Hello,
I am back to comparing transformation matrices from mcflirt and air and I have a question...
oh, well...
from some previous postings, I understand that fsl uses different coordinate system for
"voxel" and for "mm". voxels are described with respect to image storage and the origin, or
(0,0,0) voxel is the first (corner) voxel in the dataset. The mm coordinates are voxel
coordinates multiplied by voxel dimensions. Also, transformation matrices are always given
with respect to mm coordinates. But, the origin of mm coordinates (at least with regards to
transformation matrices output by mcflirt) is not set to (0,0,0) voxel, but to the "center of
mass" of the image.
So, in other words, if I wanted to find a position of a voxel after transformation I would have
to do the following:
1. transform old voxel coordinates into mm coordinates: old_mm=T*old_vox;
2. transform old mm coordinates into new mm coordinates: new_mm=XFM*old_mm
where XFM is the transformation matrix from the .mat file. But what is T? Is it possible to find
out what is the center of mass that mcflirt uses for each transformation?
I believe that is the piece that I am missing in order to compare air and fsl transformations,
because it seems that the only difference in the matrices is that air transformation matrices
are always with respect to the first voxel in the dataset.
Thanks!
zrinka
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