Ultrasound and Doppler
Ultrasound and Doppler
Ultrasound has many uses in areas of medicine associated with anaesthesia. It is used for imaging, visualisation of needle and catheter positioning, therapy and, together with the Doppler effect, for measurement of flow velocity. Real-time information can be obtained with ease, and with the low energies used, diagnostic equipment exposures are not thought to be a safety issue, either for the operator or the patient. Ultrasound is a form of mechanical energy that consists of high frequency vibrations at frequencies above human hearing range (> 20 kHz) and up to frequencies in the tens of MHz range. The frequencies used are dependent on issues such as the penetration and resolution required. It is thought that low intensity ultrasound passes through living tissue without altering tissue function. Higher energy can produce heating and cavitation, both of which can alter cell function. Ultrasound is generated by electrically inducing a deformation in a piezoelectric crystal, which compresses and decompresses the medium to which it is coupled at a rate equal to the frequency of the driving voltage. The pressure changes travel through the medium in a longitudinal direction and the distance between the points of maximum pressure, or compression, is known as the wavelength. Figure 10.2 shows the relationship between the period of the wave and the wavelength. The length of this distance is dependent on the elasticity (compressibility) and the density of the medium, and the delay between the movement of adjacent particles in the medium. As shown in the figure, the wavelength (λ) and the transmission frequency (f) are related to the propagation velocity c by c = f λ. The magnitude of the wave is the difference between the maximum and minimum pressure values. The wave propagates by movement of particles: it cannot travel in a vacuum and it does not ionise the medium through which it travels. The propagation of an ultrasound wave is not constant throughout the body. Various parts allow the passage of the wave at different velocities. Also the wave is attenuated differently by the various tissue types. For example, in soft tissues the ultrasound wave has a velocity of between 1460 and 1630 m s−1 whereas in bone it is 2700–4100 m s−1.
Keywords: Doppler effect, Doppler velocimetry, absolute blood flow measurement, colour flow Doppler imaging, duplex Doppler, four-dimensional ultrasound, high power ultrasound, resistive index (RI), two-dimensional B-scanning ultrasound
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