The main forms of Doppler used pulse wave Doppler(PWD), Color Doppler (CD) and Power Doppler (PD). Modern ultrasound equipment with CD felicities usually offers conventionally B-mode real-time scanning. CD, which shown color representation of blood flow towards and away from probe, pulsed wave spectral analysis shown as waveforms above and below a baseline and often PD.
The Doppler waveforms is constructed from a series of samples. For a given sampling rate or pulse repetition frequency (PRF) , there is a maximum frequency. Above this , the sampled waveform cannot be constructed accurately and a lower frequency signal is produced.
The sample Doppler signal adequately , at least two samples must be taken on the shortest cycle for the correct frequency to be determined. This is called Nyquist limit. This can al,so be expressed as the sample rate being at least 2 x maximum frequency in the Doppler signal. If the sample rate, controlled by a PRF, is less than the essential twice per cycle, then a interpretation of the high frequencies above one half 0f the sampling frequency. Because the signals is not sampled quickly enough, it appears below the baseline. The PRF therefore should be at least twice the maximum values of Doppler frequency to be measured in order to avoid aliasing.
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