Doppler Physics

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

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

Reflection off a static surface

Reflection off a moving surface

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

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

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 Direction of movement Alignment of beam

Change in F d with Angle f d =f t *2u*Cos(  )/c c*f d / 2*f t *Cos(  ) =u

Doppler Modes Color Power Spectral

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.

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…

Uterine Doppler

Umbilical Doppler

Common Carotid Artery