Thanks for getting back to me. 
My hp is the same as for the volumetric analysis. I want to measure thickness 
differences between patients and controls while controlling for major 
confounders (head size included). However , since thickness doesn't scale with 
eTIV as much as volume how can I control for head size variability? What would 
you suggest me to do? I am not that familiar with thickness analysis and I 
would not add unnecessary noise. 

Best, 
Gloria 

(n.b. in my sample, eTIV is significantly different between groups) Best, 



Da: "Douglas Greve" <dgr...@mgh.harvard.edu> 
A: "freesurfer" <freesurfer@nmr.mgh.harvard.edu> 
Inviato: Martedì, 10 aprile 2018 16:24:42 
Oggetto: Re: [Freesurfer] how to control for head size differences when 
measuring global atrophy? 



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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