Doppler Principles Flashcards
Who first described the doppler effect?
Christian Andreas Doppler
When was the theory of doppler proposed?
1842
What was the theory proposed by Christian Doppler?
A perceived change of the colour of stars as they moved, red towards and blue away.
Where is the doppler theory used?
Sonar, radar (speed traps), cloud direction/precipitation, ultrasound
What is the doppler effect?
A perceived change in frequency.
(As a train moves past you the pitch changes and the emitted noise sounds different when it is directly beside you versus moving away from you BUT the sound is actually the same and it is only a perceived change)
What creates the change in pitch during the doppler effect?
Relative motion.
Movement of the sound or the receiver
Compare sound as it travels towards you versus away.
Moving towards = wave crests are close together, higher pitch
Moving away = wave crests further apart, lower pitch
What is doppler usually used to assess in UT?
Blood flow (qualitative and quantitative)
What information can Doppler give about flow?
- Presence
- Direction
- Quality
What are the 3 ways doppler information is given?
- Spectral tracing
- Colour image
- Audible sound
What does it mean when doppler is used and no colour appears?
There is no flow
What is duplex scanning?
When we scan a 2D image and use Doppler simultaneously the display will show the image and a spectral tracing at the same time.
What is triplex scanning?
Same as duplex but colour Doppler is also turned on.
2D image + Doppler (spectral tracing) + Colour Doppler
How does the colour relate to movement in doppler?
Usually:
Red = Towards (positive) from the probe, faster
Blue = Away (negative) from the probe, slower
BUT the colours can be inverted so you must always check the colour map.
How does the colour map distinguish the flow?
Whichever colour is on top is moving towards the probe.
How does the Doppler effect work in scanning vessels?
The sound reflects off of moving blood cells which alters the frequency that is returned to the probe, the amount of change in the frequency is related to the direction and speed of the RBC’s.
Is the frequency change in the Doppler effect within our range of hearing?
Yes.
When the sound is perpendicular to the vessel, what happens?
It is black, no doppler can be seen.
What is the diameter of RBC’s?
7 μm
What type of scatter happens in blood vessels?
Rayleigh scattering (very weak).
Why does Rayleigh scattering occur?
Because RBC’s only have a diameter of 7 μm they are smaller than a typical wavelength.
How does Rayleigh scattering indicate the quantity of blood in a sample?
The intensity of the scatter is proportional to the number of RBC’s.
Higher intensity = more RBC’s = strong signal/colour
What principle is the Doppler shift based on?
Wave interference
What is the Doppler shift?
A change in frequency. When a reflected wave varies slightly in frequency from the transmitted wave a beat frequency is produced. (Wave Interference)
What is the basic formula for calculating the Doppler shift?
Doppler shift = Received f - Transmitted f
What will the Doppler shift equal?
A positive or negative shift.
Pos = toward probe Neg = away
What is the detailed formula for Doppler shift?
Δf = [2(f-operating) x V x cosθ] / c
V = RBC velocities Cosθ = probe angle C = speed of sound in tissue
What does cos90 in a vessel equal?
Zero, it is perpendicular to the vessel and no colour doppler will be seen.
As sonographers, what is more meaningful to us than calculating the Doppler shift?
Calculating the blood velocity
What is the formula to calculate blood velocity?
V = (Δf x C) / 2(f-operating) x cosθ
Why is it important to use the correct angle in doppler?
The machine calculates the blood velocity based on the angle we give it. If the angle is off then the velocity will be off and this is dangerous because we could unknowingly be creating or hiding pathology).
What is the relationship between the doppler shift and the operating frequency?
Linear relationship
If the doppler shift increases, so does the f-o
What type of probe is needed for doppler and why?
A low frequency probe, because the return intensity from RBC’s is so week we need a probe that gives good penetration.
What type of frequency would we use when scanning in duplex?
High frequency for the image = (good resolution pic)
Lower frequency for the Doppler
What is the RBC velocity’s relationship with the Doppler shift?
Linear
As the RBC’s move faster (velocity increases), the difference in the returned frequency also increases (Doppler shift)
What does the angle of insonation impact?
The velocity calculated.
Why do we use 60 angle of insonation?
It is the easiest angle for all sonographers to replicate and the percent error is acceptable.
What is the most accurate angle for velocity calculation? Why dont we use it?
Zero degrees
We can’t get 0 degrees to a patient’s vessel without cutting them open.
How do you get the correct angle on a vessel?
Line the angle up with the vessel so that it is completely parallel to flow by steering the box and then using “heel-toe” manipulation.
The accuracy of the RBC’s velocity depends on what?
The accuracy of the angle of insonation
Can we ever use angles less than 60 degrees?
Yes, for a vessel that dives but we MUST document it.
Can we ever use an angle greater than 60 degrees?
No, because the percent error is too high.
What happens to the doppler shift as the angle of insonation increases?
As the angle increases the cosine decreases so the doppler shift decreases (doppler shift formula).
What happens to the Doppler shift as the frequency increases?
It increases
What is the definition of the Nyquist limit?
The limit reached when the RBC’s velocities are faster than the machines ability to sample.
Why is the machines ability to sample limited? (Nyquist Limit)?
Pulses can only be sent out as fast as they are returned and are limited by the speed of sound in tissue.
What is the numerical definition of the Nyquist limit?
1/2 of the PRF
What limits the PRF?
Depth
What happens when the blood cells move faster than 1/2 of the PRF?
The Nyguist limit is exceeded (our sample doesn’t move fast enough) and we get “aliasing”.
What is 1/2 of the PRF?
Usually 5-30 kHz
What is aliasing?
An artifact that occurs when the blood cells are not sampled fast enough a false reading occurs and parts of the signal wrap around the baseline.
Does Aliasing occur with continuous wave?
No, the sound is always on so nothing is being missed.
What does aliasing look like on a spectral tracing?
The peaks on the spectral tracing are in the negative underneath the base line because they “wrap around”/
How do the colours in doppler represent velocity?
The lighter colours are faster.
What does aliasing look like on colour Doppler?
The red and blue mix together and if we stop moving there is no black line between them.
What does flow reversal look like?
The same as aliasing (mix of red and blue within the vessel) but when we stop moving there is a black line between them.
What is another way to tell aliasing is causing wrap around?
The hue changes in the colours can indicate wrap around
When does flow reversal happen?
In the presence of pathology
How do we correct for aliasing?
In the lab we can:
- Increase the PRF (scale)
- Move the baseline (but not to the very top or very bottom because we could miss things)
In theory we could:
- Use continuous wave
- Lower the operating frequency
- Increase the angle
Why do we not want to use continuous wave in Doppler to correct aliasing even though it would?
We don’t have the option on our machines and it is dangerous.
Why would we not want to decrease the operating frequency to correct aliasing even though it would work?
It lowers the velocity
Why would we not want to increase the angle to correct aliasing even though it would work?
It would increase the percent error to an unacceptable level.
What type of sound is used for Doppler?
Pulsed
What is the ‘gate’ in Doppler and how do we position it?
The sample volume.
Positioned in the middle of the vessel (sag and trans) or where the highest velocity flow occurs.
How is the gate/sample volume defined?
With the range equation
What does multi gating do?
Allows several depths to be sampled simultaneously.
What is range ambuiguity?
Artifact that occurs when we pick up 2 separate flows from 2 vessels and it merges them together.
What determines the sample volume?
- beam width
- the receiver gate length
- the length of the emitted pulse (# of cycles)
What does the overall sample volume equal?
1/2 the pulse length + the gate length
As the gate length is reduced what happens to the pulse length?
It reduces as well
Why is gate length important?
A smaller gate length will improve the spectral display but if it is too small the middle velocity may be missed.
How does pulse duration effect Doppler?
Longer pulses improve the accuracy of the beat frequency (Doppler shift).
What is the minimum pulse duration required for Doppler?
4 cycles/pulse
What type of pulses do we want in Doppler, why?
Long pulses.
To see the entire cardiac cycle (systole and diastole) in order to get an accurate portrayal (so we don’t miss anything).
Is range gating possible in continuous wave?
No, because the sound is always on we don’t know where the echos are returning from.
Where is the continuous wave sample volume found?
The intersection of the transmitted and received beam.
How big is the continuous wave sample?
Big, 5-6cm
What is the zone of sensitivity?
The big area where the sample volume is found in continuous wave (intersection of transmitted and received beam).
Is it important to get parallel to flow with continuous wave?
Yes, it is required because there is no image to use an angle correct on.
Is it possible to pick up more than one vessel with continuous wave?
Yes because the sample volume is so large (would result in a very complex spectral tracing).
How is a spectral tracing produced?
When the sound intersects the flow echos are produced and many different doppler shifts are returned, a spectrum is created using the range of Doppler shift frequencies from each RBC that passed through the sample volume.
What is the spectrum?
An array of the components of a wave arranged in order of increasing frequency over time.
What is the x axis of the spectral tracing?
Time
What is the y axis of the spectral tracing?
1) Frequency of the Doppler shift
OR
2) Velocity of the RBC’s
What is the z axis of the spectral tracing?
Strong or weak signal
What is the “power” of the spectral tracing?
The Z- axis (brightness of pixels) = density of RBC’s moving at one velocity at a given time.
What does a bright waveform mean on a spectral tracing?
Many RBC’s were contained in the sample.
What is the End Diastolic velocity?
The last velocity before the next peak
What is the peak Systolic velocity?
The highest velocity
What is the window?
The dark area under the peaks
What is the Dicrotic notch?
The dip or “notch” after the the peak
What is the envelope?
The white part of the spectral tracing that represents the range of velocities.
What does it mean if a window is filled in?
Plaque could be present
If all the RBC’s were moving at the same velocity what would the envelope look like?
A single line
If there is a big range in velocities, what will the envelope look like?
Thick envelope
What is FFT?
Fast Fourier Transform
It is the way in which the machine creates a spectral trace
What does the FFT do?
It mathematically separates the individual Doppler shifts from the complex beat frequency so that all the frequency units are broken up and can be plotted.
What units is the frequency shift (y axis) expressed in?
kHz
What is the velocity of the RBC’s expressed in? (y axis)
cm/s or m/s
What y-axis is more preferred? Velocity or frequency?
Velocity because it compensates for variations in vessel alignment relative to surface and is more meaningful.
What is Antegrade?
The flow is moving towards the probe and is expressed as a positive number on the y axis
What is Retrograde?
The flow is moving away from the probe and will have a negative value on the y axis
If there is flow above the baseline does this mean it is antegrade? (Towards the probe?)
No. We have to check the values on the graph in order to know. The flow can be negative (retrograde) and still be on the top of the graph.
How can blood flow be categorized?
By it’s pulsatility (or resistance to flow)
High, moderate, low
What do we use to determine the pulsitility of blood flow in a vessel?
A spectral tracing
Does the pulsitility of a vessel ever change?
Yes, depending on physiological condition ( At rest, exercising etc.)
Describe the charcteristics of highly pulsitile or high resistance flow?
- Tall, narrow, sharp systolic peaks
- Reversed or absent diastolic flow
Where is high resistance flow found?
Arteries of the extremities
What is an example of a vessel with high resistance flow?
Common Femoral Artery
Describe the characteristics of low pulsitile or low resistance flow?
- Broad systolic peaks
- Forward flow through diastole
Where is low resistance flow found?
In vessels that feed the vital organs (need blood all the time).
What is an example of a vessel with low resistance flow?
ICA - Internal Carotid Artery
What is moderate pulsitility?
Moderate resistance flow is a combination of high and low resistance.
Describe the characteristics of moderate flow?
- Tall, narrow, sharp systolic peaks
- Forward flow through diastole
Where are moderate flow vessels found?
Feed both high and low resistance vascular beds
What is an example of a moderate flow vessel?
Common carotid artery
Where are moderate pulsitility vessels commonly seen?
Carotids
What is laminar?
A flow profile where the inner layer of RBC’s travel the fastest and each layer travels slower as you move outwards toward the vessel wall.
What is PI?
Pulsitility index
It is used to quantify the impedance to flow like in the presence of a stenosis.
What is the formula for PI?
PI = (A - B) / Mean
A = Peak systole B = Reverse (below the baseline)
How does PI effect resistance?
High PI = high resistance
Low PI = low resistance
Where is PI used?
For extremities
What is the mean?
An average of all velocities in one beat.
What is the RI?
Resistive Index
Used to evaluate the resistance to flow like in the case of a transplanted organ.
What is the formula for RI?
RI = (A - B) / A
A = Baseline to peak systole B = Baseline to end diastole
Where is RI used?
Used for parenchymal organs
What would the resistance be like in a transplanted kidney that was working well?
Low resistance
What is used to quantify a CHANGE in pulsitility?
The systolic/diastolic ratio
What is the formula for the the systolic/diastolic ratio?
S/D ratio = A/B
A = Baseline to peak systole B = Baseline to end diastole
When does pulsitility change?
1) In the presence of pathology
2) When the physiological state changes the waves depending where blood is needed (at rest, exercising, before/after eating) (Normal)
When is it normal for the pulsitility of the extremities to change from high to low?
After exercise
What does prepandial and postpandial mean?
Before and after eating
The slow acceleration of the blood in systole can be an indicator of what?
Disease.
What is the normal acceleration of blood in systole?
Rapid and produces a vertical deflection.
What is tardusparvus?
“Late”
Acceleration will be slowed after a stenosis.
How is acceleration calculated in the spectral tracing?
Acceleration = ΔV/ΔT
ΔV = peak systole (highest velocity) ΔT = time from smallest systolic velocity to the highest
What is spectral broadening?
The vertical thickening of the spectral trace or envelope (when a greater range of velocities is present spectral broadening occurs).
What causes spectral broadening?
Disturbances to flow:
- bifurcations
- anastomoses
- curves in vessels
- Pathology (plaque)
What is an anastomoses?
The coming together of vessels, opposite of bifurcations.
What is false spectral broadening?
An artifact where spectral broadening occurs
What are reasons for false spectral broadening?
1) 2D Doppler Gain is too high
2) If the sample volume gate is large in comparison with the vessel
3) Not lined up on the “true” middle
4) Little bit of plaque (unlikely)
