 |
 |
I am processing data from an fMRI paradigm acquired from a 1.5T scanner and have some questions related to motion parameters. In the case of some subjects movement is fairly low (within 1 mm and 1 degree in all directions and rotations) except for a few quick, jerky movements. I have read this can be problematic for Realign to correct so I would like to address this potential problem. In talking to someone they suggest a method described by Tom Nichols involving adding the 6 motion parameters as coregressors and a 7th column that includes 0s and 1s for each volume. I have been unable to find documentation on this method and I'm not sure to which volumes should a 1 or 0 be attributed (my guess would be 0s for the bad volumes and 1s for the good ones). Any help in directing me to a description of this method or elucidatig the finer points would be greatly appreciated. I have another more theoretical question about adjusting for motion parameters. We currently use a threshold of 2-3 mm and degrees of movement and rotation as the point at which we discard a scan due to excessive movement. With a study we do involving kids we use a more relaxed threshold (~4 mm and degrees) since children tend to have greater trouble remaining still. Since coming across the idea of using motion parameters as co-regressors I have begun to wonder if it's logical to broaden the acceptable motion range if one includes MPs as regressors. I understand including MPs can reduce true activation signal if that signal is correlated with motion as well. Is there a ceiling at which there is simply too much motion that signal cannot be interpreted legitimately even after Realign and covarying for the MPs? If one generally does not include MPs does it make sense to include them in the case of particular subjects with high motion to better account for signal that may be motion induced and examine all these subjects together at the group level? Thanks in advance, Patrick