hi spm
we have been experimenting with the normalization tool for fMRI in situations
where there are a reduced number of slices available. when we
attempt to normalize 4 realigned slices with 7mm voxels on the z
axis, the output images invariably result rotated by several degrees,
and the modelled activations, although roughly in the expected area,
seem to be out of place. This occurs despite the fact that we have
used "header edit" and "check reg" to ensure that the origin and
image dimensions are equivalent (by visual inspection) to those of the EPI.img
template. The error message: "warning: field of view is too small to
allow linear registration" (or something similar) appears during the
normalization process.
Other than taking more slices with better resolution, is there a way
to correct this error? We have experimented using a bounding box
appropriate to the acquired slices, but the images remain rotated
within the confines of the bounding box. We also cut the internet SPM
example images (the fMRI auditory paradigm) to
roughly like ours (14 slices x 2mm voxels, including AC-PC slice) and tried to normalize
these, but we get the same error message, and the same rotation of
the output images.
We would like to know what
is the minimum number of slices necessary to acheive adequate
normalization with SPM, with the AC-PC origin included for
appropriate registration with the template image. Is there a
reasonable minimum? Or is it that SPM should theoretically be able
to normalize any number of slices, and we are simply doing something
wrong? We have looked at past correspondence on this list, but have
not found any references to this problem.
Thanks in advance for any help.
Chris Summerfield
dept psiquiatria i psicobiologia clinica
universitat de barcelona
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