By using a 1–2-MHz pulsed Transcranial Doppler (TCD), it is possible to penetrate the skull bone at special sites (windows) and register the flow velocity (FV) in the insonated artery at well-defined depts. In this way, the FV can be registered in the central arteries as well as in some of the veins. A normal FV in an artery normally indicates an adequate circulation to the territory it supplies. High and low FV does not necessarily have correlations to the CBF since the diameter of the measured vessel is unknown. If the FV is high, a differentiation between hyperaemia and vasospasm can be obtained by performing a Lindegaard Index (LI) which is the correlation between the FV in the middle cerebral artery and the internal carotid artery.