-Abstract-
Transcranial Doppler ultrasonography (TCD) is a non-invasive and relatively cheap
method of evaluating cerebral hemodynamics, allowing repeated measurements and
continuous monitoring. Use of low frequency (2 MHz) ultrasound to penetrate bony
window, and application of pulsed-wave Doppler mode allowing determination of the
depth of the insonated vessels are two technical points. TCD measures velocity rather
than flow, and therefore provides an estimation of cerebral blood flow only if vessel
diameter remains unchanged. Mean flow velocity (MFV) and pul-satility index (PI) are
the most commonly used parameters, which are influenced by various physiological
variables and pathologic conditions. In clinical practice, detection of stenosis of the basal
intracerebral arteries (>50-60%) is widely used, which is specific though not sensitive.
Identification of occlusion or recanalization of vessels in acute stroke may provide useful
information for thrombolytic therapy and prognosis. Monitoring of microembolic signal
(MES) is a challenging field to detect the high-risk patients in asymptomatic carotid
stenosis, and to evaluate the efficacy of anti-thrombotic therapy: CO2 or
acetazolamide reactivity test is a useful method to detect significantly impaired
hemodynamic reserve in patients with carotid occlusion, in which surgical
revascularization may be beneficial. Newly developed echo-contrast agents and power
Doppler instrument, which are expected to overcome the current technical limitation, and
further progress of the transcranial color-duplex sonography also light up the future of
neurosonology.
|