I'm afraid that distortion correcting the images does not eliminate the
dropout problem. If no signal is collected from a region, then there is
no way of correcting the images in order to recover it. New scanning
protocols are being developed that may help in the future, but these are
not being widely used yet.
One freely available distortion correction algorithm that I know about is FUGUE,
which uses a field map, and is freely available from:
http://www.fmrib.ox.ac.uk/fsl/index.html
I've had a quick look at your images, and the EPI volume looks a bit squashed
in the slice-select direction. In computing the voxel sizes, have you taken
any inter-slice gap into consideration?
Movement-by-distortion interractions may sometimes be a problem. Jesper's EPI
Unwarping Toolbox may help though:
http://www.fil.ion.ucl.ac.uk/spm/toolbox/unwarp.html
Best regards,
-John
> Our data presents EPI distortion in frontal lobe, which includes a priori
> hypothesis of our study:
> http://www.directnet.com.br/users/griselda/EPIdistortion.html
>
> We are seeking for advice as whether the amount of distortion should cause
> a loss of signal, and be a problem to detect effects of interest in SPM
> statistical analysis.
>
> As pointed in
> http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0110&L=spm&P=R4662, SPM will
> not correct this distortion. Is there any free utility (field map or
> registration approach) that can be used to correct our data?:
>
> TR=2000 ms
> TE=40 ms
> matrix = 64 x 64
> BOLD-sensitive GE 1.5 T sequence with RTIP
> 15 interleaved 7mm/0.7 space slices from cerebellum to vertex
> axial AC-PC alignment (usually slice 6 or 7)
> 230 volumes per series were acquired
--
Dr John Ashburner.
Functional Imaging Lab., 12 Queen Square, London WC1N 3BG, UK.
tel: +44 (0)20 78337491 or +44 (0)20 78373611 x4381
fax: +44 (0)20 78131420 http://www.fil.ion.ucl.ac.uk/~john
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