Dear Helmut,
> From: [log in to unmask]
> To: [log in to unmask]; [log in to unmask]
> Subject: Re: The limitation of correcting head motion
> Date: Wed, 20 Apr 2016 14:21:11 +0000
>
> Dear Chih-Hao,
>
> Assuming you refer to two separate fMRI sessions acquired with one EPI sequence each, and with subjects moving during the break between the two sessions:
I'm a little confused about session and run in SPM, but it seems that both of session and run are called as session in SPM?
Do you mean that I acquired two sessions from one EPI sequence?
Sorry, I didn't describe my question exactly.
I got these two session from two different EPI sequence, but for some reasons, I input two sessions into one session in preprocessing.
But I usually input each session as a different session under proprocessing & modeling (ex. realign).
I don't think treating two separate session as one session is better way to analysis, but I don't know what the difference is.
According SPM8 manual, SPM align the first image from each session to the first image of the fitst session, then other images within each session are aligned to the first image of the session.
Dose it mean that all image from all session are aligned to the first image of the first session?
If I don't misunderstand, what's different with treating all session as one session in proprocessing?
> MRI images are no exact copies of the objects/subjects under investigation, instead the geometry is just reconstructed. Distortions in magnetic field strength will lead to geometric distortions in the reconstructed image. Different positions inside the scanner will result in somewhat different geometric distortions. However, the usual realignment procedure assumes rigid-body transformations to be sufficient to realign one EPI image onto another. If EPI images differ to a large extent one would need non-rigid transformations to map one image or session onto another. So basically you will have to check your data whether the output from the combined preprocessing looks reasonable or not (I guess it will). It's not just motion between the sessions, other parameters might have been changed as well (e.g. exact orientation of the field of view, encoding directions, possibly leading to systematic differences with regard to geometric distortions, signal drop).
>
> Due to the rigid-body transformations there's another practical limitation, the realignment procedure can't correct for motion during the acquisition of a particular volume, and it can't correct for corresponding signal artefacts associated with "fast motion" like that.
>
> Best
>
> Helmut
>
I'll check the output.
Thank you for your detailed reply, it's helpful to me.
Best
Chih-Hao
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