Doppler Effect

Doppler effect

A change in frequency caused by the motion of the sound source, receiver, or reflector

Doppler shift

Change in frequency caused by motion

+ doppler shift/returns at a higher frequency

Source is approaching source and receiver

- doppler shift/returns at a lower frequency

Source is moving away from source and receiver

Doppler frequency axises

x-axis: time

y-axis: represents doppler shift or velocity

Doppler shift is _____ to the blood flow speed

Proportional

Increased velocity = increased doppler shift

Doppler shift is _____ to operating frequency

proportional

Doppler ultrasound

• Doppler effect is a result of motion of blood, the flow we wish to measure, or motion of tissue

• Minimum: a few mm per sec

• Max: determined by aliasing

• Doesn’t exceed a few m/s

Types of flow information provided by doppler US

• Presence

• Direction

• Speed-slow or fast

• Character-laminar or turbulent

Forms of doppler US presentation

• Audible sounds

• Strip-chart recording (print out)

• Spectral display

• Color doppler display

Audible doppler

• 200-15,000 Hz

• Rayleigh scattering

• Intensity of sound is proportional to the number of RBCs. and the quantity of blood in the sample volume

Rayleigh scattering

• By small particles in all direction with RBCs, this is how doppler shift is measured

• Is frequency dependent

• Echo intensity for RBC is approximately 30 dB less than surrounding tissue echo intensity

Doppler angle: 180

Direction of sound propagation is exactly opposite the flow direction, the max, and doppler shift is obtained (towards transducer)

Doppler angle: 0

If the flow speed and propagation speed directions are the same, the max doppler shift is obtained

If the doppler shift increases, the calculated scatterer speed ____

Increases

If the source frequency ____, the calculated scatter speed decreases

increases