Secondary exploratory aims of the proposal are to determine:the correlation between large vessel flow measured by QMRA and tissue level perfusion measured by MR perfusion in the posterior circulation, and the predictive value of each; other predictive factors for stroke in this population; hemodynamic effects of varying degrees of vertebrobasilar stenosis; changes in hemodynamic status of patients on medical therapy over time; utility of QMRA as a non-invasive screening and monitoring tool in VBD.