TCD is the abbreviation of transcranial ultrasound Doppler, and there are three examination sites in TCD, including temporal region, occipital region and orbital window. TCD is mainly used to reflect the general condition of intracranial blood vessels based on blood flow velocity and the pulsatility and resistance indices of blood vessels. TCD can also be used to detect vascular spasm, especially in cases of subarachnoid hemorrhage, and to assess the presence of vascular spasm through increased blood flow velocity and changes in vascular parameters. TCD can also be used to monitor cerebral arteries for the presence of microemboli.