Dear SPMers,
I'm trying to understand the Initial Import step for DARTEL a little
better, and I'm hoping someone here can help me out.
Compared to the gray matter from the segment algorithm (c1*.nii), the
imported segments (rc1*.nii) seem to be aligned slightly differently
(see attached "misalignment.png"). The DARTEL guide indicates that some
sort or rigid tissue alignment is happening in the import step, so this
isn't too surprising, though I am kind of lost as to what is being
aligned to what. Are the tissue classes rigidly aligned to some sort of
SPM template?
Referring to the imported images, the DARTEL guide also says that "the
headers of these files encode two affine transform matrices, so DARTEL
tools are still able to relate their orientations to those of the
original T1-weighted images". Am I then correct in understanding that
the flow fields map the transformation from the original space of the T1
images to the group template? To test it out, I warped a set of T1
scans before and after coregistration to the rc1*.nii images (see
"avg_anat.png" attached) using flow fields (u_rc1*.nii) from the dartel
imported GM segments (rc1*.nii). anat.nii shows the mean of the warped
original T1 images, and ranat.nii shows the mean of T1 images that had
been coregistered to the rc1*.nii images prior to warping (rc1*.nii =
ref, c1*.nii = source, T1 = other) . anat.nii looks much cleaner than
ranat.nii, implying that flow fields map from original space to group
template space.
Am I to take it then, that when creating modulated GM segments for
t-tests, I should use the original segments and NOT the segments from
DARTEL's initial import? Comparing averages of warped GM segments from
before (c1*.nii) and after (rc1*.nii) import, it looks like it's the
c1*.nii that warp together better (see attached "avggray.png"). If so,
then perhaps there is a slight mistake in the DARTEL guide, which
indicates that the rc1*.nii files (not the c1*.nii) should be selected
when running "Create Warped". Or is it that "Create Warped" and the
Deformations toolbox are not doing the same thing?
Any clarification would be much appreciated. This is using an updated
SPM8 on Matlab 2008b.
Regards,
Neil
Neil Chatterjee
Research Assistant
Stanford Systems Neuroscience and Pain Lab
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