the differences is mainly due to the new Partial Volume Estimation (PVE). With this PVE model two additional mixed classes are estimated (GM-WM and GM-CSF). The final results is estimated for the 3 main tissue classes (GM/WM/CSF) and is now expressed as fraction (or amount) of each pure tissue that is present in this voxel. The effect is mostly visible in the basal ganglia, which are a mix of GM/WM. The final GM segmentation is around 50% because these voxels also contain 50% WM. Although this value is lower compared to non PVE approaches this is a more accurate segmentation.
For your data there are probably more voxels containing mixed classes. Additionally, older segmentations with SPM5/VBM5 showed a tendency to binarize the segmentations. Furthermore, the old approaches were based on a bayesian approach with tissue priors of control subjects that often not reflect the analyzed sample. The new segmentation algorithm in VBM8 does not use any priors, thus for subjects that deviate from healthy control subjects large differences in the segmentations might occur. However, your values for GM/WM volume are quite different and I would rather check the raw volumes that are saved in a text file.
Christian Gaser, Ph.D.
Assistant Professor of Computational Neuroscience
Department of Psychiatry
Friedrich-Schiller-University of Jena
Jahnstrasse 3, D-07743 Jena, Germany
Tel: ++49-3641-934752 Fax: ++49-3641-934755
e-mail: [log in to unmask]
On Fri, 28 May 2010 01:52:43 +0800, lion gao <[log in to unmask]> wrote:
>Dear Christian Gaser,
>Thanks for your great work on VBM8. It make segmentation easier. Previously
>VBM5 or SPM5, SPM8 may fail to segment some T1 images (about 10%, e.g. gets
>strange C2. images), Now VBM8 seems work well and never fail to segment so
>One problem I find is that when I compare the images segmented by VBM5 and
>VBM8, they looks quite different. and indeed their volumes differ a lot.
>Mainly, the grey matter volume in VBM8 is smaller than VBM5, whereas the
>white matter volume is bigger. the TIV changes little. I segmented several
>subjects from different groups. e.g. from young, old group and patient
>group, this trend is similar among the groups. please note that I use the
>default setting in both VBM5 and VBM8, which are normalized modulated
>Attached pls find a figure for your reference. I wonder is there any reason
>for this. and which result may be better. BTW, another thing is that I find
>the density in white matter from VBM8 can be 2.0778, which those in VBM5 are
>all less than 1.0. So what's the meaning of 2.0778.
>Would you please help me to understand these issues. Sorry that if you have
>demonstrated these points in VBM8 manual. I have not read it thoroughly.