Doppler Flashcards
First person to describe Doppler
Christian Andreas Doppler
What is the Doppler effect
A change in pitch resulting from relative motion of the source of the sound or the receiver
Explain Doppler in relation to yourself
When a sound source is moving towards you the wave crests are closer together and the pitch is higher, or if moving away from you the crests are further apart and the pitch sounds lower
What is Doppler primarily used for?
To access blood flow bot qualitatively and quantitively
What information can Doppler give you?
- Presence of flow
- Direction of flow
- Quality of flow
When we get information from Doppler, how do we receive this information?
- Spectral tracing
- Colour image
- Audible sound
What is duplex scanning?
When scanning both the 2D image and Doppler simultaneously.
During duplex scanning what does the display side show?
The image and a spectral tracing at the same time
What is triplex scanning?
If colour Doppler is also turned on during duplex scanning
What happens when sound reflects off moving blood cells in Doppler?
The frequency of sound is altered during the return, this change in sound is within our hearing
What is Rayleigh scattering?
The kind of scattering that occurs when RBC is scanned, this is due to the smaller size of the RBC compared to a typical wave
What is the typical size of a RBC?
7 micrometers
During Doppler, the intensity of scattered sound is proportional to what?
To the number of RBCs and thus indicates the quantity of blood in the sample
What is the formula for Doppler shift?
Doppler shift = received f - transmitted f
What is Doppler shift based off of?
The principle of wave interference
What is a more detailed formula for Doppler?
Change in F = (2Fo * v * coso)/ c
What relationships does the Doppler equation show?
Relationship between doppler shift, operating frequency, RBC velocities, probe angle, and the speed of sound
The operating frequency has a linear frequency with what?
Doppler shift
As the operating frequency shift what happens to doppler?
It increases
Higher frequency probes gives better resolution at the expense of what?
Penetration
Why do we use lower frequency during doppler?
Since RBC’s return very little intensity
During duplex scanning, the frequency used to obtain the image and the one to obtain doppler are what?
Lower and higher respectively
RBC velocity has a linear relationship with what?
Doppler shift, just like frequency
What happens if RBCs move faster during doppler scans?
The difference in the returned frequency increases, therefore increasing the doppler shift
Is angles in doppler important?
Yes
Why is the angle in doppler important?
The cosine of 90 is zero, so if the beam is perpendicular to the vessel, no doppler shift can be calculated.
The angle of insinuation impacts what during doppler?
It impacts the velocity calculated in the equation and requires consistency for proper patient care
What is the most accurate angle for velocity?
0 degrees, because if the angle of intonation increases, so does the percent error for velocity.
What is the best angle that a sonographer can achieve during a exam?
60 degrees
How can we achieve the 60 degree angle for doppler?
We will heal/toe the probe
What is the Nyquist limitation?
The limit reached when the RBC velocities are faster then the machines ability to sample
The Nyquist limit is equal to what?
1/2 the PRF
During Nyquist limitations pulses can only be sent out as fast as they can be returned and are limited by what?
The speed of sound in tissue
1/2 the PRF usually falls between what frequency?
5-30 kHz
What is aliasing?
An artifact known as aliasing occurs if the Nyquist limit is exceeded
During Nyquist limitation blood cells are not being sampled fast enough so what happens?
A false reading occurs and parts of the signal are wrapped around the baseline
Correcting the aliasing for Nyquist limitation can be accomplished in what ways?
- Move the baseline *
- Increase the PRF*
- In crease the doppler angle (Heel or toe probe)
- Lower the operating frequency
- Change the continuous wave
Since pulsed sound is used for Doppler it is possible to do what?
Define a sample volume or gate
The gate for sample volume should be positioned how?
In the middle of the vessel or where the highest velocity flow occurs
Multi-gating allows for what?
Several depths to be sampled simultaneously
The sample volume is created because of what?
The range equation
The sample volume is determined by what? 3
- The beam width
- The receiver gate length
- The length of the emitted pulse (# of cycles in pulse)
The overall sample volume is equal to what? (Formula)
1/2 the pulse length plus the gate length
As gate length is reduced in sample volume what happens?
Pulse length must also reduce
A small gate length will do what to spectral display?
Improve the spectral display
In terms of doppler shift longer pulses are required to do what?
Improve the accuracy of the doppler shift
In terms of beat frequency, the longer the pulse duration what happens?
The more accurate the beat frequency
A minimum of what pulses are required for doppler shift?
4 cycles/pulse are required.
This is opposite to 2D scanning where a short pulse duration is best
Range gating is not possible with what?
Continuous wave doppler
The continuous wave doppler sample volume is found where?
The intersection of the transmitted and received beam and tends to be large (5-6 cm)
What is the zone of sensitivity?
The intersection of the transmitted and received beam.
Without an image, what is not possible? And because of this what is required with CW?
Getting a correct angle is not possible, so an attempt to get parallel flow is required
With such a large sample volume the potential for what happens?
Potential for picking up more than one vessels exists
What is spectral analysis?
When the beam intersects blood flow, flow echoes are produced and many different doppler shifts are returned from the vessel
During spectral analysis, the color spectrum created is what? (D/s)
a range of generated Doppler shift frequencies from each RBC that passes through the sample volume
The spectrum is an array of components of a wave arranged how?
Components of a wave arranged in order of increasing frequency over time
The spectral tracing of doppler frequencies are generated by what? (How it’s plotted)
Moving blood that we see is plotted along the following axes
X: time
Y: frequency of velocity
Z: power
The power of the Z axis is represented by what?
The brightness of the pixels
The brightness of the pixels is proportionate to what?
The number of blood cells moving at one velocity at a given time
The brighter the waveform the greater the what?
Number of RBC’s are contained in the sample
What is the process of Fast Fourier Transform (FFT)?
The way in which the system creates the spectral trace.
It is a mathematical technique that separates the individual Doppler shifts from the complex beat frequency
The Y axis can represent what two things in spectral analysis?
Either frequency of the doppler shift or the velocity of RBC’s
Frequency shift can be expressed in what?
kHz
Velocity can be expressed in what terms?
Cm/s or m/s
Which variable is preferred on the Y axis?
Velocity is preferred since it compensates for variations in vessel alignment relative to surface and is more meaningful to the tech.
When assessing flow direction on a spectral trace it is relative to what?
The transducer
Antegrade flow means what?
Towards the probe and is expressed as positive number on the Y-axis
Retrograde flow will have what?
A negative value on the Y axis
Spectral analysis helps us to determine what?
The many aspects of blood flow
One aspect of spectral analysis is what?
The pulsatility or resistance to flow
Blood flow can be categorized as what?
High, moderate, or low pulsatility
Highly pulsatile, or resistance flow has what characteristics?
- Tall, narrow, sharp systolic peaks
- Reversed or absent diastolic flows
Where is it common to see high resistance flow in the body?
in the arteries of the extremities
What is an example of a structure with high resistance flow?
CFA
Low Pulsatile, or low resistance flow has the what characteristics?
- Broad systolic peaks
- Forward flow through diastole
It is common to see low resistance flow in what?
The vessels that feed the vital organs
What is an example of low resistance flow in arteries?
Internal carotid artery (ICA)
Moderate pulsatility is a combination of what?
Both high and low resistance
What characteristics are common to moderate pulsatility?
- Tall, narrow, sharp systolic peaks (high)
- Forward flow through diastole (low)
Moderate resistance vessels feed what?
Both high and low resistance vascular beds
What is an example of moderate pulsatility in vessels?
The common carotid is artery (CCA)
What qualitative aspects do wavelength have? 4
- Presence of flow
- Direction of flow
- Laminar or turbulent
- Spectral broadening
What are some quantitative aspects on waveforms? 5
- Pulsatility index
- Resistive index
- Doppler shift
- Peak velocity
- Mean velocity
What is the formula for pulsatility index?
PI = (A-B)/mean
What is the pulsatility index used to quantify?
The impedance to flow like in the presences of a stenosis
What is the resistive index used to evaluate?
Resistance to flow in the case of a transplanted organ
What is the Resistive index formula?
RI= (A-B)/A
What is the systolic/ diastolic ratio?
S/D ratio= A/B
What does the systolic/ diastolic ratio mean?
quantifying a change in pulsatility
Interpretation of pulsatility requires what?
A knowledge of Normal waveform characteristics and the physiologic status of circulation at the time of study
An example of high pulsatility pattern in the extermities to low pulsatility is what?
After exercise
The acceleration of blood in systole is another indicator of what?
Disease
Normal acceleration of the arterial blood in systole is what?
Rapid and produces a vertical deflection
Acceleration may be slowed in the presence of what?
A disease
Spectral broadening is what?
The vertical thickening of the spectral trace or envelope
If all velocities are moving at the same speed then what happens?
The spectral trace would be a thin line
When greater ranges of velocities is present what happens?
Then spectral broadening occurs
Spectral broadening can be the result of what?
Disturbances to flow such as bifurcations, anastomoses and curves in vessels
Pathology in the arteries in the form of plaque can cause what?
Spectral broadening
What is false spectral broadening?
An artifact that occurs when the Doppler gain is set to high
You can create the appearance of spectral broadening if what happens?
The sample volume gate is large relative to the vessel
