Hi Martin, ICV is head size. It is unlikely that anything except head growth in children changes head size. I don’t know of drugs that do that ;-).
So it is best to assume that head size is fixed for adults. Usually results should have less variance if you remove the noise from the ICV estimate, thus smaller p-values. Not sure what happens in your case, but I would not trust that. (also how large is your dataset, how far apart are the time points, and what kind of treatment will change head size??) Best, Martin > On 1. Nov 2018, at 16:13, Martin Juneja <mj70...@gmail.com> wrote: > > Hello FS experts, > > I am using longitudinal processing pipeline > (https://surfer.nmr.mgh.harvard.edu/fswiki/LongitudinalProcessing > <https://surfer.nmr.mgh.harvard.edu/fswiki/LongitudinalProcessing>) to > calculate the cortical volume (CV) over a course of treatment (two > conditions: pre condition and post condition). In fact, I am interested in > comparing normalized CV (NCV) i.e. raw CV/ ICV between pre- and > post-condition. > > (1). Now, longitudinal pipeline assumes that there is no change in ICV so it > gives me identical ICV for pre and post condition. That way, when I compared > NCV between pre and post condition, it gives me differences at significant > level of 0.16. > (2). However, when I checked ICV (from cross-sectional pipeline i.e. each > time-point separately) and compared between pre and post-conditions, ICV > values are different. Its possible that these values could be different > because of treatment (may be !). When I compared NCV between pre and post > condition (calculated by dividing raw CV from longitudinal pipeline with ICV > values form cross-sectional pipeline), it gives me significant differences at > 0.02. > > Could you please share your thoughts on this i.e. (a) whether I can use > approach 2 or (b) if I am using longitudinal pipeline, dividing raw CV from > longitudinal pipeline and dividing by ICV calculated from cross-sectional is > in correct way to do this analysis? > > Thanks. > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
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