CVR is hypothesized to be impaired in SCD patients and could account for the white matter hyperintensities seen on MRI in these patients. Cerebral blood flow (CBF) alone is not sufficient to provide insight into the hemodynamic status of the brain. CVR allows us to gain insight into the capacity of the cerebrovasculature to respond to increased CBF. Vasculopathy is common in SCD and the caliber of the vessels can be assessed with 4D Flow MRI in response to actue and chronic changes in the velocity and wall shear stress on the intracranial arteries in the Circle of Willis.