TCD Examination TCD examination will be performed at baseline, weeks 12, 24 and 36. A 2-MHz pulsed Doppler hand-held probe is used to insonate the middle cerebral artery (MCA) through temporal window above the zygomatic arch at a depth of 52-56mm, and the vertebral artery (VA) through occipital window at a depth of 64mm. The Pulsatility Index (PI) and Breath Holding Index (BHI) will be calculated. The PI is a measure of vascular resistance of cerebral vessels. The mean PI of the middle MCA and vertebral VA will be obtained. A previous study by the investigators showed that these measures highly differentiated stroke patients with and without WML on MRI (AUC = 0.85 [MCA] and 0.81 [VA], sensitivity and specificity between 70 and 82%]). According to the standard protocol, the simple breath holding procedure will be performed during the same TCD session to obtain the BHI, which will serve as a marker of CVR. A trained postgraduate research student under the supervision of a broad certified neurologist (Co-A Vincent MOK) will perform the TCD blind to participant's treatment allocation.