Hi,
The point of the "grand" in "grand mean" is that the entire 4D dataset
is scaled by a single scaling factor so that the relative scalings of
all volumes wrt each other are preserved.
In FSL preprocessing this is done so that all non-background voxels
end up with a mean of 10000 when calculated over all volumes.
Cheers.
On 30 Jan 2008, at 17:12, Christopher Bell wrote:
> FSLers,
>
> I am still slightly confused on what "grand mean scaling" does. My
> understanding was that each volume of an fmri 4D file would be
> scaled by a
> constant factor of 10000, so that each individual volume's mean of
> non-zero
> voxels would be 10000. This does not appear to be the case. Also,
> the mean
> of the entire 4D file is not 10,000. The values are always close
> though. Is
> an individual "grand mean" for each 4D file calculated? Thanks!
>
> Chris Bell
>
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
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