Hi,
I don't know if the re-calibration/shimming changes from run to run, since the b=0
images don't show this distortion Markus mentions.
On the other hand, the Stejskal-Tanner sequence has really bad eddy-current effects,
which are much less prominent in the double-refocusing sequence (Reese's).
If you scan with a high gradient cycle, I would expect the scanner to heat-up, changing
the eddy-currents. This might be the origin of the artifacts.
The part I don't know is why eddy_correct doesn't fix it...
I think the best you can do, if you can't fix the distortions, is to use the double-refocusing
pulse sequence.
-Pablo
On Sat, 29 Nov 2008 23:56:05 -0600, Kochunov, Peter <[log in to unmask]>
wrote:
>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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