![]() ![]() 1, 2 Spectral analysis can then be used to obtain measures of blood flow velocity, as well as a few other characteristics of flow within the insonated blood vessel. Because blood flow within the vessel is laminar, the Doppler signal obtained actually represents a mixture of different Doppler frequency shifts forming a spectral display of the distribution of the velocities of individual red blood cells on the TCD monitor ( Fig. ![]() The difference in the frequency between the emitted and reflected waves, referred to as the “Doppler shift frequency,” is directly proportional to the speed of the moving red blood cells (blood flow velocity). According to this principle, ultrasound waves emitted from the Doppler probe are transmitted through the skull and reflected by moving red blood cells within the intracerebral vessels. TCD ultrasonography is based on the principle of the Doppler effect. In this review, the authors summarize the basic principles and clinical utility of TCD and provide an overview of a few TCD research applications. Clinical indication and research applications for this mode of imaging continue to expand. TCD has established utility in the clinical diagnosis of a number of cerebrovascular disorders such as acute ischemic stroke, vasospasm, subarachnoid hemorrhage, sickle cell disease, as well as other conditions such as brain death. TCD can also be used to assess the physiologic health of a particular vascular territory by measuring blood flow responses to changes in blood pressure (cerebral autoregulation), changes in end-tidal CO 2 (cerebral vasoreactivity), or cognitive and motor activation (neurovascular coupling or functional hyperemia). TCD can be used to measure flow velocity in the basal arteries of the brain to assess relative changes in flow, diagnose focal vascular stenosis, or to detect embolic signals within these arteries. Transcranial Doppler (TCD) ultrasound provides rapid, noninvasive, real-time measures of cerebrovascular function. ![]()
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