MRI-based assessment of acute effect of head-down tilt position on intracranial hemodynamics and hydrodynamics

Purpose: To quantify the acute effect of the head-down tilt (HDT) posture on intracranial hemodynamics and hydrodynamics. Materials and Methods: We evaluated the intracranial physiological parameters, blood flow-related parameters, and brain morphology in the HDT (-6° and -12°) and the horizontal supine (HS) positions. Seven and 15 healthy subjects were scanned for each position using 3.0 T magnetic resonance imaging system. The peak-to-peak intracranial volume change, the peak-to-peak cerebrospinal fluid (CSF) pressure gradient (PG_p-p), and the intracranial compliance index were calculated from the blood and CSF flow determined using a cine phase-contrast technique. The brain volumetry was conducted using SPM12. The measurements were compared using the Wilcoxon signed-rank test or a paired t-test. Results: No measurements changed in the -6° HDT. The PG_p-p and venous outflow of the internal jugular veins (IJVs) in the -12° HDT were significantly increased compared to the HS (P < 0.001 and P = 0.025, respectively). The cross-sectional areas of the IJVs were significantly larger (P < 0.001), and the maximum, minimum, and mean blood flow velocity of the IJVs were significantly decreased (P = 0.003, < 0.001, and = 0.001, respectively) in the -12° HDT. The mean blood flow velocities of the internal carotid arteries were decreased (P = 0.023). Neither position affected the brain volume. Conclusion: Pressure gradient and venous outflow were increased in accordance with the elevation of the intracranial pressure as an acute effect of the HDT. However, the CSF was not constantly shifted from the spinal canal to the cranium.