Hi Gaby,
I'm not sure if there is a "best" echo offset given the variations in
scanner and field inhomogeneities. Obviously you don't want too
much phase wrapping which means a small difference, but you
want a large signal, hence a big difference. Somewhere in there
is a compromise which will work the best. Ideally, however, you
could take a range of times and fit the best curve. I'd like to
generalise fugue to allow for this, but it isn't a high priority right
now.
As for underestimating the distortion, there might be some problems
associated with point spread functions when the voxels are large,
leading to blurring of the complex signal and bad phase estimation.
To check the acquisition and analysis, I would use a phantom of
precisely
known geometric shape in with something to give some B0 inhomogeneity,
then take images with different offsets and different phase encode
directions
(especially the sign - that is, top to bottom vs bottom to top). If
there is some
systematic underestimate it should show up different when the region is
compressed or stretch, as the resultant "unwarped" image would still
have
residual compression or stretch. However, signal loss would always just
leave a dark patch. Comparing the fieldmap values at different echo
offsets
should also give you a feeling for the precision and repeatability of
the
estimate and might show a problem at short/long differences.
I know that is all pretty common sense, but hopefully it will be of
some help.
Best of luck with it Gaby, and let me know if you still can't get things
working well.
Cheers,
Mark
On Wednesday, September 3, 2003, at 06:19 am, Gaby Pell wrote:
> Dear FSL-gurus (but especially Mark!),
>
>
>
> Do you have a feeling for what is the best aysmmetric echo offset
> gives the best field map for distortion creation? I am using 10ms and
> am just about to try collecting a wider range. The corrections I am
> getting with FUGUE at the moment look reasonable but there is still a
> fair bit of distortion that is not corrected for (and this isn’t
> signal loss). Is there anything else I could be doing wrong with the
> analysis. Can you suggest any tests that would help me to localise
> this problem either to the acquisition or the analysis?
>
>
>
> Thanks,
>
>
>
> Gaby
>
>
>
> P.S. I have tried different levels of regularisation
>
>
>
> ***************************************
> Gaby Pell, PhD
> Brain Research Institute
> Ground Floor,NeurosciencesBuilding
> Austin & Repatriation Medical Centre
> Banksia Street
> West Heidelberg,
> Melbourne,
> Victoria, 3081
> Australia
> Tel: (+61 3) 9496-2868 (W)
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>
>
>
>
>
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