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36 Terms

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0 degrees

flow in the direction of sound travel

2
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Detected flow is always

< actual flow (Cos corrects for angle in the velocity calculation)

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Greater angle

Smaller component parallel to the beam

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Pulsed Wave Doppler uses

1 element that alternates between transmit and receive (1 aperture for arrays)

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PW Doppler system

sampling system

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Range gate (user operated)

can set a depth of interest based on time

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PW Doppler signal is sampled at

discrete times (sample rate = PRF)

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cw

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pw

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pw hold period using individual samples

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pw output from the sample and hold

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pw after low pass filter

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original waveform (low and high freq)

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low pass filter waveform

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high pass filter waveform

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Sampling Rate of a Pulsed Doppler System is the

PRF

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Max Frequency that can be Accurately Measured

sampling rate (PRF) / 2

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PRF is controlled by

scale (user controlled)

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Nyquist Limit

highest freq that can be measured without aliasing

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Aliasing

freq greater than the Nyquist limit are misinterpreted as lower freq (signal wraps up and around)

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above the nyquist limit at the true freq

no motion detected

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minimum sample rate to avoid aliasing

2 samples / cycle

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Nyquist Limit vs Nyquist Rate

limit is the max freq that can be represented and rate is the sampling rate required to
accurately represent the signal (2 x freq)

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to dec aliasing

inc prf, inc doppler angle (dec Fd), dec freq (dec Fd), dec depth (allows you to inc the prf more) or adjust the baseline

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inc prf

inc sample rate which will inc the Nyquist Limit

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High PRF Mode allows the

PRF to exceed the limit by transmitting a pulse while echoes are still returning

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High PRF Mode is used to

measure high velocities

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High PRF Mode downside

Doppler Range Ambiguity (artifact)

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Doppler Range Ambiguity (artifact)

Multiple range gates operate at different depths and the machine cant distinguish early echoes from the most recent pulse

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cw vs pw

cw has 2 elements, no range resolution and unlimited freq response (no aliasing) while PW has 1 element, range resolution and limited freq response

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pulsing can miss things is it doesnt

sample fast enough

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quadrature detector

demodulator specific for direction of flow by comparing the transmitted and received signals

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reconstruction of sampled signals

connects the spaces between samples to reconstruct the signal

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low pass filter

smooths things out by removing big steps

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the quadrature detector

shifts the waveform by 90deg (1/4 cycle) and the time difference tells whether the signal is towards or away (+/-)

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the quadrature detector has

2 channels (standard demodulator has 1)