Hi - the intensity normalisation isn't quite the last step in the
preprocessing - the last step is the highpass temporal filtering - so
that may change the grand mean very very slightly - so that's probably
the cause of what you're seeing. Nothing to worry about, as they say.
Cheers.
On 11 Sep 2009, at 18:30, Dan Wolf wrote:
> Hello,
> I have noticed that there are sometimes systematic differences
> between different BOLD
> runs (same subject) in the average intensity value of the mean_func
> image (averaged
> spatially across the entire brain), and would like to understand
> why. I thought the average
> intensity value was "grand mean scaled" by FSL software to 10,000;
> the values are near
> 10,000, but why are they not exactly 10,000? The grand mean scaling
> is only for non-
> background voxels, but I assume mean_func only includes non-
> background voxels? Does the
> standard deviation of the signal in the raw data affect the
> calculation of the scaled values?
> Thanks,
> Dan Wolf
>
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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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