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Transcranial Doppler ultrasonography (TCD) is a non-invasive method to evaluate cerebral hemodynamic changes in circle of Willis with Doppler principle. Two technical points that enable TCD are adoption of low frequency (2 MHz) ultrasound to penetrate bony window, and application of pulsed-wave Doppler mode allowing determination of the depth and sample volume of the insonated vessels. A standard examination technique should be applied through temporal, orbital, and suboccipital window (submandibular, if necessary) as in protocol. To shorten the time of examination, maximum power and large sample volume are recommended. It is crucial to perform a complete and through examination, although the examiner should target clinically involved arterial segment or suspected level of occlusion. Once the highest signal is found, avoid losing signal during switching the depth of insonation. Interpretation of Doppler waveform and pulsatility is important even more than flow velocity change alone. These parameters should be interpreted comprehensively with clinical information and other physiologic factors influencing TCD results.
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