Background: A head-down (HD) position is used in some stroke centers to maintain cerebral perfusion (CP) in stroke patients. Purpose: To assess CP in healthy volunteers in the supine and HD (-15 ) positions. Study Type: Prospective. Population: Eighteen healthy subjects of 53 (AE8) years old. Field Strength/Sequence: 1.5T / arterial spin-labeling (ASL) in the supine position and after 4 minutes of HD position. Assessment: Regions of interest from reconstructed cerebral blood-flow (CBF) maps: subcortical nuclear gray matter (accumbens, amygdala, caudate, hippocampus, pallidum, putamen, thalamus), cortical gray matter (cGM), and white matter (WM). We also monitored hemodynamic parameters. Statistical Tests: Shapiro-Wilk test, analysis of variance (ANOVA) tests, Student's t-tests, and Pearson correlation analysis. Results: CBF was higher in women compared to men, whatever the position (mean difference of 17% in supine, and 13% in HD position). From supine to HD position, CBF was decreased in all regions (mean decrease of -7%). Simultaneously, mean arterial pressure and systolic blood pressure increased (respectively P = 0.004 and P < 0.001). Data Conclusion: The CBF decrease, despite increased hemodynamic parameters, may indicate efficient cerebral autoregulation. Our results seem to reflect only early cerebral autoregulation stages but may open the way towards a more precise understanding of CP. Level of Evidence: 1 Technical Efficacy: Stage 2

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Regional CBF assessment, especially in HDT, is challenging with most perfusion techniques because of ionizing radiations, inherent limitations and logistical considerations. This preliminary report presents a noninvasive technique assessing regional and absolute cerebral blood flow changes in response to posture change. It can lead to further clinical investigations for a better understanding of cerebral perfusion.