Another look at the Portal Vein residual (possible) movement after the registration.  Part of portal vein was manually drawn on 6 slices of the initial timepoints.  Corresponding TAC (VIF) was generated.  If there would be a substantial movement out of plane for the PV, we would have observed  sharp iregularities in this TAC.  However, beside one bump in the last quater the curve is very smooth which indicates that PV majorly remains in the same position.