Sonography can be enhanced with Doppler measurements, which engage the Doppler effect to assess whether structures (usually blood) are moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a focused sample volume, for example a shot of blood flow over a heart valve, its speed and direction can be determined and visualised. This is remarkably useful within cardiovascular studies (sonography of the vasculature system and heart) and essential in plentiful areas such as determining reverse blood flow in the liver vasculature within portal hypertension. The Doppler information is displayed graphically using spectral Doppler, or as an image using colour Doppler (directional Doppler) or power Doppler (non directional Doppler). This Doppler shift falls contained by the audible range and is repeatedly presented audibly using stereo speakers: this produces a very distinctive, although synthetic, pulsing nouns.
Strictly speaking, most modern ultrasound machines do not use the Doppler effect to measure velocity, as they rely on pulsed swell Doppler (PW). Pulsed wave machines transmit pulses of ultrasound, and later switch to receive mode. As such, the reflected pulse that they receive is not subject to a frequency shift, as the insonation is not continuous. However, by making several measurements, the phase tuning in subsequent measurements can be used to purchase the frequency shift (since frequency is the rate of change of phase). To get the phase shift between the received and transmitted signals, one of two algorithms is typically used: the Kasai algorithm or the crosscorrelation. Older machines, that use continuous wave (CW) Doppler, exhibit the Doppler effect as described above. To do this, they must enjoy separate tranmission and reception transducers. The major drawback of CW machines, is that no distance information can be obtain (this is the major assistance of PW systems - the time between the transmitted and received pulses can be converted into a distance with familiarity of the speed of sound).
In the ultrasound community (although not in the signal processing community), the argot "Doppler" ultrasound, has be accepted to apply to both PW and CW Doppler systems despite the different mechanism by which the velocity is measured. I THINK ITS YOUR VEINS IN YOUR NECK..... Try this website..
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