In a CW doppler instrument, what does the two crystals do?
one transmits beam and one recieves echoes.
What is the advantage of a CW doppler instrument?
It has a large focal zone.
ie. large sample volume.
What is the disadvantage of a CW doppler instrument?
It hears everything in its path
What arre the typical transducers used?
4Mhz and 8Mhz
Can CW doppler determine location?
What does a CW doppler measure?
It documents changes in velocity over time.
What direction is the right speaker demonstrating?
Flow towards the brain.
Which direction is the left speaker?
Flow away from the beam.
Why isn’t the height of an analog wave not a siagnostic criteria on its own?
The waveform height shows the f shift, so the heigh of the waveform depends not only on the velocity of the blood flow, but also the transducer frequency, AND the angle the probe is held.
*lower angle= higher waveform*
*lower angle= high cosine*
What are the 3 CW dopple controls?
- Transducer choice (4 MHz vs 8MHz)
- Gain/size on machine.
- Wall filter.
What is the limitation of CW Doppler?
What does range ambiguity mean?
It means that thr location of flow cannot be determined due to no image.
What does range resolution mean?
It means the location of flow is clear.
What is range resolution also known as?
- Range specificity
- Freedom from range ambiguilty
Why is range ambiguous with CW but not with PW?
Sample volume allows for specific placement for sampling doppler signal with PW. But there’s a large sample volume with CW and its size and depth cannot be changed.
Which is ambiguous: PW or CW?
How is CW: analog tracing created?
It is created with a zero-crossing detector.
What does a CW: analog tracing display?
It essentially displays an average of the change of frequency over time.
In the CW: analog tracing, what is averaged in the display?
- Foward and reverse flow are averaged.
- turbulence is averaged.
What is the zero-crossing detector?
IT is an instrument that creates the analog tracing for a CW Doppler.
What does the zero-crossing detector count?
It counts the number of times an echo wave crosses the zero line.
How is a zero-crossing detector displayed?
It is displayed in an analog format
Why is it important to have a wall filter?
It is mportant because without a wall filter, the doppler would pick up motion from valves, wall motion, that cause low frequencies since their motion is slower than blood flow.
What does a wall filter do?
It filters out low frequencies, leaving only frequencies from blood in display.
what is the wall filter also known as?
“High Pass Filter” because only filters with high frequencies can pass through)
What is the walll filter set at?
50 Hz or less.
When is the wall filter usually needed?
It is usually needed in abdominal vasculature.
What is the frequnecy of most echo signals?
What should you set the filter for venous (low)?
less than 50 Hz
What should you set the filter to for arterial (high)?
What effects wall filters?
What happens if the wall filter is set too low?
electronic saturation artifact may occur.
High velocoties with spectral broadening often need ______ to see waveforms.
What is spectural analysis?
It is a computer process used to break down a complex signal (ie sound) into its frequency components.
What is “Fast Fourier Transform”?
It is a devicee that performs spectral analysis.
All echoes that return to the transducer from the sv are have ________.
What is demodulation?
It is the process of calculating the frequency shift.
What does demodulation extract?
It extracts the lower doppler shifted frequency off of the higher transducer frequency that is reflected.
ie. a 7.000 MHz transducer frequency is reflected as 7.004 MHz. The demodulation extracts the .004 MHz or 4 kHz.
What is phase quadrature?
(aka quadrature detection)
It is a signal processing technique to seperate positive from negative doppler frequencies.
Flow signals toward the doppler beam (positive) are displayed how?
Above the baseline.
Flow signal away from the doppler beam (negative) are displayed as what?
Below the baseline
What can control power output?
What does a large power do to the transducer?
It increased the power applied to the transducer.
What does the large output power increase?
It increases the ampitude of the transmitted beam and of the echoes. It also increases the acoustic exposure to patient.