Doppler Flashcards
Who first described the Doppler shirt/effect?
Christian Johann Doppler (1803-1853) an Austrian physicist
What is the Doppler effect?
A change in frequency of sound, light or waves caused by motion of the source or observer
What is the Doppler shift in ultrasound?
Change in frequency of sound caused by motion of RBC
Difference b/w transmitted freq and received freq
What happens to the frequency if the RBC are moving towards the transducer?
Echo freq will be larger then the transducer therefore antegrade flow
What happens to the frequency if the RBC are away from the transducer?
The echo frequency will be smaller therefore retrograde flow
What is the shift dependant on? (3)
- Transmitted freq
- Velocity of moving blood
- Angle b/w moving blood and the sound beam
What are the methods to detect and analyze Doppler shifts? (3)
Color flow
Spectral waveforms
Audible sounds
What is the most important factor that influences the calculation of the Doppler shift?
Angle of insonation
What angle would cause an ideal orientation for allowing the largest Doppler shift? What’s the COS?
Angle of 0
Cos0=1
What occurs when the blood flow when the angle is at 90deg?
There is no flow/shift detected because there is no measurable frequency
What is the angle of the transducer to the body to create blood flow?
30-60 deg to the blood vessel
Why do we want an insonation angle of 60deg?
Reduces the margin of error and is reproducible
What occurs when the Doppler angle is greater then 60deg?
Difficult to quantify because the errors in estimating the Doppler angle relative to the reflector velocity and spectral broadening which lead to errors in estimates of peak frequencies
How does the angle of insonation affect venous flow? (3)
- Angle is not important
- Velocities are not used
- Angle correct is set to zero
Examples of how the angles of insonation affects velocities
60deg - vel 124cm/s
34 deg - vel 68cm/s
79deg - vel 286cm/s
What is the simplest Doppler device?
CW Doppler
What happens to the returning signals of CW Doppler?
Returning signals are amplified and filtered therefore only allowing US frequencies or higher
Is there an image produced with CW Doppler?
No
Is it possible to set a depth and region to sample with CW Doppler? Why?
No because of the continuous nature of the transmitting frequency
What is an advantage of CW?
It can measure high velocities so there is no aliasing!
What is pulsed Doppler?
Sound pulses produced by the transducer at regular intervals
Can the operator adjust the controls to isolate signals from a desired depth in pulsed Doppler? Why?
Yes because the pulses are sent at set intervals
Can pulsed Doppler accurately receives signals from all depths?
No it is limited to certain depths
What factor limits the accurate measurement of high velocities by producing aliasing?
PRF
When does aliasing occur?
When the Doppler shift exceeds 1/2 PRF
What is the name of the limit that determines if aliasing occurs?
Nyquist limit
What is the Nyquist limit?
When the Doppler shift exceeds 1/2 PRF
Why does aliasing occur?
Insufficient time to collect signal information before the next pulse is sent out
What is Duplex Doppler?
The combination of Doppler instruments and realtime B mode
What is Fast Fourier transform?
The separation of the received Doppler shifts into individual frequency components and then displayed on a monitor
What does quadrate detection do?
Processes the signal as a + or - valve depending on direction of flow relative to the Doppler beam
What is pulsatility?
Relationship of the peak to min velocities over the mean velocity of an entire cycle
What are the types of pulsatility waveforms?
Low, moderate or high pul. Waveforms
What is resistivity?
Relationship of peak systolic velocity to end diastolic velocity
What type of resistance waveforms are there?
Low or high
Describe low resistance waveforms
Diastolic flow above zero
Describe high resistance waveforms
Shows reversed flow in early diastole
What type of waveform that provides a constant flow of blood? What organs need this?
Low resistance waveforms
Brain, kidneys, liver, spleen
What type of assessment does resistivity provide?
Qualitative and Quantitative
What is the most common type of flow pattern?
Laminar flow
Describe the laminar flow pattern
- all particles moves forward in concentric rings
- highest velocity in the centre and gradually decreases as they approach the wall
What is the average velocity of laminar parabolic flow equal to?
1/2 the maximum velocity
What profiles describe parabolic flow?
Plug and Blunt
What are factors affecting parabolic flow? (3)
- Velocity
- Changes in diameter of vessel
- Curves, bifurcations and branch origins
Describe disturbed pattern
- Friction and energy looses start to disrupt laminar flow
- Still forward flow but in diverging directions
- Mild version of turbulence
Is disturbed flow considered normal?
Yes
Describe turbulent flow
- non-linear flow
- multiple directions (back and forth)
- multiple velocity of blood particles
Is turbulent flow considered normal?
No
What does the adjutant of gain allow for?
Allow visualization without unwanted signal
What does to high of gain cause?
False spectral broadening
Background noise or mirror image
What does baseline do?
Adjusts to allow entire spectral signal to be seen
What happens when the baseline is to high?
Aliasing
What does a wall filter do?
Eliminates low frequency noise
How should the wall filter be adjusted to optimize image?
Adjust so that low velocities are not missed on arterial or venous flow states
How should scale, PRF and velocity range be adjusted?
Adjusted so all peak and min velocities are seen
What are the 2 main methods to display Color?
- Standard method color
2. Power Doppler
What is power Doppler?
Power or intensity of the signal so measured rather then Doppler shift
What is power Doppler based on?
Density of RBCs not speed
Does aliasing occur with spectral Doppler?
No
Is power Doppler more or less sensitive of the Doppler angle?
More sensitive and independent
What is power Doppler used to access?
Small vessels
Tissue perforation
Slow flow
Why is power Doppler the preferred method when contrast is used?
Less subject to blooming
Why is power Doppler not suitable when motion of tissue or pt is unavoidable?
Because it has a very slow FR
Can flow direction be detected with power Doppler?
Not usually - power directional Doppler must be used to determine direction of flow
What is color Doppler?
Stationary reflectors make up the gray scale portion of the image
Moving reflectors generate a Doppler shift and are shown within boundaries of color box
What creates the Doppler shift in color Doppler?
Interference with moving RBCs
What does color Doppler represent?
Mean frequency shift
What is the significance of color coding in color Doppler?
Corresponds to the direction and velocity
How does color Doppler create a real time image of frequency shifts related to movement?
Uses hundreds of scan sites along multiple scan lines
What is autocorrelation?
Uses 6-20 pulses per scan line to provide info about received echos
What information does a returning echo provide?
- direction
- average frequency shift
- power or amplitude
- variance
Is color Doppler qualitative or quantitative?
Qualitative
Why does color mode degrade the B mode image?
Because color mode reduces the PRF
When adding color flow to imaging, what happens to the frame rate?
FR decreases
Does the height affect the FR?
No
What happens to the FR when the box is narrowed?
FR increases
What is the color flow derived from?
Weak echos from RBCs
What type of bar or wheel color maps are available?
Shifting hue
Changing shade
Variance maps
What is the most common color map?
Shifting hue
What is another name for changing shade?
Saturation
Which color map has the ability to “tag” certain frequencies?
Variance maps
How do you optimize velocity range?
High flow/ high PRF
Low flow/ low PRF
How does the FOV (depth) affect color optimization?
Greater depth to an area of interest degrades the image and ability to display flow
What happens when the Color scale (PRF) is to low?
Aliasing
What happens to the image when the Color scale (PRF)to to high?
Flow not being detected and/or poor color fill in
What is the most underused control?
Gain
What results when insufficient gain is used?
Poor color fill in
How do you properly adjust for gain?
Increased to a point of blooming/bleeding artifact then back to the thereshold
What’s the threshold for gain?
Gain level just before blooming occurs
What is color priority?
Image processing priority- color vs grey scale
What happens when the wall filter is set to high?
Eliminate low flow information (near baseline)
Why should the baseline be adjusted to the middle of scale?
To accommodate more red or blue velocities (not usually done)
When is power/angio used?
In cases of trickle flow or low flow states
What is mirror image artifact?
Reflection of the image produced below
Seen in grey scale, color and spectral
See pg. 17
What can occur if your color gain is set to high?
May obscure pathology
When would color flash occur?
When transmitted pulses or adjacent motion will cause color to flash outside the vessel (heart motion)
What is visible bruit?
Soft tissue vibration adjacent to area of high flow
