Dear SPMers,
while trying to calculate the size/volume change due to affine registration in the segmentation
approach of SPM5 I have noticed that the calculation of the volume change in spm_write_sn.m
always results in a value of 1. I have checked this using the formula:
detAff = det(prm.VF.mat*prm.Affine/prm.VG(1).mat)
and indeed this always returns 1. However the normalisation function (outside segmentation) is
providing the right value and the effect seems to be restricted to the normalisation used in the
segmentation approach. This will affect the calculation of the modulated images. Only local
volume changes due to non-linear normalisation will be applied and the affine parts will be
omitted (which makes sense for most cases, because usually people are interested in analyzing
local volumes which are corrected for whole brain volume). This is in contrast to the SPM2
approach, where the affine (as constant value) and the non-linear volume changes were included
in the modulation step.
If this is correct (and intended), the statistical analysis of modulated images should be always
applied without total or GM volume as nuisance parameter (which makes life easier). Is this
correct?
Best regards,
Christian
____________________________________________________________________________
Christian Gaser, Ph.D.
Assistant Professor of Computational Neuroscience
Department of Psychiatry
Friedrich-Schiller-University of Jena
Philosophenweg 3, D-07743 Jena, Germany
Tel: ++49-3641-935805 Fax: ++49-3641-935280
e-mail: [log in to unmask]
http://dbm.neuro.uni-jena.de
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