Using a principle axis system allows automatic evaluation of glymphatic flow with ALPS in imaging space and corrects for head and fibre orientation

Motivation: Reducing the dependence of ALPS on manual region selection, head and fibre orientation. Goal(s): Robust and automated ROI placement and ALPS-index calculation on acquired data. Approach: Development of a workflow for automatic ROI placement in DWI space, validated with in vivo measurements under varied head rotations. ALPS-index values using the principal axis system (PAS) and the laboratory frame were compared across MNI and DWI spaces. Results: Both methods yielded ROIs in anatomically meaningful places and corrected for head rotation. In addition, ALPS-PAS accounted for fibre orientation, did not require data registration and yielded higher indices. Impact: Accurate, automated placement of ROIs for ALPS-PAS yields a stronger ALPS effect and reproducible ROIs without registration of the acquired data, ideal for longitudinal studies.