Hi,
Are your shim settings the same for the fieldmap and
diffusion scans? If the shim changes then that would
explain it. Alternatively, it could be some grad warp
issue as that would change how a shift in voxels related
to a shift in field locally. If you are only looking at b0 to
a non-distorted (e.g. T2 FSE) image then I wouldn't
expect it to be anything to do with eddy currents,
unless you've already applied eddy current correction
and the registration has done something to your images.
I'd be interested to see some examples of what you get
to see the problem myself. It would also be good to know
exactly what commands you are running.
All the best,
Mark
Marenco, Stefano (NIH/NIMH) [E] wrote:
> Yes, we have observed the same thing in our scans, and we don't
> understand why that would be either. We are using GE equipment. What
> scanner do you have? Could this have to do with imperfect grad warp
> corrections ? Stefano
>
> -----Original Message-----
> From: Drew [mailto:[log in to unmask]]
> Sent: Tuesday, September 16, 2008 3:50 PM
> To: [log in to unmask]
> Subject: [FSL] Field Map EPI correction
>
> Hello FSL usersarolina/
>
> We are trying to use FUGUE to correct the B0 inhomogeneity EPI
> distortions
> in our DTI datasets.
> We are measuring success by rigidly registering the
> non-diffusion-weighted
> (b=0) image, before and after FUGUE correction, to a standard T2 Fast
> Spin
> Echo image, to see which one looks better.
>
> For the most part, it seems to have succeeded in "stretching out" some
> of
> the distortions we were finding at the back/bottom (posterior, inferior)
> of
> the brain. However, we've noticed that there seems to be a small, but
> noticeable shear in the corrected image that appears to make some of the
> more superior slices actually LESS good than before correction.
>
> Looking at our B0map, there does seem to be a corresponding difference
> in
> the B0 field, so we are more or less confident that everything is
> working well.
> So I guess the question is why would the correction result in a poorer
> alignment?
> has anyone else run into this? Could there be a problem with our field
> map
> acquisition/calculation? Or is this merely a "nothing's perfect" sort of
> issue?
>
> Any insights would be appreciated.
>
> Thanks
> Drew
>
>
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