volumenormalize reverse mapping

[Marie Smith]

Dear Jan-Mathijs + fellow fieldtrippers,

Sorry for my delayed response to your mail. In the end the
volumenormalize (with nonlinear = 'no') works fine with a newer
version of fieldtrip, and I am able to output the normalised (and non-
normalised) functional data to AFNI.  Now that I have this working,
and can identify group maxima, i would like to be able to transform
back into the CTF voxel space.

I know the final transformation applied to the data in
volumenormalize (norm.cfg.final T2). I had thought that to move from
the new space (vox2) to the original (vox1) would would be simple and
that I would just have to invert the final transformation matrix as

vox1 = inv (T2) * vox2

but this does not seem to be correct.  So I am wondering where I am
going wrong. Does the norm.cfg.final give the full transformation
from ctf to spm space? including flipping any axes, scaling etc.

Alternatively, I tried to work out the relationship by using the
affine transformation matrix of the original mri in CTF space
(MRI_T), and the new transformation matrix of the normalised mri
(norm.transform, NORM_T) and some other transformation to go from SPM
to CTF co-ords (CTF2SPM) as:

NORM_T * vox2 = mm (SPM) = CTF2SPM (mm (CTF)) = CTF2SPM (MRI_T * vox1)
i.e. vox1 = inv(MRI_T)*inv(CTF2SPM)*NORM_T*vox2

but again, i think there is a problem somewhere. I am getting the
CTF2SPM transformation from the one used inside volumenormalise for
this purpose, but perhaps it is incorrect.

I would appreciate any suggestions,

Thanks,

Marie

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.