> I acquired functional images (EPI) with an isotropic voxel size
> 3.5 x 3.5 x 3.5 and a slice gap equal to 0.5 mm. When I use MRIcro
> or SPM Display utility to display these images I get a no-isotropic
> voxel size 3.5 x 3.5 x4. My questions are:
> 1) Do MRIcro and SPM compute a new voxel size as 3.5 x 3.5 x
(3.5+0.5)?
Yes. The voxel size used is the distance between the centres of voxels.
> 2) What should I specify in Normalise-Write options-Voxel sizes in
spm5
> (or in Defaults for writing normalised in spm2)? 2 x 2 x 2 (default)
or
> 3.5 x 3.5 x 3.5 or 3.5 x 3.5 x 4?
It depends how much disk space, computer power and memory you have. The
smaller the voxels in the spatially normalized images the better
(because the discrete approximation to a continuous random field becomes
closer), but with much smaller voxels you don't really gain anything by
going any smaller. Personally, I would just go with the defaults of
2x2x2.
Note that if the voxel sizes of the spatially normalized images are much
bigger than those of the original data, then some of the original data
may not be used. Smoothing should first be done in such cases. To
understand this a bit more, type "help decimate" in MATLAB.
> 3) What should I specify in Normalise-Write options-Bounding box in
spm5
> (or in Defaults for writing normalised in spm2)? Are default values
ok?
These defaults should be ok.
Best regards,
-John
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