Hi all,
It seems to me that either: affinely registering the original repeat
scans and averaging them; or averaging the (s)mwc images, should be
valid*. But in both cases averaging throws away the information about
the re-scan variability, so (with the necessary disclaimer that I am
definitely not a statistician) I wonder if it might be better to keep
all n*k scans in an anova model?
This is like having longitudinal data, except that because the time
between scans is minutes rather than months the main effect of time
will not be interesting. It seems to me that the main effect of group
in such an analysis would be a valid effect to test for with these
repeat scans included. The contribution of the repeats will be a
combination of some SNR improvements where they are well-aligned, some
extra variance where they aren't, and the extra degrees of freedom
from having more scans.
Statisticians: please shout me down as appropriate!
Best,
Ged.
*(Marko, responding to your reply, don't you think that if the
intra-subject registration can't be relied upon then the idea of VBM
itself is questionable, since it relies on acceptable inter-subject
registration -- something much harder to achieve!)
Ahmed _ wrote:
> Dear SMPers
>
> An extending to this question.
>
> What if I have 'k' T1 images per subject (total 'n' subjects). And instead of averaging per subject right in the begining, I treat them to be coming from n*k subjects. Run VBM. And then calculate subject average Gray/White/CSF images.
>
> Thanks in advance.
>
> Regards
>
> +ahm
>
>
>
>> ----- Original Message -----
>>
>> Hi SPMers,
>>
>> I have two sets of T1 images per subject and was wondering if a simple
>> average ((i1+i2)/2) before input to the SPM VBM workflow is valid at
>> all and if it is would it be expected to increase the overall strength
>> of the results ?
>>
>> Thanks
>> Mithun
>>
> --
> Ahmed,
> Graduate Student
>
> E204, CoIL, AI Lab
> Department of Computer and Information Sciences
> University of Hyderabad
> P.O. Central University, Gachibowli, Hyderabad
> Andhra Pradesh 500046, INDIA
>
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