Hello,
there was a very good and 'practical' study on the effect of customized
templates, optimized VBM (i. e. normalisation of gm to GM template instead of
using wholehead images) and on the effect of modulation: Keller et al,
NeuroImage 2004. It may be worth looking at this systematic comparison.
In brief, modulation seems essential whereas the creation of a customized
template and priors did not have a large effect. Isn't it rather recommended
to use own priors and templates in study populations with brains very
different from the MNI template as subjects with (gross) atrophy or pediatric
populations?
From the point of view of the mentioned paper it might be worth trying a more
simple approach, i. e. use GM in native space (segmented using standard
priors) for normalisation onto the standard GM template, apply it on
wholebrain, segmentate using the standard priors and modulate the GM results.
It would be very interesting to see if also in your study this way is the
most sensitive method to reveal hippocampal volume differences.
Have you compared this 'standard' way of optimized vbm to your approach which
used two modulation steps?
The reason to produce customized priors is to achieve improved agreement of
the variable cortices - to my understanding the shape of the GM partitions is
the essential information which should be translated into the prior rather
than than the volume information.
Best regards,
Philipp G. Saemann
Max-Planck-Institute of Psychiatry
NMR Study Group
Kraepelinstr 2-10
80804 Munich
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