Perhaps this various with software. I usually use SPM for preprocessing,
and you have the option to motion correct to a (session-wise) mean image
or single volume (the first, or whichever).
Volume order within a session definitely matters: I had a dataset in
which the volumes were entered in the wrong order (due to a numerical
vs. alphabetical file naming mixup) and the motion correction was
extremely abnormal and poor. Once the file naming was fixed the motion
correction looked fine.
Jo
On 7/12/2011 4:02 PM, Yaroslav Halchenko wrote:
On Tue, 12 Jul 2011, Mike E. Klein wrote:
before being run a second time on the concatenated file. While there is
very little head motion within each session, there looks to be
considerably more between sessions, which probably comes as no
surprise. A test of running motion correction a single time (after
concatenation) looks like it does not perform very well: there is still
a large amount of motion visible to the naked eye.
ha -- that is interesting to me: afaik (correct me if I am wrong) motion
correction is just a re-alignment/slicing to a reference 3D volume extracted
(or computed, e.g. mean of representative volumes), i.e. no temporal
order between volumes should matter... and now you are getting worse results
if you concatenate them all into a single 4D series (which is what I usually
do, to eliminate double-smoothing/error others pointed out) :
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