Towards Lissajous navigator-based motion correction for MR-PET

Introduction With the combination of Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) into a single combined system, a novel imaging modality has become available. Exciting new study designs investigating different aspects of the exact same biochemical process are finally possible thanks to simultaneous MRI and PET acquisition. Due to long acquisition times of up to hours, motion poses a significant problem for PET images. Previously this problem has been addressed in PET by using external devices which track the motion of a marker attached to the subject’s head [1]. The recorded data are incorporated into the PET reconstruction process to correct the PET data for motion [2]. Due to the magnetic field and the confined space in the MR-PET scanner, the use of an external tracking device is not easily feasible. However, the MRI scanner is an ideal tool for delivering motion data with minimal overhead during combined MR-PET measurements. Three-dimensional navigators – short MR acquisitions optimised for rigid body motion detection – are a promising tool for motion detection and estimation in MRI [3-5]. A novel navigator trajectory, the Lissajous navigator, which delivers motion parameters with six degrees of freedom, has been developed. By acquiring Lissajous navigators during a simultaneous MRI and PET measurement, motion data are available which can be utilized to motion-correct the PET data.