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

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

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” (fd)given by:

fr-ft=fd=ft*2*u/cWhere fr=received frequency

ft=transmitted frequencyu=relative velocity of source and reflectorc=speed of sound

Geometry

Maximum Doppler shift hereMoving 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 o

f beam

Change in Fd with Angle

-1.5

-1

-0.5

0

0.5

1

1.5

0 30 60 90 120 150 180Series1

fd=ft*2u*Cos()/c

c*fd/ 2*ft*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