Dear All,
interesting discussion evolving here :)
> It seems to me that either: affinely registering the original repeat
> scans and averaging them; or averaging the (s)mwc images, should be
> valid*
I still believe that averaging before segmentation will decrease your
resolution as the accuracy of the fit (see below) will be lower than the
original resolution of the data.
> I wonder if it might be better to keep
> all n*k scans in an anova model?
Sounds more intuitive to me.
> 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.
Then the question is, does the SNR improvement outweigh the extra
variance. In this case, which does without averaging, I would tend to
agree with you.
> Statisticians: please shout me down as appropriate!
Yes, please do :)
> *(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!)
True, but spm has never tried to be really good at matching scans in the
first place but rather chose to cover up the inevitable inaccuracies by
smoothing the hell out of the data. I think it is an approach that has
worked well.
Andreas:
> It can be a useful strategy if you have a patient population that has a hard time keeping still, becaues you can discard individual volumes that are messed up.
I see that you had six image volumes per subject, and I believe that
averaging those will increase SNR, but does that not result in even
visible blurring of structures? Plus, if you discard some volumes in
some subjects, does that not systematically bias your analyses w.r.t.
more signal to noise in subjects lying still longer? This is true anyway
but if you have more scans in some subjects I would be hesitant to use 6
scans in one subject and 1 scan in the next.
It's interesting to see how different opinions can be, and I would love
to hear that somebody has compared different approaches and can tell us
"the truth" :)
Best,
Marko
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Marko Wilke (Dr.med./M.D.)
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Universitäts-Kinderklinik University Children's Hospital
Abt. III (Neuropädiatrie) Dept. III (Pediatric neurology)
Hoppe-Seyler-Str. 1, D - 72076 Tübingen
Tel.: (+49) 07071 29-83416 Fax: (+49) 07071 29-5473
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