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Katell, Bonjour. I have looked into and performed similar analyses to make similar conclusions. But what I also found was that by segmenting the brains, thresholding, combining and then summing to calculate TIV some voxels are included twice. The segmentation process gives probabilities so one voxel can with be 50% gray and 50% white and therefore be included twice with a threshold <0.5. What I did find was that if the voxels are restricted to only being classified as a single tissue type the thresholding makes much less difference. What I have done is to classify voxels based on their maximal probability. Did you do this? All the best, Jason. ----- Original Message ---- From: mevel <[log in to unmask]> To: [log in to unmask] Sent: Tuesday, June 5, 2007 5:43:13 AM Subject: [SPM] Total intracranial volume: for information Hi ! As expected in our previous post we have performed some tests to assess the effect of different threshold methods and values on total intracranial volume estimation from segmented and modulated MRI data sets. We have used either different thresholds (from 0 = no threshold, to 0.9) applied on each of the brain partition, or differently thresholded wholebrain masks (again using thresholds from 0 to 0.9). We thus calculated the total intracranial volume of 19 healthy controls and 28 slightly atrophied patients in these 9*9 conditions, and assessed their inter-correlations. As expected, all values were significantly correlated. Highest correlations (r>0.97) were observed for thresholds from 0 to 0.5 (where TIV values decrease from 0 to 35%, respectively). Thresholds above this value lead to decreased correlations (0.95 < r < 0.70) and more than 49% decrease of the maximum TIV value. We thus conclude from this preliminary analysis that the use of <0.5 threshold values would lead to more consistent TIV estimation, and this threshold could be indifferently applied to either each brain partition or the whole brain image. Best regards, Katell ****************************************** Katell MEVEL Doctorante email : [log in to unmask] Tél : +33 (0)2 31 47 02 32 Neuropsychologie Cognitive et Neuroanatomie Fonctionnelle de la Mémoire Humaine Equipe INSERM EMI0218 - EPHE- Université de Caen Basse normandie Laboratoire de Cycéron Blvd Becquerel BP 5229 14074 Caen cedex 5 Tél : +33 (0)2 31 47 02 32 Fax : +33 (0)2 31 47 02 75 ************************************************