17 Doppler Flashcards
What is Doppler shift or frequency?
Change or variation in frequency of sound as a result of motion between the sound source and receiver
What is a positive change or shift?
When source and receiver are approaching each other
What is a negative change or shift?
When source and receiver are moving apart
How is Doppler created?
When sound reflects off of RBCs
What is the range of Doppler shift?
20Hz - 20 kHz
What is demodulation?
Extraction of the Doppler frequency from the transducer frequency
How is bidirectional Doppler analyzed?
With phase quadrature processing
What is the equation for Doppler shift?
Doppler shift = received frequency - transmitted frequency
FD = FR - FE, FE = emitted freq, FR = reflected freq
What is THE Doppler equation?
Doppler shift = (2 x reflector speed x incident frequency x cos(angle)) / propagation speed
What is Doppler shift directly related to?
Blood cell speed
Frequency of the transducer
Cosine of the angle between flow and the sound beam
What is Doppler inversely related to?
Speed of sound in the medium
What does Doppler measure and not measure?
Measures velocity, not speed
What is the equation for measured velocity?
Velocity (measured) = true velocity x cos(angle)
What is the # of crystals in a continuous wave doppler transducer?
2 crystals
1 is continuously transmitting and the other is continuously receiving
What is range ambiguity?
Echoes that arise from entire length of overlap between the transmit and receive beams
What is the advantage of continuous wave doppler?
High velocities are accurately measured
How many crystals are in a pulsed wave doppler?
1 crystal that alternates between sending and receiving
What is the advantage of a pulsed wave doppler?
Echoes come only from the area of interrogation, the sample volume or gate
What is the disadvantage to pulsed wave doppler?
Aliasing, errors in measuring high velocities
What is the horizontal and vertical axes of a Doppler spectrum?
Horizontal - time
Vertical - frequency or velocity
What is aliasing?
When high velocities appear negative with pulsed Doppler
High velocity measurements are inaccurate if PRF is too low in comparison to the measured Doppler shift
What is Nyquist frequency?
Doppler frequency at which aliasing occurs
= to 1/2 the PRF
How is aliasing eliminated?
Use continuous wave Doppler
Select a transducer with a lower frequency
Select new view with a shallower sample volume
Increase the scale, same view
Baseline shift
What will reduce Doppler shift and shrink the spectrum?
Lower frequency transducers
What occurs with a shallow sample volume?
PRF and therefore Nyquist frequency is higher
What is the relationship between sample volume size and doppler spectrum?
Smaller sample volumes (gates) create Doppler spectra with cleaner spectral window
What are the gray shades of a spectrum related to?
Amplitude of the reflected signal
Number of RBCs creating the reflection
What is color flow Doppler?
Form of 2-D multigated Doppler where the shifts are coded into colors and superimposed on a 2-D image
What is color Doppler based on?
Pulsed ultrasound
What is color Doppler subjected to?
Range resolution or specificity
Aliasing
What is velocity vs variance mode?
Look-up tables that color doppler uses to convert measured velocities into colors
Velocity - colors on map do not vary side-to-side
Variance - colors on map vary side-to-side
What do the different colors tell us on velocity mode?
The colors are present information on flow direction only
What do the different colors tell us on variance mode?
The colors provide information on flow direction and the presence or absence of turbulence
What does the appearance of the colors located on the either side of the color bar mean in variance mode?
The right side shows turbulent or disturbed flow that is random and chaotic
The left side shows laminar or parabolic flow which is uniform and smooth
What are Doppler packets?
Multiple ultrasound pulses, or ensemble length
What is associated with a small packet?
Less accurate Doppler
Less sensitive to low velocity flow
Higher frame rate, improved temporal resolution
What is associated with a large packet?
More accurate Doppler
More sensitive to low velocity flow
Lower frame rate, reduced temporal resolution
What are advantages to more pulses in a packet?
Greater accuracy of the velocity measurement
Sensitivity to low flows is also increased with many pulses in the packet
What are disadvantages to more pulses in a packet?
More time is required to acquire the information
Frame rates and temporal resolution are reduced
What is color power Doppler?
Dim-witted color Doppler where the Doppler information regarding direction and actual velocity is not calculated
What are advantages of color power Doppler?
Increased sensitivity to low flows
Not affected by Doppler angles unless the angle equals 90°
No aliasing
What are limitations to color power Doppler?
No measurement of velocity or direction
Lower frame rates
Susceptible to motion of transducer, patient or soft tissues = flash artifact
What is the role of continuous wave Doppler?
Identifies highest velocity jets anywhere along the length of the ultrasound beam; range ambiguity; no aliasing
What is the role of pulsed wave Doppler?
Accurately identifies the location of flow; has good temporal resolution; range resolution, aliasing
What is the role of color flow Doppler?
Provides 2-D flow information directly on anatomic image
Poor temporal resolution because of multiple packets; pulsed ultrasound, subject to range resolution and aliasing
What is the role of power mode Doppler?
Allows the use of color with low velocities or small volumes of blood flow; greatest sensitivity
What is spectral analysis?
Extraction or identification of the individual frequencies making up the complex signal of an echo returning after striking mass of moving blood cells
What are current methods of spectral analysis?
Pulsed or continuous wave Doppler - Fast Fourier Transform
Color flow Doppler – autocorrelation or correlation function
How does anemia affect Doppler?
Hemodynamics may differ from normals
A Doppler exam can always be successfully performed regardless of the patient’s hematocrit
At what angle will a transducer measure the greatest Doppler shift?
0° or parallel
Where must a transducer be located to measure a reflection with the greatest amplitude?
Blood cells that are located closest to the transducer, which also undergo less attenuation
What is ghosting?
Doppler artifact that results from low velocity motion from pulsating vessel walls that can also produce small Doppler shifts that “bleed” into the surrounding anatomy
What is a high pass filter?
Eliminates low magnitude Doppler shifts that are created from moving anatomy rather than red blood cells
What do wall filters serve?
A “reject” for Doppler
They exclude low-level Doppler shifts around the baseline while having no effect on large Doppler frequency shifts
In what type of flow do wall filters eliminate color?
Slow flows
They do not change fast flows
How do you optimize the following color Doppler images?
Too much color – color throughout image, confetti
Too much color – color extending outside of vessel lumen
Color Doppler gain too high – decrease color gain
Color assigned to low velocities – increase wall filter
How is the following color Doppler image optimized?
Right color, color in lumen only – rainbow, multiple colors in lumen
Color aliasing – increase color scale
How are the following color Doppler images optimized?
Too little or no color
90° angle between flow and sound – steer color box or reposition transducer
Color Doppler gain too low – increase color gain
Scale too high, making system insensitive to slow flows – decrease scale
Wall filter too high when actual velocities are slow – decrease wall filter
