Anemia, Sickle Cell/complications; Carotid Stenosis/diagnostic imaging; Carotid Stenosis/prevention & control; Stroke/diagnostic imaging; Stroke/prevention & control; Ultrasonography, Doppler, Transcranial/methods; Adolescent; Age Factors; Brain/blood supply; Brain/pathology; Brain/physiopathology; Brain Infarction/etiology; Brain Infarction/pathology; Brain Infarction/physiopathology; Carotid Artery, Internal/diagnostic imaging; Carotid Artery, Internal/pathology; Carotid Artery, Internal/physiopathology; Carotid Stenosis/etiology; Child; Child, Preschool; Female; Humans; Magnetic Resonance Angiography; Male; Mandible/anatomy & histology; Mass Screening/methods; Neck/anatomy & histology; Predictive Value of Tests; Prospective Studies; Sensitivity and Specificity; Stroke/etiology