Transcranial Doppler ultrasonography (TCD) can identify children with sickle cell disease at risk for stroke and in need of blood transfusion. Data from the Stroke Prevention Trial in Sickle Cell Anemia (STOP) shows that persistently increased TCD velocities in the distal internal carotid and/or middle cerebral arteries indicates a high risk for first time stroke. TCD detection of abnormal time averaged maximum mean flow velocities in two consecutive exams, followed by blood transfusion resulted in 90% risk reduction for first time stroke. Transcranial color duplex imaging (TCDI) can also be used to perform these exams, but variations in velocities have been reported when compared to TCD studies.