This is from the help for 3dvolreg. If you use 1Dfile instead of dfile, you get just the 6 columns of motion parameters.

-dfile dname Save the motion parameters in file 'dname'.
The output is in 9 ASCII formatted columns:

n roll pitch yaw dS dL dP rmsold rmsnew

where: n = sub-brick index
roll = rotation about the I-S axis }
pitch = rotation about the R-L axis } degrees CCW
yaw = rotation about the A-P axis }
dS = displacement in the Superior direction }
dL = displacement in the Left direction } mm
dP = displacement in the Posterior direction }
rmsold = RMS difference between input brick and base brick
rmsnew = RMS difference between output brick and base brick