A diffusion-weighted arterial spin labeling (DW-ASL) technique has been proposed to non-invasively measure water exchange rate (kw) across the BBB. kw was compared with GBCAs permeability (Ktrans) in aged subjects at risk of small vessel disease. A positive correlation was found between kw and Ktrans only in the caudate, suggesting different BBB mechanisms probed by kw and Ktrans. Significant increase of kw was found in subjects with diabetes or high vascular risk while no Ktrans difference was observed. Water permeability could be a sensitive biomarker to study glymphatic function and vascular diseases before detectable BBB disruption occurs.