The apparent change in frequency produced by relative movement between a sound emitter and receiver is called the Doppler effect (Christian Doppler 1805-1853). This principle is used in ultrasound systems to provide information about blood flow. This may simply be in the form of audio information, a spectral display or as colour Doppler displays. Two main ways of obtaining Doppler information can be used:
1. Continuous wave ultrasound beam - information presented as either an 302 audio signal and/or a spectral display.
2. Pulsed wave ultrasound beams whereby audio, spectral and colour display of data are possible.
Duplex Doppler is a term used to refer to a combination of grey scale and pulsed-Doppler display. This produces a grey scale image with a line of sight along which there is a Doppler acquisition marker/gate. The position and size of the gate can be varied within the ultrasound image, so that it lies within the vessel or area of interest. When the machine is switched into Doppler mode, a real-time Doppler spectral display appears on the monitor. This can be used to assess flow direction, resistance, evidence of spectral broadening and peak velocity.
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