Transcranial Doppler ultrasound (TCD) is a non-invasive technique that utilizes the weak part of the skull as the acoustic window and applies the principle of the Doppler effect to study the vascular condition, and is mostly used for the examination of cerebrovascular diseases in the clinic. Transcranial Doppler ultrasound is generally used to assess whether stenosis or occlusion of intracranial and extracranial arteries has occurred, and is suitable for the assessment of intracranial vascular conditions in patients at high risk of stroke, such as patients with diabetes mellitus, hypertension, coronary artery disease, and hyperlipidemia. Transcranial Doppler ultrasound can also be used for microembolism monitoring, evaluation of right-to-left shunting, evaluation of cerebral vascular diastolic responsiveness, assessment of changes in ambulatory blood pressure and dynamic regulation of cerebral blood flow, detection of vasospasm due to subarachnoid hemorrhage, and assistance in the diagnosis of brain death. Transcranial Doppler ultrasound is suitable for people at high risk of cerebrovascular lesions and is mainly used to assess the general condition of cerebral blood vessels. The test is non-invasive and patients do not need to be overly nervous.