Could it be possible that your scanning sequence, field strength, etc
has changed over the years, or maybe your scanner software has been
upgraded? The behaviour of the segmentation is likely to vary slightly
with different sequences. There will also be a bit of random variation,
so if one segmentation systematically under- or over-estimates cortical
thickness by a tenth of a voxel, then that could easily explain the 3%
increase you are seeing.
Manual segmentations are still considered to be the gold standard, so I
guess the best way to understand what is going on would be to eyeball
the images, along with the segmentation. Coregistering them and
creating a subtraction image may also be worth trying.
All the best,
-John
------- Original Message -------
Thanks John - I'm now running VBM The New Recipe.
Is it valid, after the initial segmentation, to do a quick-and-easy
analysis using the c[12]* images, or are they no good?
I'm asking this, because I get weird results doing that. Both the grey
matter and white matter maps of the same persons seem to show a greater
tissue volume in follow-up scans than in the initial scans!
>> n1_1999=nifti('c1f100911_1999.nii');
>> n1_2009=nifti('c1f100911_2009.nii');
>> n_1999=n1_1999.dat(:,:,:,:);
>> n_2009=n1_2009.dat(:,:,:,:);
>> sum(n_1999(:)).*prod(n1_1999.hdr.pixdim(2:4))
ans =
7.6070e+05
>> sum(n_2009(:)).*prod(n1_2009.hdr.pixdim(2:4))
ans =
7.8465e+05
I'm sure that can't be right, but this is what the segmentations
suggest. Or is this due to the stretching etc and do we need to wait for
DARTEL to undo those?
Many thanks
Alle Meije
--
John Ashburner <[log in to unmask]>
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