Chapter 19 Pt. 2 Flashcards
The most common error associated with Doppler Ultrasound
Aliasing
High velocity flow in one direction that is incorrectly displayed as traveling in the opposite direction is?
Aliasing
The very top of the display is called?
Nyquist limit
Part of the spectrum appearing at the bottom
Aliasing
The highest Doppler frequency or velocity that can be measured without the appearance of aliasing
Nyquist frequency (limit)
Nyquist limit is ___________ of the pulse repetition frequency (PRF)?
half
Two ways to avoid aliasing are:
- raise the nyquist limit
2. reduce the Doppler shift
Higher frequency transducers create________ aliasing,
more
Higher frequency transducers create _________Doppler shifts and lower frequency transducers create ____ Doppler shifts
Higher, lower
5 techniques used to avoid aliasing
- Adjust the scale to its max
- Select a new ultrasonic view with a shallower sample volume
- Select a lower frequency transducer
- Use baseline shift
- Use continuous wave doppler
Color Doppler measures. . .
average or mean velocity
Spectral Doppler (continuous and pulsed) measures
peak velocity
In Velocity mode the colors above the black stripe indicate?
Blood cells moving towards the transducer, Positive Doppler shift
Colors below the black stripe indicate?
Blood cells moving away from the transducer, Negative Doppler shift
The black region in the middle of the color map indicates:
No Doppler shift
Variance mode distinguishes
laminar and turbulent flow
In variance mode, colors of the left side of the map indicate?
Laminar flow
In Variance mode, colors on the right side of the map indicate?
Turbulent flow
Multiple pulses used in color Doppler to accurately determine blood velocity
Packet or ensemble
Power mode is
Nondirectional Doppler
Power Doppler is also called
energy mode or color angio
On Spectral displays, low frequency Doppler shift artifacts are called
Clutter
With color Doppler low frequency Doppler shift artifacts are called?
Ghosting
Wall filters are used to eliminate what?
Clutter
a special form of “mirror” image artifact, that arises only with spectral Doppler
Crosstalk
a tool that breaks the complex signal into its basic “building blocks” and identifies the individual velocities that make up the reflected Doppler signal
Spectral analysis
Two methods that spectral analysis is currently used for?
- Fast Fourier Transform (FFT)
2. Autocorrelation
A digital technique, used to process both pulsed and continuous wave Doppler signals
Fast Fourier Transform (FFT)
Two advantages of FFT
- exceedingly accurate
- displays all individual velocity components that make up the complex reflected signal
Spectral display of an FFT distiguishes
laminar from turbulent flow
a wider range of velocities and Doppler shift within the sample volume
Spectral broadening
a digital technique used to analyze color flow Doppler
Autocorrelation
Identifies the highest velocity jets anywhere along the length of the ultrasound beam
Continuous wave Doppler
Accurately identifies the location of flow
Pulsed wave
Provides two dimensional flow information directly on anatomic image
Color flow
Used with low velocity or small volume blood flow
Power mode
With what Doppler modality does Range ambiguity occur?
Continuous wave Doppler
With what Doppler modality does Range resolution occur?
Pulsed wave Doppler, Color Flow, and power mode
Doppler modality that is most sensitive?
Continuous wave
Pulsed wave Doppler is ____________ sensitive?
Moderately
Moderately sensitive and the size of the color jet is most affected by color Doppler gain settings
Color flow
What Doppler modality has greater sensitivity than color flow?
Power mode
What Doppler modalities have Very good temporal resolution?
Continuous wave and pulsed wave
Color flow has reduced temporal resolution due to ___________?
multilple packets
What Doppler mode has the lowest temporal resolution?
Power mode
__________has no aliasing
Continuous wave
This modality is subject to aliasing
Pulsed wave
Power mode is subject to __________ artifact, not to aliasing
flash
Based on pulsed ultrasound, color flow is subject to
aliasing
Which Doppler measures mean velocities
Color flow
Which Doppler measures Peak velocities
Continuous wave and pulsed wave
Which Doppler measures no velocities
Power mode
Doppler shift is directly related to
velocity & transducer frequency
Doppler shift is _______________ related to the cosine of the angle between the directions of flow and the direction of sound
directly
CW transducer do not use __________
Backing material
No use of backing material results in
- undampened transmitted signal
- narrow bandwidth
- high quality factor
- higher sensitivity
CW transducers have ___________and they detect ___________ amplitude reflections and small Doppler shifts
increased sensitivity , low
Pulsed Doppler Transducers are ________ transducers and contain__________
imaging, backing material
Since PW transducers have backing material they have
- low quality factor
- lower sensitivity
- wide bandwidth pulses
Nyquist limit formula
nyquist limit(Hz) = PRF (Hz)/2
- slower blood velocity
- lower frequency
- shallow gate (high PRF)
Less aliasing
- Faster blood velocity
- Higher frequency
- deep gate (low PRF)
More aliasing
Doppler shift formula
Doppler shift (Hz)= reflected frequency- transmitted frequency
What two techniques to avoid aliasing have no disadvantages?
Selecting a new ultrasonic view, and lower frequency tranducer
