Transcranial Doppler (TCD) is a noninvasive ultrasound (US) study used to measure cerebral blood flow velocity (CBF-V) in the major intracranial arteries. It involves use of low-frequency (≤2？MHz) US waves to insonate the basal cerebral arteries through relatively thin bone windows. TCD allows dynamic monitoring of CBF-V and vessel pulsatility, with a high temporal resolution. It is relatively inexpensive, repeatable, and portable. However, the performance of TCD is highly operator dependent and can be difficult, with approximately 10–20% of patients having inadequate transtemporal acoustic windows. Current applications of TCD include vasospasm in sickle cell disease, subarachnoid haemorrhage (SAH), and intra- and extracranial arterial stenosis and occlusion. TCD is also used in brain stem death, head injury, raised intracranial pressure (ICP), intraoperative monitoring, cerebral microembolism, and autoregulatory testing. 1. Introduction Transcranial Doppler (TCD), first described in 1982 [1], is a noninvasive ultrasound (US) study that involves the use of a low-frequency (≤2？MHz) transducer probe to insonate the basal cerebral arteries through relatively thin bone windows. TCD allows dynamic monitoring of cerebral blood flow velocity (CBF-V) and vessel pulsatility over extended time periods with a high temporal resolution. It is relatively inexpensive, repeatable, and its portability offers increased convenience over other imaging methods, allowing continuous bedside monitoring of CBF-V, which is particularly useful in the intensive care setting [2]. The technique is however highly operator dependent, which can significantly limit its utility [3–6]. It also has a long learning curve to acquire the three-dimensional understanding of cerebrovascular anatomy necessary for competency [3]. Furthermore, approximately 10–20% of patients have inadequate transtemporal acoustic windows [2, 4, 7]. Current applications of TCD in adults and children include vasospasm in sickle cell disease [8], subarachnoid haemorrhage (SAH) [9], intra- and extracranial arterial stenosis and occlusion [10, 11], brain stem death [12], head injury, raised intracranial pressure (ICP) [13], intraoperative monitoring [14], impaired vasomotor function [15], and cerebral microembolism in right to left cardiac shunts [16]. TCD has also been widely used to investigate cerebral pressure autoregulation [17]. Combined with waveform morphology, indices derived from flow velocity readings such as Gosling’s pulsatility index (PI) and the Lindegaard ratio (LR) allow identification of increased
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