Transcranial Doppler mainly utilizes ultrasound through the skull to check the general condition of intracranial blood flow as follows. 1. To check the speed of blood flow, to check the blood flow in the internal carotid artery, vertebral artery and ophthalmic artery, and to detect lesions in the relevant blood vessels, which can be used to determine whether there is atherosclerosis, atheromatous plaque formation, stenosis or occlusion of blood vessels. 2. For the determination of vascular malformations, whether the patient has arteriovenous malformations and arteriovenous fistulas. 3. For assessing whether the patient has subclavian artery steal and whether it is due to atherosclerosis resulting in high resistance type vascular spectrum changes. 4. To monitor microemboli and provide a diagnostic basis for the etiology of cerebrovascular disease. In summary, regarding the main role of transcranial Doppler, in addition to neurology, patients in cardiology, endocrinology, brain surgery, ophthalmology, general surgery, orthopedics, obstetrics and gynecology, otolaryngology, and other departments may use the test. It is recommended to prescribe the test report and interpret the results under the guidance of a professional physician.