# Frame displacement

Dear AFNI experts,

I am trying to calculate the frame displacement (FD) according to the formula given in Power et al. 2012. The goal is to compare FD values between two groups. However, I have some doubts regarding how I can implement this with the output I have access to (the data is already analyzed). If I understand correctly, frame displacement represents the absolute value of displacement changes in the x, y, z directions as well as rotational changes (roll, pitch and yaw–once converted to mm) between adjacent volumes.

I only have access to the motion files (2 runs) and the .1D matrix from 3dVolreg. The base for registration was the 10th volume of the 2nd run. With the output that I have, can I calculate an FD value per participant? If I understand correctly, the motion parameters in the motion file represent the displacement between the volume and the base volume used for registration, which are not adjacent.

Sincerely,

Isabelle

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