Dear Marko and Max,
Just to add some "confusion". ;-)
I clearly remember Karl stating that there is nothing like "too much
movement" (repeated at several SPM courses). Still movement will affect
your statistical results, as the signal is affected by all the
preprocessing steps. You can include the the movement parameters as
regressors in your GLM to account for soem movement related variability.
A key criteria would rather be how fast did the subject move, i.e. the
time derivative of the movement parameters. Large movement over a single
(or couple of) scan(s) is likely more harmful than a slow regular drift
over a few hundred scans leading to displacement of several voxel sizes...
Scans that are clearly affected by movement artefact, e.g. a hiccup, can
always be modeled with separate regressors in the design matrix:
accounting for the weird signal in that/those scans while preserving the
continuity of the time series. (BTW this would resemble the derivative
of the movement parameters.)
Lastly if the movement is correlated with the task itself, you're a bit
more in trouble... See the discussion about using "realign & unwarp"
without including the movement parameters in the GLM.
Best,
Chris
Le 6/10/2010 8:32, Marko Wilke a écrit :
> Hello,
>
> this question keeps on coming up on the list but there does not seem
> to be a definitive answer as yet. A rule of thumb is "motion exceeding
> voxel size", but that ignores that in-plane is likely different from
> through-plane motion, and that motion correlated with the task is
> worse. John addressed the question in a more general way at
> https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind03&L=SPM&P=R511591&I=-3&d=No+Match%3BMatch%3BMatches
>
> Cheers,
> Marko
>
> Max Gunther wrote:
>> Dear SPMers,
>>
>>
>> We have been debaiting what is a good threshold for movement is (i.e.
>> how much is too much that we should throw the scan out).
>>
>> We have a series of functional scans in an elderly clinical population
>> using 2 x 2 x 3mm EPI with a 2 second TR on a Philips Achieva.
>>
>> Is there any consensus in the literature regarding how much movement is
>> too much?
>>
>> Many thanks,
>> Max
>>
>>
>> Max Gunther, PhD
>>
>> Vanderbilt University Medical Center
>> Departments of Psychiatry and Medicine
>> Center for Health Services Research
>> Nashville, TN www.ICUdelirium.org <http://www.ICUdelirium.org>
>>
>
|