Hello,
In fact in FreeSurfer's wiki they say:
"The method employed in Freesurfer is taken from a procedure described by Buckner et al., 2004. Please see this manuscript for the details of this procedure:
A unified approach for morphometric and functional data analysis in young, old, and demented adults using automated atlas-based head size normalization: reliability and validation against manual measurement of total intracranial volume Buckner et al. (2004) NeuroImage 23:724-738.
Basically, total intracranial volume is found to correlate with the determinant of the transform matrix used to align an image with an atlas. The work demonstrates that a one-parameter scaling factor provides a reasonable TIV estimation."
http://surfer.nmr.mgh.harvard.edu/fswiki/eTIV
So I understand that it's only an estimation. As a result it's not very surprising that the measurement of SPM by means get_totals provides more accurate values.
cheers
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