Dear Ramy and Antoine,
I think we should be a little careful with "hard and fast" rules for "too much motion". We can all agree that less motion is better, but I would hesitate to advice someone to throw data out based on a rule like this.
One thing is that different types of movement result in different residual effects after motion correction. A (close to) pure translation is easier to correct than a rotation around the x- or y- axis as the latter will cause changes to the field that will
change the pattern of distortions and drop-out, something that isn't dealt with in most registration algorithms.
It is also the case that a slow gradual movement is easier to correct than a sudden one (though in the latter case one can often fix it through a stick-regressor in the subsequent modelling).
Finally there is the important distinction between movement which are unrelated to the task and that which is caused by/correlated with the task, where the latter is a much bigger problem than the former.
So, Antoine, where I am concerned there is no easy rule and you will have to use your best judgement in each study.
Jesper
wrote:
Hi Antoine,
Generally, the rule of thumb is that you don't want the head motion to exceed half a voxel size. So if you selected voxel sizes of 1mmx1mmx1mm, motion should not be greater than .5mm.
Cheers,
Ramy