17 Doppler Flashcards
What is Doppler shift or frequency?
Change or variation in frequency of sound as a result of motion between the sound source and receiver
What is a positive change or shift?
When source and receiver are approaching each other
What is a negative change or shift?
When source and receiver are moving apart
How is Doppler created?
When sound reflects off of RBCs
What is the range of Doppler shift?
20Hz - 20 kHz
What is demodulation?
Extraction of the Doppler frequency from the transducer frequency
How is bidirectional Doppler analyzed?
With phase quadrature processing
What is the equation for Doppler shift?
Doppler shift = received frequency - transmitted frequency
FD = FR - FE, FE = emitted freq, FR = reflected freq
What is THE Doppler equation?
Doppler shift = (2 x reflector speed x incident frequency x cos(angle)) / propagation speed
What is Doppler shift directly related to?
Blood cell speed
Frequency of the transducer
Cosine of the angle between flow and the sound beam
What is Doppler inversely related to?
Speed of sound in the medium
What does Doppler measure and not measure?
Measures velocity, not speed
What is the equation for measured velocity?
Velocity (measured) = true velocity x cos(angle)
What is the # of crystals in a continuous wave doppler transducer?
2 crystals
1 is continuously transmitting and the other is continuously receiving
What is range ambiguity?
Echoes that arise from entire length of overlap between the transmit and receive beams
What is the advantage of continuous wave doppler?
High velocities are accurately measured
How many crystals are in a pulsed wave doppler?
1 crystal that alternates between sending and receiving
What is the advantage of a pulsed wave doppler?
Echoes come only from the area of interrogation, the sample volume or gate
What is the disadvantage to pulsed wave doppler?
Aliasing, errors in measuring high velocities
What is the horizontal and vertical axes of a Doppler spectrum?
Horizontal - time
Vertical - frequency or velocity
What is aliasing?
When high velocities appear negative with pulsed Doppler
High velocity measurements are inaccurate if PRF is too low in comparison to the measured Doppler shift
What is Nyquist frequency?
Doppler frequency at which aliasing occurs
= to 1/2 the PRF
How is aliasing eliminated?
Use continuous wave Doppler
Select a transducer with a lower frequency
Select new view with a shallower sample volume
Increase the scale, same view
Baseline shift