Hi Gabriel,
It is not the case that the hippocampal subfield segmentation routines simply subdivide the aseg hippocampus segmentation - instead, it only uses the aseg results for initialization purposes, and computes everything else directly from the original MR data from that point onwards. Therefore, it's perfectly possible find differences between the two estimates.
My recommendation would be to look at the segmentations visually rather than simply relying on the quantitative tables. Do the subfield segmentations look reasonable?
I should add the we have a recent paper (http://nmr.mgh.harvard.edu/~koen/IglesiasMICCAI2012.pdf) showing that the global hippocampal volume computed by adding the subfield volumes yields better discrimination between AD patients and controls than the standard FreeSurfer aseg volumes.
Hope this helps,
Koen
On Tue, Jul 10, 2012 at 9:36 AM, Gabriel Gonzalez Escamilla ggonesc@upo.es wrote:
Dear Fresurfer experts and Koen Van Leemput
I have made the hippocampal subfield segmentation of a group a subject in which is supposed to take the hippocampus from the aseg segmentation and calculate the new subfields segmentations inside this hippocampus of each hemisphere, am I right? If so, The volumes between the aseg and the summed subfields are supposed to be nearly the same, I have checked the volume outputs from both the aseg and the subfield segmentations, but even when some of them seem to change subtely, some others seem to be quite different, more than I did expect.
Here are some examples: summ-segmentaseg --> Volume in 1mm^3 2696.63932660 --> this difference is the lowest I'd get ~37 2116.854 2592 --> aseg volume greater than summed volumes from subfields ~475 4002.35354764 --> aseg volume greater than summed volumes from subfields ~762 2110.32351914 --> summed volumes from subfields greater than the aseg vol ~196
I'm just wondering if this is normal, or if it could be some kind of trouble when making the subfield segmentations of my subjects? can any one exaplain to me why the differences can be so random between my subjects?
Many thanks in advanced, Gabriel.
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