Chapter 19 Flashcards
Creation of everyday doppler shift
The change in frequency as a result of relative motion between the sound source and the receiver
Doppler shifts
are created when transmitted sound waves strike moving red blood cells
Positive Doppler Shift
When red blood cells move toward the transducer
The reflected ƒ is higher than the transmitted ƒ
Negative Doppler Shift
When red blood cells move away from the transducer
The reflected ƒ is lower than the transmitted ƒ
The 2 represents the fact that there are two Doppler shifts during an ultrasound exam
First Shift: Receiver (RBC) moves towards or away from the sound source (probe).
Second Doppler Shift: The RBC is hit with the beam and reflects it back towards the probe, thus becoming the sound source while the probe is now the receiver
Nondirectional Doppler systems
simply measure the presence of moving blood cells by detecting a Doppler shift
Bidirectional Doppler
Bidirectional distinguishes the direction of flow
Doppler spectrum
Flow towards is displayed above the baseline
Flow away is displayed below the baseline
Phase quadrature (quadrature detection)
is the commonly used signal processing technique used to identify direction in bidirectional Doppler systems
Continuous Wave Doppler
Requires two crystals:
One constantly transmits while the other continuously receives the reflected signals from RBC
Advantage: because it is constantly sampling the motion of the red blood cells, CW can accurately measure very high velocities
Disadvantage: Doppler shifts and velocities are arising from the entire length of the beam; therefore, uncertain from where the highest velocity came
A second disadvantage is reflections from deeper RBCs will have lower amplitude
range ambiguity
Doppler shifts and velocities are arising from the entire length of the beam
Duplex imaging
When there is a 2D reference image and PW or CW Doppler graph/display on the screen
interval display
Older systems periodically briefly interrupt the Doppler display (thick black line seen) to update the 2D reference image
simultaneously (display)
both the 2D and the Doppler display are live (the reference image is not frozen and no black update lines interrupt the Doppler display)
dedicated CW or “Pedoff” probe
Cross shaped wand with a spherical footprint
2 crystals in the shape of a semicircle
Pulsed Wave Doppler
One crystal alternates between sending and receiving ultrasound pulses
Advantage: Selecting the exact location of Doppler interrogation called range resolution, range specificity or freedom from range ambiguity artifact
Disadvantage: inaccurate measurement of high velocity signals
Continuous Wave Transducers
While performing CW Doppler with a conventional imaging array probe, a select pair of crystals (and associated electronics) are used for the Doppler
Pulsed Wave Doppler Transducers
is accomplished by a single crystal in the imaging probe
Aliasing
presents as high velocities being cut off and wrapped around our Doppler baseline and displayed as if they are moving in the opposite direction
With audio, aliasing presents as sound arising from the incorrect speaker
the Nyquist Limit
when aliasing will occur in PW Doppler
these things will reduce aliasing
Imaging shallower (increase PRF)
Lower velocities (decrease DS)
Using a lower frequency probe (decrease DS)
Five techniques may be used to avoid aliasing artifact
1: Increase the scale to its maximum
#2: Select a new view with a shallower sample volume
#3: Select a lower frequency probe
#4: Shift the baseline so that the entire velocity scale is devoted to one direction
#5: Switch to continuous wave Doppler
Color Flow Doppler
reports average or mean velocities
Color Maps
Color Doppler uses a “legend” or “key” to assign the mean velocities at each location a color
Color Map: No flow
black line and the color black represent no Doppler shift
Velocity Mode Color Maps
inspect any horizontal line on a velocity map there is no change in color from side to side
Variance Mode Color Maps
inspect any horizontal line on a variance map there is a change in color from side to side
colors to the left represent laminar flow, colors to the right turbulent flow
Color flow does not angle correct
only the mean velocity of parallel flow is close to the velocity indicated on the color map
Aliasing occurs in color flow imaging
When color flow aliases it does so in a wrap around fashion relative to the color bar
help to remove aliasing from a color Doppler display
-Adjust the color scale (PRF) to its maximum (low velocities will no longer get assigned a color)
-Select a lower frequency transducer
-Try to obtain a shallower color sector
-Baseline shift may suffice to unwrap the signal on the display
Transverse Flow
(cross sectional flow) requires that we look at the direction of flow relative to scan plane
“BART”
the standard map where Blue = Away and Red = toward
Flow Reversal
flow cuts across the color map baseline
Doppler Packets or ensemble
The number of pulses sent down each scan line in your color sector
Larger packets (larger ensemble) have advantages
Main: color fill in
More accurate velocity information
More sensitive to low flow
The disadvantage to larger packet sizes (sending down more pulses)
is decreased frame rate and temporal resolution
Color Line Density
Choosing a high line density when in color modality, increases the number of lines in the color region of interest
Power Doppler
A color is assigned based on the amplitude (signal strength) of the returning Doppler Shift
AKA: Color Angio, Energy Mode
Advantages to Power Doppler
-Increased sensitivity to low or slow flow such as venous or small vessel flow
-Unaffected by Doppler angles unless the angle is exactly 90 degrees
-No aliasing since the velocity information is ignored
Disadvantages to Power Doppler
-No velocity or direction information
-Low PRF are used to detect slow flow therefore frame rates are lower than conventional color flow, and the lowest of all modalities
-Susceptible to motion of the probe, patient, soft tissue which may result in a burst of color = flash artifact or ghosting
Temporal resolution lowest to highest:
Power Doppler
Color Doppler
2D
single line frames CW/PW Doppler and M-Mode
Crosstalk
Appears as identical Doppler spectrum above and below the baseline
A type of mirror artifact
clutter
small Doppler shifts may be produced by moving anatomy, these low velocity signals appear on the Doppler spectral tracing
ghosting artifact
small Doppler shifts may be produced by moving anatomy, When these low velocity signals are assigned a color
Wall filters
-eliminate small Doppler shifts or slow flow around the baseline on the Doppler spectrum
-eliminate color arising from slow velocity reflectors such as tissue
Tissue Doppler
At times we do determine the Doppler shifts and velocities coming off the walls of the left ventricle
Spectral Analysis
Reflections arising from a mass of moving blood cells will have many Doppler shifted
identifies the individual Doppler shifts
Two methods of digital spectral analysis are currently used
- Fast Fourier Transform (FFT)
- Autocorrelation
Fast Fourier Transform
-is a digital computer-based technique used to process both PW and CW Doppler signals
-exceedingly accurate
-It displays all individual components (shifts) that make up the complex reflected signal (every shift/velocity gets a brightness dot)
Several situations will increase spectral broadening:
- Turbulent flow
- Continuous wave Doppler
- Pulsed Doppler with a large sample gate
The Doppler shift
The difference in frequency between what is transmitted and received
