Influence of orientation, size and shape of the region of interest in diffusion MRI along perivascular spaces index

Abstract Objective Diffusion-tensor imaging (DTI) and diffusion-weighted imaging (DWI) along perivascular spaces (ALPS) index have been proposed as noninvasive techniques to indirectly evaluate the glymphatic system function. However, these techniques are sensitive to examination parameters, limiting inter-study comparability. The definition of the region of interest (ROI) has been identified as the primary weakness of the ALPS method. Therefore, we aimed to determine which ROI characteristics would best promote consistent ALPS index analysis. Methods We examined 13 healthy volunteers using DTI and DWI to calculate the ALPS index, and compared and determined correlations among 11 different ROI configurations, and tested inter-method reliability. Results We found significant differences between different ROI configurations in the ALPS index calculation. Considering ROI characteristics and inter-method reliability, a squared ROI is the most suitable. The ICC between ROI configurations showed good-to-excellent inter-method agreement (mean ICC = 0.83). We did not find significant inter-method differences. Conclusion It is important to standardize the ROI characteristics for consistent ALPS index calculation.