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Doppler Physics

Doppler Physics. Waves from a static source. Wave peaks evenly spaced around the source at 1 wavelength intervals. Waves from a moving source. Old positions of source. Source moving this way. http://www.kettering.edu/~drussell/Demos/doppler/doppler.html. Reflection off a static surface.

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Doppler Physics

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  1. Doppler Physics

  2. Waves from a static source Wave peaks evenly spaced around the source at 1 wavelength intervals

  3. Waves from a moving source Old positions of source Source moving this way

  4. http://www.kettering.edu/~drussell/Demos/doppler/doppler.htmlhttp://www.kettering.edu/~drussell/Demos/doppler/doppler.html

  5. Reflection off a static surface

  6. Reflection off a moving surface

  7. Back to waves • If a wave reflects off a moving object the reflected frequency is changed • This is called the Doppler effect • “Doppler shift” (fd)given by: fr-ft=fd=ft*2*u/c Where fr=received frequency ft=transmitted frequency u=relative velocity of source and reflector c=speed of sound

  8. Zero Doppler shift here Maximum Doppler shift here Geometry Moving reflectors – Blood cells

  9. Angle effects • Maximum Doppler shift at 0 degrees minimum at 90 degrees – proportional to the Cosine of the angle between the beam and direction of travel Alignment of beam Direction of movement

  10. Change in Fd with Angle fd=ft*2u*Cos(q)/c c*fd/ 2*ft*Cos(q)=u

  11. Doppler ModesColor Power Spectral

  12. CW doppler • Always transmitting and receiving • Just look at the difference in frequency • So – don’t know depth ! • Can avoid this (somewhat) by using intersecting beams.

  13. Pulsed wave Doppler • Pulses – just like real time scanning • Can find depth • Need to “gate” analysis of received pulse, so we know where the moving objects are…

  14. Uterine Doppler

  15. Umbilical Doppler

  16. Common Carotid Artery

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