9 The magnitude of gender difference in this age group is ~10 to 15%. 7– 9 Women have been shown to have higher flow velocities than men between 20 to 60 years of age, 7 and this difference may be explained by the lower hematocrit in premenopausal women. All studies should be conducted with the patients at rest-not speaking or moving their limbs.īlood flow velocities in the basal arteries of the brain decline an average of 0.3 to 0.5% per year between 20 to 70 years of age. Therefore, it is important to remember that during the course of a TCD study, any measured differences in blood flow velocity should be interpreted in the context of these variables. The most robust of these variables are age, gender, hematocrit, viscosity, carbon dioxide, temperature, blood pressure, and mental or motor activity. Physiologic Determinants of Blood Flow Velocity and IndicesĪ number of physiologic variables can impact blood flow velocity as measured by TCD. However, the basic assumption for TCD measurements is that the diameter of the insonated vessel does not change during the course of a study and it remains constant in response to various physiologic variables such as blood pressure or changes in sub> has also been shown/sub>, which may easily occur as a result of treatment. In addition, the velocity of blood flow through a vessel is proportional to the fourth power of the vessel radius. Therefore, it is important to minimize this angle to less than 30 degrees to keep the error below 15%. The larger the angle, the larger is the cosine of the angle hence, the greater is the error in our velocity measure. If the angle is zero, or the emitted wave is parallel to the direction of flow, the cosine of zero is 1, and we have achieved the most accurate measure of flow velocity. Theta (θ) is the angle of insonation or the angle of the emitted wave relative to the direction of vessel (blood flow). The propagation speed of a wave is a constant that can be obtained for various mediums (speed in soft tissue is 1541 m/s). Reflector speed ( cm / s ) = ( Doppler Shift ) × propagation speed 2 × Incident frequency × cos ( θ ) 3 The V mean is a continuous trace of peak velocities as a function of time and in most TCD instruments, it is calculated and displayed automatically. The specific parameters obtained from this spectral analysis include peak systolic velocity (Vs), end diastolic velocity (Vd), systolic upstroke or acceleration time, pulsatility index (PI), and time-averaged mean maximum velocity (V mean). 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.

Transcranial doppler machine