Two major advancements since the inception of TCD have been in relation to duplex sonography and the power-motion mode Doppler (PMD). For TCD the frequency of the USG waves used is 2 MHz while it is 4 MHz or more for the Doppler studies of extracranial vessels. One basic difference is in the frequency of the probe used for the Doppler studies. However, the USG machine can be used for structural imaging of the tissues as well as for study of the blood vessels, including the arteries and the veins. The TCD machine is specifically designed to study the flow velocities of intracranial arteries. How Does Transcranial Doppler Differ from Conventional Ultrasoundīoth the TCD and conventional ultrasonographic (USG) machines are used in the intensive care units for various studies on human body. Another study done in post-cardiac arrest patients undergoing hypothermic therapy, however, does not support a relationship between temperature and flow velocities. An inverse relationship has been found between temperature and BFV in a study. BFV can be higher in patients with higher systemic blood pressures in spite of the presence of an intact autoregulatory system. There is an increase in BFVs by ~20% with a decrease in haematocrit from 40 to 30%. Haematocrit and viscosity are inversely related to cerebral BFV.
However no difference is observed after the age of 70 years between males and females. These are higher in females as compared to males between 20 to 60 years of age. BFVs decrease by 0.3 to 0.5%/year between 20 to 70 years of age. Thus during the course of a TCD study, the measured differences in BFV should be interpreted in the context of these variables. The physiologic determinants of BFV when measured by TCD includes age, gender, viscosity, haematocrit, temperature, carbon dioxide, blood pressure, motor and mental activity. Physiologic Determinants of Blood Flow Velocity and Indices The reflected waves received by the TCD probe generates electrical impulses which are then processed to calculate ΔF, V as well as in the production of a spectral waveform consisting of peak systolic velocity (PSV) and end-diastolic velocity (EDV). BFV is calculated using the Doppler formula: V = ΔF × C/2 Fi × Cos θ, where V = velocity of the moving target, ΔF = Doppler shift, C = frequency of sound in tissues, Fi = transducer frequency and Cos θ = cosine of angle of insonation of ultrasound waves. It works on the principle that with arterial narrowing there is an increase in the blood flow velocity (BFV) within the vessel. This can be represented by the following equation which forms the basis of calculation of cerebral blood flow velocity (CBFV) using TCD. This change in frequency of the reflected wave from that of the incident wave is called as the Doppler shift (ΔF). TCD works on the principle of Doppler effect, which states that when a sound wave of a particular frequency (incident wave) strikes a moving object, for example, an erythrocyte, the frequency of the reflected wave changes proportionally to the velocity of the reflector (V).
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