Assessing cerebral microvascular volumetric with high-resolution 4D cerebral blood volume MRI at 7 T

Arterial pulsation is crucial for promoting neurofluid circulation. Most previous studies quantified pulsatility via blood velocity-based indices in large arteries. Here we propose an innovative method to quantify the microvascular volumetric pulsatility index (mvPI) across cortical layers and white matter (WM) using high-resolution four-dimensional (4D) vascular space occupancy (VASO) and arterial spin labeling (ASL) magnetic resonance imaging (MRI) at 7 T with simultaneous pulse recording. We assessed aging-related changes in mvPI in 11 young (28.4 ± 5.8 years) and 12 older (60.2 ± 6.8 years) participants and compared mvPI with large artery pulsatility assessed by 4D-flow MRI. mvPI peaked in the pial surface (0.18 ± 0.04). Deep WM mvPI was significantly higher in older participants (P = 0.006) than young ones. Deep WM mvPI correlated with large artery velocity PI (r = 0.56, P = 0.0099). We performed test-retest scans, non-parametric reliability test and simulations to demonstrate the reproducibility and accuracy of the method. In conclusion, our non-invasive method enables in vivo fine-grained measurement of mvPI, with implications for glymphatic function, aging and neurodegenerative diseases.