Aim (1). By separately magnetically labeling blood water in different feeding arteries (8), the investigators will use a tested vessel-selective arterial spin labeling (VS-ASL) approach and a calculated perfusion asymmetry index to noninvasively assess changes in collateral CBF patterns in patients with IC stenosis. IC stenosis (n=90) patients will be monitored for two years during which their known stroke incidence is 20%. Stroke incidence will be recorded and separate correlations between perfusion asymmetry and stroke risk will be assessed.