Hello Sarvy, basically, TIV is the sum of total GM, total WM and total CSF (with CSF containing the internal and external CSF spaces). You can calculate the 'total' of an individuals GM, WM or CSF from the sum of its segmentated maps in native space, or the sum of its modulated maps in normalized space - these results should be equivalent (by definition). So, simple in general, but one significant problem arising: The CSF maps a considerable amounts of voxels that are misclassified as CSF and that lie outside the intrancranial vault (mostly parts of the skull). We (here, locally) use a 'tiv' mask to simply mask these voxels out of the CSF maps (and the GM and WM maps, too) and by this make the total CSF value more reliable. Such a 'tiv' mask can be generated in the following way (just one solution, of course): you add up all (modulated or non-modulated, does not make a big difference here) normalised GM, WM and CSF images to a preliminary TIV image (of your study sample, as many subjects as possible), and then find a threshold that gives you a nice margin that just follows the inside of the skull (e. g. use the rcolin brain for comparison) but includes the external CSF spaces and the basal cisterns. Alternatively a sume of the segmentation priors can be used to create such a mask. So, assuming your mask is labelled e. g. 'tiv_mask.nii' (i4), your tiv image of a subject can be calculated using the imcalc function: tiv_image = (i1+i2+i3).*i4 (with i1, i2 and i3 being modulated GM, WM and CSF, respectively, and i4 being the mask). The total of this tiv_image would be your TIV. Also, see the link http://dbm.neuro.uni-jena.de/vbm/vbm2-for-spm2/calculate-raw-volumes/ to scripts for this purpose (although I could not tell you how Christian Gaser's script handled the problem of the rather unreliable margins of the CSF maps), best wishes, Philipp Max Planck Institute of Psychiatry NMR Research Group Kraepelinstr. 2-10 80804 Munich Mail: [log in to unmask] Phone: 0049-89-30622-413