Hello,
> I conducted a study that contains 3 independent runs. But what
> frustrated me were 6 of 16 of my participants’ head motion between the
> runs after realign appeared > 3mm (they rested for about 1 min between
> each run), but within each runs, the result is fine.
That should not frustrate you too much, as it is both not so bad and
taken into account. Motion during sessions is much worse than motion
between sessions.
> So, I was wondering whether I can pre-process my data separately as 3
> independent runs then converge data together when doing modeling?
No, you should not, as you would then not have the functionals in the
same space (each session would remain being shifted from the previous
run). For exactly this reason, spm brings all runs into one space (the
first image of the first session, usually) when doing realign. This
allows you to normalize all images usng the same set of normalization
parameters later-on, for example.
> Is there any particular reason that we should pre-process our data in
> one session even if we have several runs?
You should not provide it as one session but as several sessions of the
same subjects. SPM will then know what to do. You could, conceivably,
realign them independently from each other and then coregister them all
to the same target, but due to the above, this is not necessary (and
likely not more exact either, although I haven't explored this
systematically).
Hope this helps,
Marko
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PD Dr. med. Marko Wilke
Facharzt für Kinder- und Jugendmedizin
Leiter, Experimentelle Pädiatrische Neurobildgebung
Universitäts-Kinderklinik
Abt. III (Neuropädiatrie)
Marko Wilke, MD, PhD
Pediatrician
Head, Experimental Pediatric Neuroimaging
University Children's Hospital
Dept. III (Pediatric Neurology)
Hoppe-Seyler-Str. 1
D - 72076 Tübingen, Germany
Tel. +49 7071 29-83416
Fax +49 7071 29-5473
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http://www.medizin.uni-tuebingen.de/kinder/epn
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