Hi,
just my two pennies (is that the right expression?)
to the discussion:
To my experience
(I tried out different designs with or without MPs modelled for years:
in this case the simple 6 parameter model - translation/rotation)
it is not very easy to compare methods
if one does not take into account the amount of motion in total
and (very likely to be of importance:) the amount of motion
correlated to the paradigm or event regressors.
If there is nearly no motion, entering the Mps will conly cost
degrees of freedom. On the other hand, if there is moderate
motion, entering the 6 MPs (or the more complex models)
will explain a lot of variance.
However, the 6 parameter MP model has its limitaitons:
With high T-field scanners or excessive (instead of moderate) motion
as in the case of children or patients
excess motion cannot be modelled satifactorily.
At least my results were abysmally bad in these cases and I
excluded the subjects in question - which is a darn pity.
Excessive motion means (in a 3T field): movement of 0.5 mm
between two consecutive scans (twiching) - this seems to
be bad enough for statistics.
I missed in Brett's abstract a mention of the degree of motion
in the data used for his comparison.
I would therefore like to hear of people who have experience with
this (excessive motion or moderate motion and high field scanners).
Maybe the mode complex models of motion are doing
much better than the simple one in these special case.
Best,
Anja
Dr. Anja Ischebeck
Innsbruck Medical University
Clinical Department of Neurology
Anichstrasse 35
A-6020 Innsbruck - Austria
tel.: +43 (0) 512 504 23661
>>> Matthew Brett <[log in to unmask]> 05.10.2005 17:45 >>>
Hi,
> I don't think it is the speed of the movement that increases the
effect
> of artefacts, it really is the displacement. The artefact is due to
the
> spin excitation history of the voxel, i.e. energy transmitted to a
brain
> region before thatt region was at its current voxel location (maybe
you
> should ask an MR physicist instead of reading this...).
Just a question - my impression was that the current thinking is that
spin-history is a rather minor factor in the motion-related variance.
Is that true? Certainly there can be quite large effects from motion
by distortion interactions - this is stuff Chloe Hutton and Jesper
Andersson have worked on.
I did a very tiny study of including movement parameters up to the 24
regressor spin-history model (which will include the effects modeled
by the difference of the parameters) and found, like Tom Johnstone,
that only the movement parameters themselves seemed to be robustly
useful:
http://www.mrc-cbu.cam.ac.uk/~matthew/abstracts/Moves/moves.html
The link points to my HBM2005 abstract.
Best,
Matthew
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