Doppler Flashcards
What is a doppler shift?
A change in sound frequency caused by motion between the transmitted and received frequency
Difference between the transmitted and received frequency
Doppler shift = received - transmitted
What is another word to describe doppler shift?
Doppler frequency
What does a positive doppler shift indicate?
The source is moving to the transducer
The reflected frequency is higher than transmitted frequency
What does a negative doppler shift indicate?
The source is moving away from the transducer
The reflected frequency is lower than transmitted frequency
What are typical values for Doppler shift?
20-20,000 Hz
Audible
What is demodulation?
It extracts the lower frequency doppler shift from the higher frequency transducer
What is the doppler equation?
Doppler shift = (2 x velocity of blood x transducer freq. x cos0)/prop. speed
How is doppler shift related to velocity?
Doppler shift directly related to velocity
Faster the velocity, the greater the doppler shift
How is doppler shift related to frequency?
Doppler shift directly related to transmitted sound frequency
Transducer frequency doubled, doppler shift doubled
What happens if doppler angle is not parallel (0 or 180) to the direction of flow?
The doppler shift is not 100%
Velocity is determined by the cosine of the beam angle compared to motion
What is the equation for measured velocity?
Measured velocity = true velocity x cos0
What is the cosine of 0?
1
What percent of doppler shift is measured at 0 degrees?
100%
What is the cosine of 60?
0.5
What is the percentage of doppler shift at 60 degrees?
50%
What is the cosine of 90?
0
What is the percentage of doppler shift at 90 degrees?
0%
What type of signal processing is used for bidirectional doppler?
Phase quadrate or quadrate detection
Flow to transducer is above line, flow away is below line
What does speed measure?
Only magnitude
What does velocity measure?
Magnitude and direction
How many crystals does CW use?
Two
One always transmitting
One always receiving
What are the advantages of CW doppler?
Accurately measure high velocities w/ no aliasing
Higher sensitivity for detecting smaller shifts
What is the disadvantage of CW doppler?
Cannot determine the exact location or depth of the moving blood cells (range ambiguity)
How many crystals does PW doppler use?
Only one crystal must be used
Alternates between sending and receiving sound pulses
What are the advantages of PW doppler?
Select the exact location where velocities are measured
Adjustable receive gate with echoes from one area
What is the disadvantage of PW doppler?
Aliasing occurs and we cannot accurately measure high velocities
What term is used to describe systems that perform imaging and PW doppler at the same time?
Duplex imaging
When does aliasing occur?
When high velocities of PW doppler are incorrectly displayed as being negative
If PRF is too low relative to doppler shift or when doppler shift reaches nyquist limit
What is the Nyquist limit?
The doppler frequency at which aliasing occurs Nyquist limit (kHz) = PRF/2
What are the two main ways to avoid aliasing?
Reduce doppler shift
Raise Nyquist limit
What can you do to reduce doppler shift?
Change to a lower frequency transducer
What can you do to raise the Nyquist limit?
Take a volume from a shallower location because PRF is increased when depth is decreased
Increase the scale, which increases PRF
PRF increase raises Nyquist limit
How might adjusting the scale eliminate aliasing?
It maximizes the PRF, which increases Nyquist limit
How might changing depths eliminate aliasing?
When the sample volume is taken from shallower depth, PRF is increased, raising Nyquist limit and reducing aliasing
How might a lower frequency transducer affect aliasing?
Lower frequency transducers cause lower doppler shifts, which are less likely to exceed Nyquist limit or to alias
How might shifting the baseline affect aliasing?
It slides the baseline down so the entire velocity is in the same direction, eliminating the appearance of aliasing
Only changes scale appearance
What is the advantage of using CW doppler in aliasing situation?
Aliasing does not occur in CW
What two factors affect the shades of gray on the doppler spectrum?
Amplitude of the reflected signal
Number of blood cells causing reflection
Does color doppler use pulsed ultrasound?
Yes
Is color doppler subject to range resolution and aliasing?
Yes
What information does color doppler provide?
Direction or location of flow
Average velocities
What velocities does color doppler measure?
Average or mean velocities
What velocities does spectral doppler measure?
Peak velocities
What information does the color map provide?
Direction and velocity of flow
How can you know if flow has a positive or negative doppler shift based on color map?
Positive is the color above the black stripe
Negative is the color below the black stripe
What is a doppler packet?
A group of multiple pulses that are used to accurately determine RBC velocities using doppler
What are advantages to a longer doppler packet?
More accurate for measurement
More sensitive to low flow
What are disadvantages to a longer doppler packet?
More time is needed to get data
Reduced frame rate and temporal resolution
What are other terms used to describe a doppler packet?
Packet
Ensemble length
What is power doppler?
Non-directional color doppler that displays doppler shift without considering speed or direction
What are other terms for power doppler?
Energy mode
Color angio
What are advantages of power doppler?
Increased sensitivity to low flow
Not angle dependent (unless 90)
No aliasing
What are limitations of power doppler?
No direction or velocity info
Lower frame rate and temporal resolution
Soft tissue, transducer or patient movement may be displayed
What is clutter?
Low frequency shift artifacts on spectral doppler displays created by slow moving anatomy
What is ghosting?
Low frequency shift artifacts on color doppler displays created by slow moving anatomy
What does wall filter do?
Eliminates the low frequency doppler shifts from moving anatomy, but leaves those high frequency from blood cells
What is another name for wall filter?
High pass filters
What is cross talk?
Mirror artifact image that arises from spectral doppler
Shows spectral doppler above and below baseline
When does cross talk occur?
When doppler gain is too high or incident angle is near 90 degrees
What is spectral analysis?
Tool that breaks the complex signal into the basic building blocks, identifying individual velocities that make reflected doppler signal
What are the two methods of spectral analysis?
Fast Fourier Transform
Autocorrelation
What is FFT and what type of signals is it used for?
A digital technique used in PW and CW doppler
It is highly accurate and displays all the individual velocities that comprise the reflected doppler signal
What is laminar flow?
Flow in which most cells move at similar velocities, causing a clear spectral window and a spectral trace as a narrow line
What is spectral broadening?
When many different directions and speeds (wider range) are found in the sample volume
What is autocorrelation and what type of Doppler is it sued for?
It is the digital technique that is used to analyze color flow doppler
It is faster than FFT but less accurate
What will happen to color Doppler if the angle of incidence is 90 degrees?
There is no doppler shift with 90-degree incidence, either color or spectral
No color appears when angle is at 90 degrees
What is the first thing you should do if there is no color flow in a vessel?
The first to do if no color shows in a vessel is to check the angle between flow and sound, making sure it is not 90 degrees
What causes “Color confetti, and what does this look like?
Color confetti is when the color gain is set at a level that is much too high
It looks like color filling the entire color box instead of the appropriate amount of vessel
If you are using a linear probe and have already corrected for angle and there is still no color flow in a vessel what is the next setting you should adjust?
For a linear probe, after adjusting the angle, you should then adjust the color gain to try to get more color in the vessel
In a sector shaped image what is the first setting you should adjust if color flow is not seen?
In a sector image, the first thing you should adjust is the color gain
When using spectral Doppler what will cause gray scale noise to appear throughout the spectrum?
In spectral doppler, noise will be seen throughout the spectrum when the pulsed doppler gain is too high
How will aliasing appear on color Doppler?
It will appear as one color around the vessel wall, but a different color in the center of the vessel that is incorrect
What is the best way to eliminate aliasing on color Doppler?
To eliminate aliasing in color doppler, it is best to increase the velocity scale
For color Doppler how does increasing the scale affect sensitivity?
Increasing the scale decreases the sensitivity to slow or low flows
Low velocities may be lost
What affect does wall filter have on color Doppler?
Wall filter removes the color from slowly moving reflectors (blood cells, vibrating tissues) by eliminating low frequency doppler shifts
What effect does wall filter have on spectral Doppler?
Wall filters eliminate the low velocity flows near the baseline of a doppler spectrum
