Hello Pierre,
I have no obvious answer but in order to see if the change in the
defaults is to blame, why don't you try different separations at the
coregistration step? I was about to say that a separation that is too
close to the voxel size may lead to funny effects but I recall a mail
from John where he foresaw that issue and introduced a little jittering
for that parameter. Still, I would try different settings and see if a
pattern evolves.
Another point to consider is that the voxel size may not be the only
thing that differentiates the two mpr's. Inhomogeneity could potentially
be an issue, or incomplete coverage, but I guess you looked into that.
Hope this helps,
Marko
Pierre Gravel wrote:
> Dear SPMers
>
>
>
> I have a question about coregistration.
>
>
>
> I can successfully coregister mean EPI images (4 x 4 x 4 voxel size) with their associated MPR images (1 x 1 x 1 voxel size) when I use the estimation options:
>
> realignment (estimate& reslice): separation of 4 mm, smoothing of 5 mm
>
> Coregister (estimate): separation of [4 2] mm.
>
>
>
> On the other hand I get a good alignment only 50% of the time when coregister new mean EPI images (3 x 3 x 3 voxel size) and MPR images (0.94 x 0.94 x 1 voxel size) with the following estimation options:
>
> realignment (estimate& reslice): separation of 3 mm, smoothing of 5 mm
>
> Coregister (estimate): separation of [3 1.5] mm.
>
>
>
> I tried this with both SPM5 and SPM8 with the same results.
>
>
>
> Why is coregistration worse when I use smaller voxels and adjust the estimation options accordingly? Did I forget something totally obvious?
>
>
>
> Thanks in advance
>
>
>
> Pierre
>
--
=====================================================================
Marko Wilke (Dr.med./M.D.)
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Universitäts-Kinderklinik University Children's Hospital
Abt. III (Neuropädiatrie) Dept. III (Pediatric neurology)
Hoppe-Seyler-Str. 1, D - 72076 Tübingen
Tel.: (+49) 07071 29-83416 Fax: (+49) 07071 29-5473
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