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 F-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.