Dear Helmut,
One of the reasons I used the term "task" is I wanted to avoid the confounding between session and run, but it seems that it's not a better way to describe.
Your reply is very helpful to me, I'm not confused with session/runs now.
Thanks for your reply.
Best,
Chih-Hao
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寄件者: MRI More <[log in to unmask]>
寄件日期: 2016年4月21日 下午 07:12
收件者: [log in to unmask]; SUBSCRIBE SPM Chih-Hao
主旨: Re: The limitation of correcting head motion
Dear Chih-Hao,
I was just wondering as you had stated you had acquired two "tasks". Usually there's more than one "task" anyway, e.g. a simple and a complex task/condition, the task/condition "of interest" and a task/control condition, ... Leaving this aside, you could also acquire two completely unrelated "experiments" with a single EPI sequence (without stopping the scanner in between). With regard to session/runs, I tend to use "session" in case subjects have to arrive several times at the scanner (e.g. on separate days), and "run" for different sequences acquired during one day/appointment. Anyway. It's not properly defined, is it.
In your case (scanner stopped in between, no matter whether it's different "runs" or "sessions"), one would usually go with separate "sessions" during slice timing or realignment indeed. For slice timing, it's pretty obvious, you don't want the last volume of session n to be affected by data from the first volume of session n+1 and vice versa. For realignment, the first volume of one session should usually be more representative for the other volumes of that session, thus going with two separate sessions (within one realignment module) seems to be reasonable. There's still some discrepancy then when realigning the first volumes from the different sessions together, but this way, images of the second session should be misregistered to the same extent (plus of course the misregistration from within the sessions). If you go with a single session, aligning everything on the first volume of the first scanner session, then the volumes of the second scanner session might be misregistered onto the first one to a different extent - however, this way there's obviously no "misregistration within scanner sessions". I would assume the separate-session approach to be better in principle, but this is basically guessing.
Best
Helmut
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