When MR-derived motion estimates are unavailable, either in the gaps between sequences or when a given MR sequence is not amenable to motion estimation, motion can also be derived directly from the PET images themselves. Motion estimates from both the MR and the PET can be unified to provide continuous estimates of head motion throughout the duration of the scan, leveraging the higher resolution and more accurate MR-based estimates when they are available. (Levine 2017 Abstract)