Cerebral autoregulation is an important mechanism whereby cerebral perfusion is normally maintained at a constant level, over a relatively wide blood pressure range. It can be assessed noninvasively by the use of Trans Cranial Doppler (TCD). This means using ultrasound probes over both sides of the head to measure changes in blood flow in one of the main brain arteries (the middle cerebral artery) in response to beat to beat changes in blood pressure dynamic cerebral autoregulation (dCA). It is established that dCA is impaired following moderate to severe stroke, acting as a key role in the development of secondary brain damage related to brain swelling and further damage related to low blood flow. The administration of clotbusting therapy (thrombolysis), one of the main approved treatments of acute ischaemic stroke (AIS), results in recanalization of the blocked artery in over approximately 50% patients. However, due to its clot dissolving property, it may increase the risk of bleeding in the body, especially in the brain, leading to greater disability or even death. To date, there has been very little information regarding the natural history and prognostic significance of impaired Cerebral Autoregulation during and following reperfusion, especially those who receive thrombolysis. This research will use the noninvasive technique of Trans Cranial Doppler (TCD) to see how blood flow changes in AIS patient at the initiation and completion of thrombolysis, and during acute, subacute and chronic phase post stroke onset, compared with those AIS patient who did not receive thrombolysis. This study will provide important data regarding perithrombolysis blood pressure management, an important and common clinical dilemma