We propose to exam the water exchange rate (kw) change after caffeine uptake based on an optimized diffusion prepared arterial spin labeling (ASL) sequence and reconstruction method in a previous work1. 5 subjects went through two scans to test the reproducibility of the reconstruction pipeline. Another 5 subjects are scanned before and half an hour after caffeine uptake. Cerebral blood flow (CBF) and are reconstructed from the ASL images. A 26% decrease in CBF and 13% decrease of kw is observed.