Markerless motion tracking enabling motion-compensated PET in awake rats

Motion-compensated PET of awake animals has the potential to greatly improve translational neurological investigations by enabling brain function to be studied during learning tasks and complex behaviors. Previously we have demonstrated the feasibility of performing motion-compensated brain PET on rodents, obtaining the necessary head motion data using marker-based techniques. However, markerless motion tracking would simplify animal experiments and potentially provide more accurate pose estimates over a greater range of motion. Previously we have described a markerless stereo motion tracking system and associated algorithms and validated the approach in phantoms. In this work we performed a pilot study to demonstrate motion-compensated ¹⁸F_FDG brain imaging in an awake, unrestrained rat using head pose measurements obtained from the markerless tracking system. Motion compensation clearly worked, resulting in easily identifiable structures in the head. However, it was also obvious that considerable residual error remained after correction. Post analysis of the motion estimates indicated that the residual error was the result of occasional spurious pose estimates, most likely caused by features on non-rigid parts of the head contributing to the pose estimation. Moreover, the line-of-response rebinning used for motion correction resulted in a large proportion of lost events, leading to noisy and inconsistent projection data. The latter is avoided by using a direct list mode reconstruction. In summary, markerless tracking continues to show promise for motion-compensated imaging of awake animals, but further optimization is required to match the accuracy and consistency of marker-based tracking.