Hi Rick,

If I look at the time series of one of those runs without any preprocessing, when I pick one voxel within the brain sometimes it looks relatively fine (without spikes). But, if I pick a voxel near to de edge of the the signal it looks very similar to that motion parameter (see image as an example). I also attached a gif showing a video of the time series in which you will notice an abrupt and fast movement on axial view.

As you can see in the example there are some spikes and there is noticeable movement/displacement when we jump between these time points. Then, for some reason at the last abrupt movement the signal stays in the higher displacement without returning to the original position.

This situation happened in a few subjects in different experiment, but acquired with the same MRI machine.

Is it possible that the cause could be an issue in the MRI machine? or could it be an issue at the moment of estimating the motion parameters?
I would like to clarify that this occasionally happen, not always.

Thanks a lot,
Karel