Hi,
I have a question about how one should normalise EPI fMRI datasets.
There seems to be (at least) three possible approaches, and I wonder if
there is a consensus on which is best (or least worst....)
1) Take a set of high-resolution T1 weighted slices in the same plane
and with the same FOV as the EPI slices. Normalise this to the T1
template and then apply the normalisation parameters to each EPI volume.
2) Take a high-resolution multi-shot EPI image, again with exactly the
same parameters as the single -shot functional EPIs, normalise this to
the EPI Template and then apply the normalisation parameters to the
single-shot functional EPIs
3) Simply normalise the single-shot functional EPI (usually the mean
image from the realignment) to the EPI template.
In spm99b, option (3) appears to work really well, so long as the
spm_defaults are modified so that the BrainOnly option is disabled, and
it has the advantage of not requiring any other acquistions. It also has
the advantage that the spatial distortions inherent in single shot GE
EPI are corrected for. However, there is an assumption here that those
distortions *should* be corrected. Surely there are some artefacts on
such images which are dropouts and so that particular region should not
be pulled out to match the edge of the template.
We have done a quick comparison of (1) and (3) on two of our group
studies. On one of our studies we obtained more significant activations
in the final statistical analysis if we used option (3) (i.e.
normalising the single shot EPIs directly) compared to (1). This was
initially a surprise as we naively expected only the location of the
clusters to change. We then came up with the idea that probably option
(3) was bringing the group data into better register with each other -
hence the better statistics.
Of course, it had to happen, in the other study we looked at, Option (1)
gave better final statistical results than Option (3).
Any thoughts? I'd be really interested to know what other groups are
doing, and whether anybody has done a more rigorous comparison of the
various methods.
Thanks in advance,
krish
--
Dr Krish Singh, ([log in to unmask])
Magnetic Resonance and Image Analysis Research Centre, Liverpool
University
Pembroke Place, Liverpool, L69 3BX, UK. Tel 0151 7945645. Fax 0151 794
5635
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