How does one calculate the displacement from the 6 parameters?
>Torben Ellegaard Lund wrote:
>
>> In Karl's MRM paper on residual movement artefacts: Magn Reson Med.
>> 1996 Mar;35(3):346-55. Movement-related effects in fMRI time-series.
>>
>>http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=8699946&query_hl=1
>>
>> they use an expansion of the movement parameters. The expansion
>> includes in addition to the 6 rigid body movement parameters for a
>> volume acquired at time t: R(t) it also the movement parameters from
>> the previous volume R(t-1) and these terms squared: R(t).^2 and
>> R(t-1).^2, this is 24 extra parameters in your design matrix. In this
>> way you can model both linear and quadratic term of both the movement,
>> but also of the velocity of the movement as well as history effects.
>
>A combination of these methods would probably be optimal: compute the
>displacements D for each voxel, and use D(t), D(t-1), D(t).^2, and
>D(t-1).^2. That would model linear and quadratic displacement terms, as
>well as velocity and history effects. Computing the displacements as
>scalars instead of using the 6 parameters, does not introduce 5
>`virtual' degrees of freedom for each of those terms, which makes the
>analysis more powerful.
>
>Best wishes,
>Alle Meije
>
>
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