What is the doppler effect?
It is a change in frequency when a reflector or source of sound is moving.
What is frequency?
Frequency=number of events/time.
If a man runs into the ocean against the waves, how does the frequency of the waves hitting him change?
Causes an increase in waves.
Are there only positive frequency shifts?
No, there can be positive and negative frequency shifts.
What is the difference between the frequency sent vs the frequency recieved called?
Doppler echoes are evaulated by equipment for what 3 things?
- Timing. (depth)
- Strength (amplitudes)
What is depth determined by?
What will reflect the initial frequency?
A stable structure.
What will reflect a different frequency?
A moving structure.
What is the doppler equation?
Δf= (2vFo (cos Ø))/C
v=velocity of moving reflector
Fo= frequency of sound source.
Δf=frequency shift= the difference between original and recieved frequency.
cos Ø= cosine of angle between the ultrasound beam and the flow of direction.
C= the speed of sound in the medium.
What is 3 things is Δf directly related to?
- Transducer frequency (Fo)
- Velocity (V)
- Cosine of the doppler angle (cos Ø)
Δf is indirectly related to what?
Speed of sound in a medium (C).
f0 is changed when it encounters what?
Moving reflectors, such as RBC.
What is the doppler frequency shift?
It si the difference between f0 and the frequnecy recieved. (subtract them)
What is the frequency shift proportional to?
It is proportional to the velocity of the reflectors.
Blood moves at velocities that create doppler freuwncy shifts within the frequency range of human hearing, what is the frequency range of human hearing?
20 Hz-20 kHz
What kind of audible characterisitcs and measurements does the speed of RBC give?
Pitch, and it is measured as frequency.
What kind of audible characteristic and measurement does the number of RBC give?
Loudness, and it is measured as amplitude.
How is loudness seen as?
How is velocity related to pitch?
It is directly rated.
The higher the velocity the louder the pitch.
Where does the initial frequency (f0) come from?
It comes from the transducer.
What is frequency shift?
It is the change from inital to recieved frequency.
ie, a 5.000 MHz beam may be recieved as 5.002 MHz. The frequency shift here is .002 MHz
ie, a 5.000 MHz beam may be recieved as 3.994 MHz. The frequency shift here is .006
What does a positive shift indicate?
an increase in frequency, towards the doppler.
What does a negative shift indicate?
frequency decreases, away from the doppler beam.
What is Δf proptional to?
What is Δf indirectly proportional to?
If a 10 MHz Doppler beam creates an echo read by the equipment at 10.005 MHz, what is the frequency shift?
.005 MHz→ 5 kHz
If a 2 MHz probe results in an echo of 1.998 MHz, what is the f shift?
.002 MHz→-2 kHz
If a +3 kHz frequency shift was created by a reflector, the flow direction was_________?
Towards the doppler beam.
If the blood velcoity speeds up, the doppler frequency shift will?
What are the 2 doppler shifts in vascular ultrasound?
- one shift when the moving RBC meets the sound beam.
- one shift as it sends the echo back
What does the velocity equal?
The velocity of RBC
What does the velocity tell us?
It tells us how tight a narrowing is in an artery. As % of stenosis increases, the velocity increases.
What is the velocity equation?
v= c ( Δf)/2 f0 (cosØ)
What happens when the velocity of the blood changes?
The doppler frequency shift changes.
What happens when the transducer frequency changes?
The doppler frequency shift changes.
What is the relationship between f0 (transducer) and the frequency shift?
It is directly proportional because the higher the frequency probe, the greater the frequency shift.
What changes when f0 changes?
What is the doppler angle?
It is the angle between the doppler beam and flow direction.
What should the angle be between the doppler beam and flow direction?
less then 90°
What is the cosine of 0°?
What is the cosine of 30°?
What is the cosine of 45°?
What is the cosine of 60°?
What is the cosine of 90°?
What is the relationship between the angle and cosine?
They are inversely related.
What is the relationship between cosine of an angle and the doppler frequency shift?
What happens to the cosine as the angle decreases?
the cosine angle increases, so the doppler frequency shift increases and the velocity measurement decreases.
What happens to the cosine angle as the angle of the beam increases?
The cosine angle decreases, so the doppler shift decreases, but the velocity measurement increases.
What are 2 doppler angle errors?
- Cosine table differences
- Error in angle cursor setting with larger angles are greater than errors at smaller angles.
Velocities should never be made at angles greater then what?
What does doppler ultrasound mean in the vascular lab?
It means to evaluate blood flow.
What does a doppler ultrasound show?
It shows flow patterns.
What does a doppler ultrasound measure?
Measures velocities of blood flow.
- Peak systolic velocity
- end diastolic velocity
- Velocity ratios
- Resistance calculations (pulsatility)
- Flow estimates.
In the doppler equation, the ________ is a number that corrects for measuring at an angle.
Why can we hear the doppler frequency shifts from blood flow?
Because it has a frequency shift that is within human hearing. (20 Hz-20kHz)
The doppler angle is the angle between the ______ and _______.
Doppler beam and Flow direction
If you change transducers from an 8 MHz to a 4 MHz, how will the waveform be affected?
Decrease in amplitude.
If the frequency of the doppler beam is 9 MHz and the frequency of the returning echo is 8.995 MHz, then the flow is going?
Away from the doppler beam.
If the blood flow increases, the doppler shift__________
Why don’t we get a waveform with a doppler angle at 90°?
B/C cos 90° is zero, which means there is no doppler shift.
Why don’t we measure velocities at angles greater than 60°?
Because the cosine changes that occur for small changes in angle measurement can result in larger errors in velocity measurement if the angle is not perfectly in line with flow direction.