Joe,
Thank-you for your quick response! So if you are skeptical about
removing volumes and replacing them with an average, does that also
apply to using the volumes in question as a covariate of no interest?
Unfortunately I am largely scanning an aged population, some of whom
are cognitively impaired. These people tend to be squirmy, and to not
tolerate a bite bar well, so fixing the acquisition problem may not be
possible.
Thank-you,
Meredith
On Friday, September 24, 2004, at 10:57 AM, Joseph Devlin wrote:
> Dear Meredith,
>
> I don't have any definitive answers but I'll give these a shot...
>
> I'm pretty skeptical about removing volumes and replacing them with a
> mean
> of the temporally adjacent scans -- particularly in any event-related
> design. I suppose in an extreme case it might be ok to do it but if I
> had
> to replace more than about 1 volume in 100 then I almost certainly
> wouldn't
> use this procedure. If you're replacing that many volumes my guess
> would
> be that there is some serious acquisition problem (drop-out, scanner
> instability, bad motion, etc) and personally I'd throw the data set
> out and
> try to fix the basic problem in acquisition rather than at the
> post-processing stage. This is a fairly conservative approach but
> probably
> a safe one.
>
> I use the same approach for correlated head motion. If the head
> motion is
> significantly correlated with the stimulus paradigm, I throw the data
> set
> out as there is no way to unconfound these factors in the GLM. If the
> motion is really small (e.g. much less than the minimum voxel
> dimension)
> then you probably don't need to worry about it anyway, and in that
> case I'd
> just do realignment ("motion correction") and proceed normally. Jesper
> Andersen has a really nice tool for correcting motion x susceptibility
> interactions which can be very useful for stimulus correlated motion.
> For
> the moment it is implemented as a toolbox in SPM2 but you can use it
> for
> realignment and take the results back to FSL, if you want to use it.
>
> Hope this is some help.
>
> Joe
>
> --------------------
> Joseph T. Devlin, Ph. D.
> FMRIB Centre, Dept. of Clinical Neurology
> University of Oxford
> John Radcliffe Hospital
> Headley Way, Headington
> Oxford OX3 9DU
> Phone: 01865 222 738
> Email: [log in to unmask]
>
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