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Hi Wei, Paul,
 
on a side note: this is due to warming of the gradients and B1 fields that are increasingly off, and hence a slight continuous drift in real space. We see this mainly in fast scans and high acquisition bandwidth scans, eg more energy and hence warming of the gradients.
 
No need to worry when it comes to realigment, SPM usually does a good job correcting this drift. you might have some more interpolation noise at the edges of the brain, but this is usually negligible in comparison to physiological noise, real movement noise, inter subject variability when doing 2nd level stats, etc.
 
Cheers,
 
Bas
 
 
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Van: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] Namens Paul Smeets
Verzonden: maandag 4 januari 2010 11:19
Aan: [log in to unmask]
Onderwerp: Re: [SPM] Abnormal Realignment Result??

Dear Wei,

I cannot open your .fig 's but this sounds familiar. Does it look like the attached file (a 12-min run)?
This 'movement' is due to a drift effect. There is an option to prevent this on Philips scanners, I think it is called 'fMRI echo stabilization', this may have been switched off in your case. It inserts a very short (~10 ms) measurement of the resonance frequency f0 in your sequence and adjusts accordingly as things warm up (with a lag of a few dynamics).
In any case, your registration is correct; the head does move, although the subject doesn't ...

Best,

Paul

Huang, Wei (Psychiatry) wrote:
<pre wrap>Dear All,

I've done realignment (estimate and reslice) on 246 EPIs (taken in one
continuous scan), and the result showed a total 7mm continuous movement
in the y direction (see attached 1st.fig), which led me doubt the
quality of images and/or realignment.  I went ahead to do another
realignment based on the already realigned 246 scans and got the result
of very small movement in all directions (see attached 2nd.fig), which
made me believe the 1st realignment job was fine.  However, I have never
got such big continuous translation in one direction.  Is this set of
images reliable?  Is there another way to check if anything is wrong
with my images?

Thanks,
Wei
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Paul A.M. Smeets, PhD
Image Sciences Institute
University Medical Center Utrecht
Heidelberglaan 100, Rm Q0S.459
3584 CX Utrecht, The Netherlands
Tel: +31 88 75 53172
Fax: +31 30 251 3399

Division of Human Nutrition
Agrotechnology and Food Sciences Group
Wageningen University and Research Centre
Tel: +31 3174 80759 (usually on Fridays)

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