I would probably correct total GM, total WM, and total ventricular CSF
by head size. It is not surprising that they covary a lot (as you point
out). But what you want is what is left over after you remove the
effect. You can also divide by the eTIV rather than regressing it out,
but the results will be similar.
As for thickness, many people regress out the mean thickness, but this
changes the hypothesis that you are testing (and so the interpretation
of the results). As such, this is a question that you will have to
answer yourself.
On 4/9/18 10:11 AM, Maria Gloria Rossetti wrote:
Dear freesurfers
my study aims to measure volumetric differences between patients X and
controls. As dependent variables, I have (i) a-priori ROIs and (ii)
global brain measures (total gm, total wm and CFS). In the ROIs
analysis, I use eTVI to control for for head size variability. Now my
question is: should I control for head size when measuring overall
brain atrophy? Theoretically speaking I'm not sure. Practically
speaking there is a high positive correlation (as expected) between
global brain measures and eTVI in my sample so I think it wouldn't be
correct to use eTVI. But still, is global atrophy affected by head
size? should I control for it?
Second quick question: I'm planning to replicate the analysis
measuring thickness differences. Should I control for tot mean
thickness then?
Thanks in advance for your help.
Gloria
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