PET Motion Tracking with Radioactive Fiducial Markers

Anesthetics used to keep small animals motionless during PET studies are a major confounding factor in the uptake and retention of radiotracers, particularly in the brain. This study proposes the use of radioactive fiducial markers to track and recover motion using only data from the PET scanner allowing anaesthetized imaging. Three small Na-22 fiducials (1mm, 10uCi) were attached to the outside of a micro hot-rod cylinder. To measure the accuracy with which fiducial markers can be tracked in the presence of a radioactive background the cylinder was filled with 1mCi F-18 and translated through the field of view at a known speed. For further validation the cylinder was filled with 0.3mCi F-18 and waved in a random motion path for 10 min. To process the listmode data is first gated into short time frame (100ms) images using simple ray casting. Each time frame image was registered (Mutual Information) to a reference frame to recover motion transforms. The listmode data is reprocessed transforming each event and binning into a single motion corrected sinogram which is reconstructed using standard programs. In the controlled motion study the fiducial markers were tracked with high precision (mean abs. error = 0.34mm). The motion recovered images were artifact free, showed good image quality, and a minimal loss of resolution. These studies suggest that motion tracking with radioactive fiducials is possible, and is suitable to enable anaesthetized brain imaging in a rat.