Hello, I used the default longitudinal stream to calculate ventricular volumes over time. When I examined the difference score (longitudinal-baseline) I noticed that for some participants ventricular volumes *decreased*. The same site, scanner and scanning protocol was used for both baseline and longitudinal scans. I processed the cross, template and longitudinal time points on the same computer using the same Freesurfer version (v 5.1). Furthermore, this decrease in ventricular volume is evident before and after manual corrects were made to the aseg file. Is it common to see an *decrease *in ventricular volume over time? Can this be due to Freesurfer variability or slight differences in the scanner over the two time points?
Thank you in advance for your help. Best, Tamara
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