Spatial normalisation is not exact - especially as only about 1000 parameters
are used to define the warps. Therefore the warps have to be smooth, and if
a structure is reduced in size, then all its neighbouring structures are also
reduced.
Ventricles tend to be rather large in elderly subjects, and there is a large
contrast between CSF and WM. The mean sum of squares cost function used by
SPM spatial normalisation can therefore be greatly reduced by making the
ventricles smaller in these subjects. The end result is that neighbouring
structures are also reduced in volume.
The differences you see are likely to be due to ventricle size differences
rather than volumetric differences in grey matter structures.
This problem can be reduced by spatially normalising the images using a
procedure similar to that used by Tina Good in her recent NeuroImage
paper:
"A Voxel-Based Morphometric Study of Ageing in 465 Normal Adult
Human Brains". Catriona D. Good, Ingrid S. Johnsrude, John
Ashburner, Richard N. A. Henson, Karl J. Friston, and Richard S. J.
Frackowiak. NeuroImage 14, 21 36 (2001)
Best regards,
-John
On Tuesday 16 October 2001 21:30, Christopher Summerfield wrote:
> hi John Ashburner & spm
>
> a question about the warping routines in normalization. In a VBM study, I
> have some "activations" which pop up in the "central" grey matter areas eg
> hippocampus, thalamus, cingulate, etc. the patient sample are an elderly
> group whose scans were normalized to the standard SPM T1 template.
>
> is it possible that warping in the normalization process could have
> "pulled" the tissue out from central brain areas, reducing differences in
> cortical tissue and inflating differences in subcortical/limbic areas?
> how reliable are these results, given that I have normalised to the T1? I
> have tried normalizing to an AD template but normalization repeatedly
> fails producing a variety of oddly-shaped brains.
>
> are these activations likely to be unreliable?
> thanks
>
> chris summerfield
> dept. psychology
> columbia university
--
Dr John Ashburner.
Wellcome Department of Cognitive Neurology.
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
mail: [log in to unmask]
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