Doppler Principles Flashcards
Who first described the doppler effect?
Christian Andreas Doppler
When was the theory of doppler proposed?
1842
What was the theory proposed by Christian Doppler?
A perceived change of the colour of stars as they moved, red towards and blue away.
Where is the doppler theory used?
Sonar, radar (speed traps), cloud direction/precipitation, ultrasound
What is the doppler effect?
A perceived change in frequency.
(As a train moves past you the pitch changes and the emitted noise sounds different when it is directly beside you versus moving away from you BUT the sound is actually the same and it is only a perceived change)
What creates the change in pitch during the doppler effect?
Relative motion.
Movement of the sound or the receiver
Compare sound as it travels towards you versus away.
Moving towards = wave crests are close together, higher pitch
Moving away = wave crests further apart, lower pitch
What is doppler usually used to assess in UT?
Blood flow (qualitative and quantitative)
What information can Doppler give about flow?
- Presence
- Direction
- Quality
What are the 3 ways doppler information is given?
- Spectral tracing
- Colour image
- Audible sound
What does it mean when doppler is used and no colour appears?
There is no flow
What is duplex scanning?
When we scan a 2D image and use Doppler simultaneously the display will show the image and a spectral tracing at the same time.
What is triplex scanning?
Same as duplex but colour Doppler is also turned on.
2D image + Doppler (spectral tracing) + Colour Doppler
How does the colour relate to movement in doppler?
Usually:
Red = Towards (positive) from the probe, faster
Blue = Away (negative) from the probe, slower
BUT the colours can be inverted so you must always check the colour map.
How does the colour map distinguish the flow?
Whichever colour is on top is moving towards the probe.
How does the Doppler effect work in scanning vessels?
The sound reflects off of moving blood cells which alters the frequency that is returned to the probe, the amount of change in the frequency is related to the direction and speed of the RBC’s.
Is the frequency change in the Doppler effect within our range of hearing?
Yes.
When the sound is perpendicular to the vessel, what happens?
It is black, no doppler can be seen.
What is the diameter of RBC’s?
7 μm
What type of scatter happens in blood vessels?
Rayleigh scattering (very weak).
Why does Rayleigh scattering occur?
Because RBC’s only have a diameter of 7 μm they are smaller than a typical wavelength.
How does Rayleigh scattering indicate the quantity of blood in a sample?
The intensity of the scatter is proportional to the number of RBC’s.
Higher intensity = more RBC’s = strong signal/colour
What principle is the Doppler shift based on?
Wave interference
What is the Doppler shift?
A change in frequency. When a reflected wave varies slightly in frequency from the transmitted wave a beat frequency is produced. (Wave Interference)