Furthermore, in addition to being a reliable bedside tool for monitoring arterial recanalization during systemic thrombolysis, TCD is known to enhance the rates of arterial recanalization. However, its use has expanded immensely over the past three decades and TCD has emerged as a noninvasive and cost-effective tool for evaluating cerebral arterial patency, detecting stenosis, collateral flow patterns, and embolization. TCD was initially introduced for detecting the vasospasm following subarachnoid hemorrhage. The PMD or M-mode Doppler had the advantage of simultaneously displaying the intensity and direction of intracranial blood flow with over 6 cm or more of intracranial space, which simplified the examination technique. Mark Moehring, in 2002, invented the transcranial power-motion mode Doppler (PMD) with 33 sampling gates. It was a single gate spectral TCD system that required considerable expertise to obtain flow signals from intracranial vessels. Rune Aaslid, in 1982, introduced the transcranial Doppler (TCD) for detecting blood flow in the basal intracerebral arteries. This article describes the basic ultrasound physics pertaining to TCD insonation methods, for detecting the flow in intracranial vessels in addition to the normal and abnormal spectral flow patterns. Advanced applications of TCD help in the detection of right-to-left shunts, vasomotor reactivity, diagnosis, and monitoring of vasospasm in subarachnoid hemorrhage and as a supplementary test for confirmation of brain death. TCD is a noninvasive, cost-effective, and bedside tool for obtaining information regarding the collateral flow across various branches of the circle of Willis in patients with cerebrovascular disorders. Currently, TCD is the only diagnostic tool that can provide real time information about cerebral hemodynamics and can detect embolization to the cerebral vessels. During evaluation of patients with acute ischemic stroke, TCD combined with cervical duplex ultrasonography provides physiological information on the cerebral hemodynamics, which is often complementary to structural imaging. Since its introduction in 1982, by Rune Aaslid, TCD has evolved as a diagnostic, monitoring, and therapeutic tool. Transcranial Doppler (TCD) can be aptly called as the doctor’s stethoscope of the brain.
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