Hi Matthew,
Thanks for your quick reply. I am actually extracting GM values from the smoothed GM image (sm0wrp1*).
How should I interpret this value? As a probability of GM in the mask? Is there a way to get the mean volume?
Thanks for your time.
Cheers,
Yann
> On 13 Sep 2015, at 1:37 pm, Matthew Brett <[log in to unmask]> wrote:
>
> Hi,
>
> On Sat, Sep 12, 2015 at 8:27 PM, Yann Quidé <[log in to unmask]> wrote:
>> Dear all,
>>
>> We are conducting a VBM analysis (using the VBM8 toolbox) and decided to use a ROI approach. We used Marsbar scripts to extract individual mean grey matter values for our ROIs.
>>
>> But what exactly represent this value? I initially thought it would extract the mean grey matter volume, but the numbers I get do not seem to confirm that (seems to be in a 0-1 range). Does it rather represent the grey matter density or concentration within the ROI mask? What would be the unit, % of grey matter?
>
> Are you extracting data from the output of an SPM classification? In
> that case the images will either be (at each voxel) the probability of
> the given tissue type, or that probability modulated by the Jacobian.
> So, if you are extracting from a GM volume the voxel values will be
> probabilities that that voxel is in GM.
>
> Best,
>
> Matthew
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