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Dear Gelareh, The units of volume and both voxels (first number) and mm^3 (second number). The best way to calculate the volume is to multiple the mean value by the volume (get the output of -M and multiply by the second output of -V) which will then give you the most accurate estimate of GM volume in mm^3, as it takes into account all of the partial volume contributions. The result from just the -V output is an overestimate as it counts the entirety of any voxel that contains any non-zero amount of GM partial volume. All the best, Mark On 19 Apr 2012, at 07:21, Gelareh Ahmadi wrote: > Dear FSL experts, > > I ran VBM for my images. During its process all brain-extracted images are segmented into GM, WM and CSF. So the image *_struc_GM.nii.gz is the GM images. How can I calculate the volume of the gray matter by this image? > > Is it correct to use fslstats? is it correct to write below script: > fslstats -t *_struc_GM.nii.gz -V >> gary_matter_volume.txt > > And what is the unit of this volume?? > > If I use -M, instead of -V then it will give me the mean of the gray matter not the whole volume, is it correct? > > I would highly appreciate if you could help me with this regard, > > Thank you in advance for your kind assistance, > > Best Regards, > > Gelareh >