Module 5- Just Doppler Imaging Flashcards
What are other names for the Doppler effect?
- Doppler Shift
- Frequency Shift
- Doppler Frequency
- Beat Frequency
What is the Doppler effect?
A change in the observed/received frequency of a wave due to the motion of the source, the observer, or both (relative to each other)
What is the Doppler effect used for?
Used to determine:
- The velocity of the moving reflectors (red blood cells)
- Direction of flow
What is a positive Doppler shift?
Flow towards the transducer
What is a negative Doppler Shift?
Flow away from the transducer
What happens if the receiver is moving towards the stationary source?
- Increase in frequency
- sound source sends out a wave
- the receiver meets each wave-crest more quickly than if stationary
- the receiver detects more complete wavelengths per second (so a higher frequency)
What happens if both the sound source and receiver are stationary?
No change in frequency (between the transmitted and received frequency)
What happens if the receiver is stationary and the source is moving away from it?
- Decrease in frequency
- Creates a longer wavelength and a lower frequency
What happens if the receiver is stationary and the source is moving towards it?
- Increase in frequency
- creates a shorter wavelength and a higher frequency
What happens if both the source and the receiver are moving in the same direction at the same speed?
-No Doppler shift is observed
no change in frequency
How can a Doppler shift occur if the source and receiver are both moving in the same direction?
If they are moving at different speeds
What are the advantages of continuous wave Doppler?
- No upper limit to velocity
- No aliasing
- Small probe sizes
- Ability to use higher frequencies
What is a disadvantage of continuous wave Doppler?
Range ambiguity ( can’t tell where the echo is coming from)
In continuous wave Doppler, when is the change in voltage detected?
When the wave returns to the receiver
What is continuous wave Doppler?
- Uses 2 crystals
- The beam is directional, with an area of overlap which is where the Doppler signals will read
How is the frequency of CW Doppler determined?
The applied voltage
What is the advantage of Pulsed wave Doppler?
Range resolution (knowing where the Doppler signals are coming from)
What is the disadvantage of Pulsed wave Doppler?
Aliasing
What is pulsed wave Doppler?
- 1 crystal
- A sample volume or gate is placed in the vessel from which the Doppler info is desired
What is range resolution?
- Only signals (reflectors) from within the sample gate will be processed
- Allows the sonographer to know exactly where the Doppler information is being derived from
- The opposite of range ambiguity
What is the Nyquist limit?
1/2 PRF (the PRF of the Doppler)
-The level at which aliasing will begin to occur
TRUE/FASLE: Pulses used in Pulsed Doppler are longer than pulses used in imaging systems.
TRUE
Are longer or shorter pulses needed to properly determine Doppler frequency of returning echoes?
Longer
What is the formula for Doppler Shift (∆F)?
c
(Fo being incident frequency)
(v = velocity of RBCs)
(cosθ being the cosine of the incident angle)
What is the cosine of 0°?
1
What is the cosine of 45°?
.70 (.76 to Sherrill)
What is the cosine of 60°?
.5
What is the cosine of 90°?
0
What is the Doppler shift formula written to calculate the reflector speed?
2 x Fo x cosθ
(∆F being the Doppler shift)
(Fo being the incident frequency)
If there is a lower receiving frequency than sending frequency, which direction is it going?
Away (final is greater than initial)
If there is a higher receiving frequency than sending frequency, which direction is it going?
Towards (final is less than initial)
What is the relationship between the Doppler shift (∆F) and the angle (θ)?
- ∆F increases
- θ decreases
What must be know to calculate an accurate velocity?
The cosine of the angle
What scenario would you not need the cosine of the angle to determine the velocity of the flow?
If you are dead on to the flow (coming straight at you)
What is the relationship between Doppler shift (∆F) and Incident frequency (Fo)?
DIRECT
- ∆F increases
- Fo increases
What is the relationship between Doppler shift (∆F) and velocity of the reflector?
DIRECT
- ∆F increases
- v increases
What is the relationship between cosθ and Doppler shift (∆F)?
DIRECT
- cosθ increases
- ∆F increases
What is the relationship between the cosθ and the angle (θ)?
INVERSE
- cosθ increases
- θ decreases
TRUE/FALSE: The Doppler shift frequency does not occur in the audible range.
FALSE
it is within audible range, so sonographers can hear the change in velocity on the speakers
What happens if there is a high Doppler shift?
Aliasing
A _____ angle yields a greater percent error in Doppler?
Higher
the bigger the angle the bigger the mistake
What is the relationship between the PRF and depth?
Deeper depth = lower PRF
can’t detect higher velocities
When do you get range ambiguity artifact?
If a pulse is sent out before sufficient time has passed for returning echoes to be received (there will be uncertainties about the range from which the signal came from)
To measure velocity accurately, PRF must be:
At least twice as fast as the Doppler shift at its highest velocity
What is the spectral analysis?
- Allows visualization of the Doppler signal
- Provides quantitative data that can be used for evaluating the Doppler shift
What is aliasing?
-Occurs when the Nyquist limit is exceeded
-Creates a misleading pattern of abnormal flow
(wrap-around pattern)
-Frequencies appear both above and below the baseline
What is the quantitative and qualitative analysis of spectral Doppler?
Range and distribution of frequencies obtain
What are the methods that are utilized with spectral Doppler analysis?
- Zero crossing detection
- Fast Fourier transforms (FFT)
What is zero crossing detection?
- Counts the number of times the oscillating Doppler signal crosses the “zero volts line” during small time intervals
- Often applied to physiological recording devices and displayed along with other traces such as ECG and arterial pressure signals
What is Fast Fourier transforms (FFT)?
- Performs signal processing or spectral analysis
- Digital computer technique
- Current method
- Separates complex signals into each individual frequency
- Separates signals into how many speeds, and how many at each speed
For the spectral analysis, what is on the horizontal axis and what is on the vertical axis?
Horizontal: time
Vertical: velocity or frequency
What are the quantitative and qualitative aspects of the spectral analysis?
Quantitative: amplitude (with a value)
Qualitative: flow characteristics
What is another name for a wall filter?
Reject or threshold control
TRUE/FALSE: The waveform magnitude/brightness does not represent the Doppler velocities or frequency shift.
TRUE (it is just the amplitude that the operator changes, and not something that the signal is returning)
If the RBCs are moving towards the transducer:
- Final frequency is greater than the initial
- Positive Doppler shift
If the RBCs are moving away from the transducer:
- Final frequency is lower than the initial
- Negative Doppler shift
What does CFM stand for?
Color flow mapping
What does CFI stand for?
Color flow imaging
What velocities are shown in color flow imaging?
Mean/average velocities
TRUE/FALSE: Color flow imaging is a form of pulsed wave Doppler.
TRUE
In Color flow Doppler, what is a packet?
- Area of interest, where many pulsed Doppler lines and gates are over
- Typically composed of 3-30 pulses per line of color
TRUE/FALSE: Lighter colors/hues are represent slower flow.
FALSE
lighter hues represent faster flow, darker hues represent slower flow
What is variance mode?
- Red and blue are in the vertical orientation
- Additional colors are added in a side-to-side orientation (usually green or yellow)
- Used to tag turbulent flow
What is the disadvantage of color Doppler?
Aliasing (limited by the Nyquist limit)
What are other names for power Doppler?
- Color angio
- Color power
- Energy mode
What is color power/energy mode?
- Estimates the total strength of the Doppler signal within each gated region
- Relies mainly on the amplitude strength of the Doppler signal
- The net signal and the value displayed are related to the number of RBCs moving within the gated region regardless of the velocity
What are the advantages of color power/energy mode?
- Does not rely on velocities or reflectors
- No aliasing
- Better edge definition
- Better at demonstrating flow continuity
- Not angle dependent
- Increased sensitivity to very low flow such as venous flow and flow in smaller vessels
What displays blood flow velocities in two dimensions?
Color flow mapping
What is called the multi-gated Doppler technique?
Pulse echo technique
What is saturation?
- The purity of the hue
- More saturated means it doesn’t have as much white (slower flow)
- Less saturated means the color has been diluted with white (faster flow)
_____ Doppler shifts are displayed by decreased color saturation.
Higher
_____ Doppler shifts are displayed by increased color saturation.
Smaller
Will higher or lower velocities be displayed with a more saturated color?
Lower
the more true (saturated) the color, the slower the flow is
What is the hue associated with?
The wavelength of the light
What is aliasing sometimes called?
- Onion skin effect
- Candle flame
What kind of flow causes a mosaic color Doppler appearance?
Turbulent flow
What kind of color will have a pure hue, and only one wavelength?
Saturated color
What type of flow has layers of blood that travel at individual speeds and are aligned in a parallel fashion?
Laminar
Types: Plug & Parabolic
Parabolic= bullet shaped profile where the fastest velocities are found in the center
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What type of flow has chaotic pattens with patterns in many different directions at different speeds?
Turbulent
Will be seen DISTAL to stenosis within a vessel
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What type of flow has all blood cells and layers traveling at the same velocity and in a parallel fashion?
Plug
Found in larger diameter vessels (Aorta)
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What is the range-gate?
- “sample gate” “sample volume”
- isolates signals from the desired depth
How are Doppler signals received?
-Signals enter the receiver
-Signals are amplified and sent to the demodulator
(if the reflectors within the gated area are moving, then the echoes collected are of different phase and this difference shows up in the demodulator)
What is the phase quadrate detector?
- Compares the phase of the two waves (one transmitted one received)
- The resulting phase tells the machine what direction the Doppler shift was
What is luminance?
- Corresponds to greyscale brightness
- Represents echo amplitude
The faster a velocity is, the ______ white is added.
More
What is ensemble length?
Number of pulses sent out for each scan line
What is dwell time?
- Time required to interrogate one line of site
- The longer the dwell time, the higher the frame rate
What is the imaging field made up of with the pulse echo technique?
- Line segments
- Each line divided into many gated regions
What are the advantages of autocorrelation techniques when evaluating color Doppler?
- More accurate velocity measurement
- More sensitive to low flow
- Frame rate is lower thus decreasing temporal resolution (very fast collection method)
What is time domain processing, and how is it accomplished?
- Determines the velocity of a moving reflector by measuring the change in spatial location during a know time interval
- Uses a wide bandwidth which improves spatial info on image
- Sampling along a single scan line is accomplished with a high PRF to maintain adequate temporal resolution
- Doppler angle must be known
What is an asynchronous system with color flow imaging?
- Collects images and flow data at different times
- Superimposes the information
- Field of view is scanned TWICE for each frame (so frame rate is reduced
What is the advantage of using an asynchronous system with color flow imaging?
Transducer frequency and transmit power can be adjusted separately
What is the disadvantage of using an asynchronous system with color flow imaging?
Worse frame rate (because each frame is scanned twice
What is the advantage of using a synchronous system with color flow imaging?
Higher frame rates are possible
Aside from hue and saturation, what is the third attribute used to describe the color in an image?
Luminance
How does autocorrelation work?
- Series of pulses are transmitted
- Once they return, the color flow detector compares the phases of signals from one pulse with the phases of signals from the previous pulse at each pixel
What is the significance of autocorrelation?
- Self comparison among measurements for the same reflector
- Each echo is correlated with the corresponding one from the previous pulse
- Determines the motion that has occurred during each pulse period
- Gives a value for the average flow velocity, direction of flow, and spread about the mean
