Doppler Instrumentation Part One
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What is Continuous Wave Doppler?
- Continuous generation of U/S waves coupled with continuous U/S reception.
- A two crystal tdx accomplishes this dual fxn with one crystal devoted to each fxn.
- Un-damped crystal so narrow bandwidth and high Q factor
What is disadvantages of CW?
- Poor range resolution: don't know which vessel the shifts are coming from.
- Lack of TGC: reflections from deeper depths will have lower amplitudes
What is advantages of CW?
- Accurately measures high velocities
- No aliasing
- High sensitivity (detects low amplitude reflections and small doppler shifts)
What is Pulsed Wave Doppler/
- The Txd alternates between sending and receiving pulses.
- Color Doppler is also pulsed u/s technique.
- Wide bandwidth pulses with low quality factor compared to CW.
What is sample gate?
(Sample Volume, range gate and gate)
- is where doppler takes place in vessel.
- Doppler signal is only taken from the sample volume area.
What is advantages of PW Doppler?
- Range Resolution or Range Specificity
- Seeing the exact vessel that you are examining.
What is disadvantages of PW doppler?
- Inaccurate measurement of high velocity signals (appears as aliasing)
- Lower sensitivity than continuous wave (CW)
What is Duplex?
- Duplex units combine real-time imaging with CW or PW Doppler.
- The machine does both B-mode and spectral doppler, alternating between the two.
What is Amplifier?
In the receiver amplifier, the returning signal is boosted in strength and then applied to the demodulator.
What is demodulator?
- is where the doppler shifted frequencies are extracted from the high ultrasonic frequencies.
- Demodulation produces what is called the "beat" frequency which is the doppler shifted frequency.
What is Sample and Hold
- For a single reflector, a series of echoes from successive transmitted pulses are obtained over time and temporarily stored in the sample and hold unit.
- It assembles the measured values obtained from multiple pulses to form the doppler frequency signal
- Doppler signal is built up gradually over time in the sample and hold unit.
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