As far a I know, thickness does not vary with head size, so we don't 
traditionally make a correction


On 04/10/2018 01:52 PM, Maria Gloria Rossetti wrote:
> 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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