Dear Bianca,
> I heard that it is best to avoid using interleaved EPI sequences. Another
> recommendation was to use one long session (no matter how long) instead of
> breaking up the experiment into runs of <10 mins.
>
> Is this information correct and what are the reasons for these
> recommendations?
I suspect that there is a range of opinions on both of these topics,
so it may not be possible to clearly state the "correct" thing to do.
However, here are some of the reasons you may have heard this advice
(which is reasonable):
1) Because adjacent slices in an interleaved acquisition are not
collected nearby in time, this type of sequence may be more affected
by subject movement. This is particularly true if slice timing
correction is used (see
http://imaging.mrc-cbu.cam.ac.uk/imaging/SliceTiming).
2) From a purely data-centric approach, having fewer longer sessions
(compared to more, shorter sessions) is better because every time a
new scanning session is started, global signal levels differ. This
means that sessions need to be modeled separately (the default in
SPM); that is, within the same model, but using different columns for
regressors, and including columns for session effects. But, this also
means that less data is available to estimate each parameter estimate.
Having a single long session would get around these issues, and
practically be slightly easier to set up (fewer columns in the design
matrix). However, long sessions may also result in subjects being
distracted or asleep, which would probably negate any of these
potential gains! In addition, if there are any technical problems,
breaking an experiment into multiple sessions often makes it easier to
restart the experiment. So, there are tradeoffs both ways.
Hope this helps! I would be interested to hear from others,
particularly on preferred EPI acquisition schemes! :)
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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