What is doppler shift AKA?
doppler frequency
When does the frequency of sound change?
When the sound source and receiver move closer together or farther apart
What is the Doppler shift?
a low frequency that rides on top of the much higher transducer frequency
What is demodulation?
Process of extracting the low doppler frequency from transducer’s carrier frequency
When is doppler shift positive?
- blood cells moving toward the transducer
- reflected freq higher than transmitted freq
What is the equation for doppler shift frequency?
Doppler shift = reflected freq - transmitted freq
When is doppler shift negative?
- reflected freq lower than transmitted freq
- blood cells moving away from transducer
What is the doppler equation?
Doppler shift = (2 x velocity of blood x transducer freq x cos) / propagation speed
What is the relationship between the velocity of blood and Doppler shift?
Directly related, faster velocity = greater Doppler frequency
Why is there a 2 in the doppler equation?
There are 2 doppler shifts
- First occurs when sound wave from transducer strikes moving blood cells
- Second results from transducer’s reception of sound wave from moving RBCs
If doppler shift is measured with units of frequency, how do we measure the velocity of blood?
- frequency diff b/w received and transmitted sound waves is measured
- system uses equation to convert freq, Hz to velocity, m/s
What are the x and y axis of a doppler spectrum?
X = time
Y= doppler shift or velocity
What is the relationship between the transmitted frequency and the doppler shift?
- doppler shift is directly related to frequency of transmitted sound
- directly related to velocity
A doppler exam is performed with a 2 MHz transducer, doppler shift is 3 kHz. If the identical exam is performed with a 4 MHz transducer, what is the doppler shift?
6 kHz
What does the measurement of doppler frequency depend on?
Relationship b/w direction of blood flow and direction in which sound wave propagates
What happens when the beam is parallel to blood flow?
- entire velocity is measured
- velocity measurement is 100% accurate
- RBCs are moving directly toward or away from transducer
What happens when the sound beam is at an angle to blood flow?
velocity measured is less than true velocity
