Hi Bhavani,
It depends on the question that you want to ask. I believe that many
people when doing AD research will use intracranial volume as a
covariate. That basically controls for head size when the subject was
young, before cortical atrophy. However, what if the atrophy is global,
and you want to know whether the atrophy in a given local region is
above and beyond the degree of global atrophy? For that, you would then
use brain volume as the covariate. Frankly, the two approaches provide
different information, and I think that the results of both approaches
should be reported in AD research. In your case, the absence of
differences between AD and control when using brain volume as the
covariate suggests that the temporal and parietal cortex atrophy is not
sufficiently greater than global atrophy to be statistically
significant.
cheers,
-Mike H.
On Fri, 2010-06-04 at 19:44 +0530, Bhavani shankara wrote:
> Dear iM experts,
>
> I am performing the VBM analysis (using VBM5 toolbox) in my subjects
> with Alzheimer's dementia (AD) comparing with controls at p<0.05 FDR
> corrected , While i am using intracranial volume as one of the
> covariate in the statistical design i am getting the results as i
> previous reports of VBM (in temporal and parietal cortex atrophy) .
> But when i used the Total Brain volume (GM volume + WM volume) . I am
> not getting any difference between AD and control.
>
> Please let me know which one to use Total brain volume or Intra
> cranial volume (ICV) as a covariate? And Why?
>
> When we use total brain volume are we removing what ever real atrophy
> difference in my subjects?
>
> Thanks for your response
>
> --
> Dr BHAVANI SHANKARA. B
> MBBS, (PhD in Clinical Neurosciences, NIMHANS) Bangalore,
> INDIA-560029
> email: [log in to unmask]
> [log in to unmask]
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