Hi Isabelle,
I would not expect FD to be based on alignment between
successive pairs of volumes, but to be on the same
alignment to a single base volume.
You can verify these things, but...
From what I recall, FD is the sum of of the absolute
differences between the motion parameters at successive
time points (we do not compute this, because it seems
very strange to use this over the Euclidean distance).
But it should be easy to compute this using the motion
parameter files output by 3dvolreg (via either -dfile
or -1Dfile).
Between 2 time points, just add up the absolute values
of the differences between the six terms. There may
be a preferred scalar to apply to the rotation terms,
so maybe you can look that up.
- rick