> P = spm_get(Inf,'*.img');
> V = spm_vol(P);
> gl = zeros(length(V),1);
> for i=1:length(gl),
> gl(i) = spm_global(V(i));
> end;
> gl
>
> I am trying to caluculate the ratio of the white matter ROI value (by
> MRIcro) to global CBF (by above formula) using H2O-PET data. Sometimes,
> global CBF values are a little less than ROI values of cerebral white
> matter (eg, 28.94 vs 30.24). I think this is unreasonable and global CBF
> values are a little small. I would like to know why this phenomenon can
> occur and how I have to do to obtain correct values.
I'm afraid that there is nothing sophisticated about the way that SPM
computes "globals":
% spm_global returns the mean counts integrated over all the
% slices from the volume
%
% The mean is estimated after discounting voxels outside the object
% using a criteria of greater than > (global mean)/8
This measure is dependent on many things, such as the field of view of the
images (or bounding box for spatially normalised data).
There are a number of alternative approaches that could be used for
computing a "global", but the best ones would involve using a smoothed,
segmented coregistered MR image, and solving a few linear equations
in order to estimate the average intensities for various different tissue
classes. See e.g.
Aston JAD, Cunningham VJ, Asselin M-C, Hammers A, Evans AC, Gunn RN.
Positron emission tomography partial volume correction: estimation and
algorithms. J Cereb Blood Flow Metab 2002; 22: 1019-1034.
Best regards,
-John
--
Dr John Ashburner.
Functional Imaging Lab., 12 Queen Square, London WC1N 3BG, UK.
tel: +44 (0)20 78337491 or +44 (0)20 78373611 x4381
fax: +44 (0)20 78131420 http://www.fil.ion.ucl.ac.uk/~john
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