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I believe to get the local volume, you would multiply the voxels X ROI size. If you want percentage of great matter within the ROI, you would divide the gray matter ROI volume by the total gray matter volume and then multiply by 100. ________________________________________ From: SPM (Statistical Parametric Mapping) <[log in to unmask]> on behalf of Sandra <[log in to unmask]> Sent: Thursday, April 7, 2016 3:26 AM To: [log in to unmask] Subject: [SPM] Average grey matter volume: multiply with ROI voxel count or ROI volume? Dear SPM experts, I’d like to perform VBM analysis on my structural data. I extracted an average grey matter volume value in a specified ROI using REX toolbox, derived from the segmented, modulated, warped, unsmoothed c1-image (smwc1…; all steps performed by low-dimensional SPM preprosessing pipeline as I rely on age-specific TPMs). I end with a value between 0 and 1 (which should be “volume” units as I rely on the modulated image). I know that I have to take into account the size of the ROI. What is not completely clear to me is: Do I have to multiply the extracted value by the voxel count (number of voxels within the ROI) or with the ROI’s volume (i.e., in mm^3) to get the final grey matter volume value for this ROI? Thanks for any help. Best, Sandra