Principles of Colour and Spectral Doppler Flashcards
As velocity increases what happens to the doppler shift?
It increases
What does velocity depend on?
The angle
What does the doppler shift frequency depend on?
- Angle
2. Transducer frequency
What is the nyquist limit
1/2 PRF (Between 5-30 kHz
What is autocorrelation?
A process that represents the mean or average flow of velocity in the vessel that allows for colour doppler
What is the doppler effect?
The change is frequency of sound caused by the motion of red blood cells.
What is the doppler shift?
The diff between transmitted frequency (transducer) and received frequency (reflected/echo)
What is antegrade flow, describe the shift.
RBC’s moving towards transducer (the echo frequency is larger than the transducer frequency).
What is retrograde flow?
The RBC’s are moving away from the transducer (echo frequency smaller than transducer).
What is the method used to measure the doppler shift?
Wave interference
What is beat frequency?
The reflected wave differs from the original slightly and the sum of the two gives a resulting frequency called the beat frequency.
Why are there many diff frequencies within the beat frequency?
Because the RBC’s speed up and slow down with systole and diastole and the cells closest to the centre move faster than near the walls.
What is plotted in the spectral trace?
All of the frequencies contained within the beat frequency
What is the doppler equation and what does it represent?
Change in F = 2 x Fo x V x Costheta / C
Represents: F(received) - F(transmitted)
What is the doppler shift measured in and why can we hear it?
Measured in Hertz or kilohertz
In the “audible range” so we can hear it.
Why does aliasing occur?
When the nyquist limit is exceeded the PRF is too slow and the sampling is only catching parts of the flow.
How to compensate for aliasing? (5)
- Move baseline
- Increase PRF/scale
- Increase doppler angle
- Lower the operating frequency
- Change to CW (sampling is continuous so no aliasing is possible)
What is high PRF mode?
Allows machine to send out scan lines before the first is returned.
What is a downside to high PRF mode?
Increases range ambiguity due to the machine not knowing what depth the echoes are being returned from.
What ways can doppler info be represented?
- Colour
- Spectral
- Sound
What is FFT?
Fast Fourier Transformation
The method of graphing a spectral trace by taking the beat frequency and dissecting it into its component waves (individual reflections off of an RBC at a certain point in time).
How is colour doppler info obtained?
Autocorrelation
What is the zone of sensitivity?
In CW it is the area of overlap between the continuously emitted sound waves and the reflected sound waves.
What is the benefit and drawbacks of CW?
Benefit = no limit to velocities that can be detected
Drawbacks = no range resolution/2d image
How many crystals for CW?
Two
How many crystals for PW?
One (transmits and then listens).
How is spectral broadening affected by sample volume?
Large sample volume = more spectral broadening
What does power doppler use to detect flow instead of the doppler shift?
Amplitude
What is power doppler sensitive to?
Low flow states
What is power doppler independent of?
Angle
