Hi Again,
As someone pointed out to me, I wasn't very clear on one point:
> "If you want to go further than this then, in my opinion, the head size variability generally dominates over the changes in brain size (as the latter are often only 1% or so) "
Here I did _not_ mean to say that brain size does not vary much. Instead I meant to say that the change in brain size *over and above the scaling due to head size* is small. That is, typical brain atrophy is only around 1% (normalised for head size) and so normalising for head size alone gets rid of the vast bulk of the change.
Apologies for any confusions.
All the best,
Mark
On 9 May 2013, at 23:20, Mark Jenkinson <[log in to unmask]> wrote:
> Dear Dan,
>
> This is not a straightforward question and I suspect that different people will have different opinions and that there is no unanimous "best" solution. It will depend on the biology and how big the changes are in brain size due to atrophy and how much variation in head size there is in your group.
>
> The easiest way, and what I would generally recommend, is just doing the analysis in MNI space, since that already scales the images to match the standard space brain.
>
> If you want to go further than this then, in my opinion, the head size variability generally dominates over the changes in brain size (as the latter are often only 1% or so) and the true TIV is quite difficult to calculate (as it needs to differentiate between CSF outside the brain and non-brain structures of the cranial cavity). Therefore I favour just using the more easily/robustly derived TBV or overall head-size scaling factor (as reported in SIENAX) if you are wanting an explicit measure to include as a covariate to be extra conservative. However, as I said, I suspect that people's opinions will vary over these details.
>
> All the best,
> Mark
>
>
>
> On 8 May 2013, at 10:22, Daniel Lumsden <[log in to unmask]> wrote:
>
>> Dear All
>>
>> I was hoping to get some advise regarding the best way to normalise subcortical grey matter volumes from FIRST to ensure that I'm looking at local volume relationships rather then more global whole brain changes (e.g atrophy). A convenient way to do this seems to be to use the Total brain volume (TBV) output derived from Sienax (grey matter + white matter + intraventricular CSF), though I can see many papers use total intracranial volume (TIV). I would imagine the two are pretty well correlated, so is there any advantage to using TIC over TBV?
>>
>> Many thanks
>>
>> Dan
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