Investigation of 4D PET attenuation correction using ultra-short echo time MR

In hybrid imaging systems where positron emission tomography (PET) is combined with magnetic resonance (MR) imaging, attenuation maps can be derived from MR to correct for attenuation in PET. However, MR-based attenuation correction (AC) remains challenging especially for tissue types showing poor signal in common MR sequences, such as bone. To overcome this problem, Ultra-short Echo Time (UTE) sequences have been proposed. Another challenge is given by respiratory motion which results in misalignments between the derived attenuation maps and the PET emissions. In this paper we address both issues. We compute a 4D attenuation map including bone from MR by combining a respiratory gated UTE acquisition with a subject-specific motion model derived from a second short dynamic acquisition. We demonstrate our approach on an MR-derived time-averaged PET simulation from three healthy volunteers, including three artificial lesions. The impact of bone on AC is simulated by excluding bone from the derived 4D map leading to errors of up to 26.0%. The impact of respiratory motion is simulated by using a non-moving 3D map, showing an error of up to 24.2%. These results indicate that a consideration of both bone tissue and respiratory motion is crucial to ensure accurate MR-based PET AC.