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0 degrees
flow in the direction of sound travel
Detected flow is always
< actual flow (Cos corrects for angle in the velocity calculation)
Greater angle
Smaller component parallel to the beam
Pulsed Wave Doppler uses
1 element that alternates between transmit and receive (1 aperture for arrays)
PW Doppler system
sampling system
Range gate (user operated)
can set a depth of interest based on time
PW Doppler signal is sampled at
discrete times (sample rate = PRF)
cw
pw
pw hold period using individual samples
pw output from the sample and hold
pw after low pass filter
original waveform (low and high freq)
low pass filter waveform
high pass filter waveform
Sampling Rate of a Pulsed Doppler System is the
PRF
Max Frequency that can be Accurately Measured
sampling rate (PRF) / 2
PRF is controlled by
scale (user controlled)
Nyquist Limit
highest freq that can be measured without aliasing
Aliasing
freq greater than the Nyquist limit are misinterpreted as lower freq (signal wraps up and around)
above the nyquist limit at the true freq
no motion detected
minimum sample rate to avoid aliasing
2 samples / cycle
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)
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
inc prf
inc sample rate which will inc the Nyquist Limit
High PRF Mode allows the
PRF to exceed the limit by transmitting a pulse while echoes are still returning
High PRF Mode is used to
measure high velocities
High PRF Mode downside
Doppler Range Ambiguity (artifact)
Doppler Range Ambiguity (artifact)
Multiple range gates operate at different depths and the machine cant distinguish early echoes from the most recent pulse
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
pulsing can miss things is it doesnt
sample fast enough
quadrature detector
demodulator specific for direction of flow by comparing the transmitted and received signals
reconstruction of sampled signals
connects the spaces between samples to reconstruct the signal
low pass filter
smooths things out by removing big steps
the quadrature detector
shifts the waveform by 90deg (1/4 cycle) and the time difference tells whether the signal is towards or away (+/-)
the quadrature detector has
2 channels (standard demodulator has 1)