Transcranial Doppler ultrasound (TCD) and transcranial color duplex imaging (TCDI) studies measure blood flow velocities and direction within portions of the intracranial arteries. TCD and TCDI evaluate the anterior and posterior circulation territories to assess and manage patients with cerebrovascular disease. Testing is often performed to detect and monitor vasospasm after subarachnoid hemorrhage and screen children with sickle cell disease for stroke risk. Additionally, these exams can identify intracranial stenosis and occlusion. TCD can detect intracranial emboli and assess vasomotor reactivity.