Dear Afni Experts,

What is the minimal amount of motion that 3dvolreg can detect and output in the motion params? I have a spherical water phantom that obviously cannot move much, but I 3dvolreg-ed it anyway and when I looked at the motion params it finds rotations and translations in each that are much, much smaller than the voxel size. For example, movement of .01 degrees in the yaw, or .005 mm in z, when my voxel size is 2.5 mm isotropic. I read through the Cox and Jesmanowicz 1999 MRM paper, and the README.registration file, both of which state that movement should be in the neighborhood of 1-2 mm or degrees to be detectable. Does 3dvolreg use a resampling of the grid to find these small motion values?

thanks in advance!

michael