).
Thus, looking at one acquisition separately, it looks nice, but as soon as they are interleaved there is a shaking in between the acquisitions.
This is only true for the gradient volumes, not for the b0 volume.
It appears consistently in every subject in a similar way.
Applying eddy_correct does not change it.
I tried with applying a fieldmap, but this did not change neither, however I am not sure if I did it correctly.
The sequence is a mono-polar (stejskal-tanner) ep2d dti on a Siemens TrioTIM Syngo VB15 System.
Does anybody know what this could be and what could be done against?
Obviously the problem becomes important when averaging the different acquisitions.
Thanks in advance,
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 <http://labnic.unige.ch/>
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