Carotid endarterectomy has been proven effective in prevention of cerebral vascular events in patients with severe carotid artery stenosis (symptomatic,>50%;asymptomatic,60%). But during the surgery, when the carotid artery is clipping, the patient will have the chance to have hypoperfusion or stroke. Our study is designed to determine how to use TCD to reduce the risk of death and nonfatal stroke in patients .

AHA guidelines recommend carotid endarterectomy (CEA) for severe carotid stenosis. But during CEA, there is little chance that the patient suffer from severe hypoperfusion or stroke. So we use TCD to monitor the blood flow of brain in the surgery and keep the blood pressure stable. The sudy is designed to find out whether the index which we found in previous work is more sensitive to show the blood flow of middle cerebral artery.

Inclusion Criteria:

• Carotid artery stenosis detectable by duplex ultrasound, and no previous procedure done on it, which might well need procedural treatment now with CEA or CAS.
• Already started any appropriate medical treatment (eg, statin, aspirin etc), and already recovered from any necessary coronary procedures (eg, CABG)
• Patient seems fit and willing for follow-up in person (at 1 month) and by annual letter (for at least 5 years)
• Some type of angiography (eg, MRA or CTA) has already been done that has shown that CEA .

Exclusion Criteria:

• Small likelihood of worthwhile benefit (eg, very low risk of stroke because stenosis is very minor, or major co-morbidity or life-threatening disease, such as advanced cancer)