List-mode motion tracking for positron emission tomography imaging using low-activity fiducial markers

Motion correction in positron emission tomography (PET) imaging may benefit from an accurate knowledge of the patient motion during the image acquisition. We evaluated the feasibility to detect and record the motion of external fiducial markers during PET scans. We developed an iterative algorithm that can track the three-dimensional (3D) motion of low-activity fiducial markers while surrounded by high physiological tracer activity from a patient undergoing PET imaging. Monte Carlo techniques were used to simulate a 92.5-kBq ²²Na marker moving sinusoidally in 3D. The simulated events were combined with list-mode data from patients undergoing cardiac PET imaging in order to test the algorithm. In experimental studies, three external ²²Na markers were placed on a dynamic torso phantom with an initial activity of approximately 680 MBq of ⁸²Rb in its cardiac insert. We tracked the motion of those markers while simulating breathing motion and patient drift with the phantom. Results from simulations show that a 92.5-kBq marker can be tracked in 3D at a frequency of 2 Hz with an accuracy of 1.2 mm and a precision of 0.8 mm. The phantom study qualitatively confirms that the algorithm can track both breathing and patient motion. The relative accuracy of the tracking was 0.4±1.1 mm and the precision was 0.8 mm. In conclusion, we have developed an algorithm that can track the 3D motion of low-activity positron-emitting markers during PET imaging. This motion information might prove useful in developing new motion correction schemes in clinical PET imaging.