Doppler Flashcards
What is the Doppler principle?
The physical principle used to measure the velocity of blood in circulation
Define demodulation in the context of Doppler.
The process of extracting the low Doppler frequency from the transducer’s carrier frequency
What are positive Doppler shifts?
When blood cells move toward the transducer, causing the reflected frequency to be higher than the transmitted frequency
What are negative Doppler shifts?
When blood cells move away from the transducer, causing the reflected frequency to be lower than the transmitted frequency
What does the Doppler shift represent?
The difference between the reflected frequency and the transmitted frequency
Fill in the blank: Doppler shift (Hz) = _______.
reflected freq. - transmitted freq.
What is measured velocity in Doppler?
Defined by a magnitude and a direction
What is the formula for Doppler shift?
Doppler shift = 2 x velocity of blood x transducer freq. x cosine
What does the cosine factor represent in the Doppler shift formula?
The angle between sound and vessels
What does bidirectional Doppler distinguish?
The direction of flow toward or away from the transducer
What does a spectral tracing below the baseline indicate?
A negative shift or flow away from the transducer
What does a spectral tracing above the baseline indicate?
A positive shift or flow toward the transducer
What is Continuous Wave (CW) Doppler?
A commonly used signal processing technique for bidirectional Doppler
What are the advantages of CW Doppler? List at least two.
- Ability to accurately measure very high velocities
- Very small Doppler shifts can be detected
What is a disadvantage of CW Doppler?
Cannot determine the exact location of moving blood cells due to range ambiguity
What does range ambiguity refer to?
The inability to determine depth of reflections due to overlap of transmit and receive beams
Fill in the blank: Simultaneous anatomic imaging and Doppler fusion is achieved with _______.
2D US + MRI/CT
What does duplex imaging/scanning refer to?
Velocities sampled every second in blood circulation
What are the two Doppler shifts in the Doppler shift formula?
- Reception of sound wave by moving RBC
- Transducer’s reception of the sound wave from the moving RBC
What is the significance of the number 2 in the Doppler shift equation?
Represents that there are actually 2 Doppler shifts involved
What does PW Doppler stand for?
Pulsed Wave Doppler
PW Doppler is a type of ultrasound that measures blood flow velocities.
What is range resolution in the context of PW Doppler?
The ability to select the exact location where velocities are measured
This is achieved by placing a small marker called the gate or sample volume on a 2D image.
What is aliasing in Doppler ultrasound?
Inaccurate measurement of high velocity signals
It can result in high velocity flow being displayed as traveling in the opposite direction.
What is the Nyquist limit?
The highest Doppler frequency or velocity that can be measured without aliasing
It is defined as Nyquist limit (Hz) = PRF / 2.
What does the color map do in Color Flow Doppler?
Converts measured velocities into colors on the image
It displays flow direction and velocity through a vertical bar of various colors.
What is the primary advantage of PW Doppler?
Ability to select the exact location for velocity measurement
This is facilitated by the use of a gate or sample volume.
Fill in the blank: The most common error associated with Doppler ultrasound is that high velocity flow in one direction is incorrectly displayed as traveling in the _______.
opposite direction
What are the two most common dictionaries used in Color Flow Doppler?
- Velocity mode
- Variance mode
True or False: Aliasing never occurs with Continuous Wave (CW) Doppler.
True
What is the relationship between the amplitude of the reflected signal and the number of blood cells?
Related to the number of blood cells creating the reflection
A higher number of blood cells leads to a stronger reflected signal.
What type of information does Color Flow Doppler provide?
Direction of flow and average or mean velocities
This is done through color-coded velocity information superimposed on a gray scale image.
What is the effect of scanning deeper regions in relation to Doppler sampling rate?
Aliasing can occur if the sampling rate is too low compared to measured blood velocities
This is particularly relevant when measuring high velocity flows.
What is the significance of the black bar in the Color Map?
Indicates ‘no Doppler shift’
It serves as a reference point for determining flow direction.
How does Color Flow Doppler differ from PW Doppler regarding the importance of angle knowledge?
Knowledge of angle is less important than in CW or PW Doppler
This allows for more flexibility in imaging techniques.
What is a characteristic of the Velocity mode in Color Flow Doppler?
Reports average or mean velocities
This mode provides a simplified representation of flow dynamics.
Fill in the blank: Color Flow Doppler is considered _______.
semi-quantitative
What is Variance Mode used for?
To distinguish laminar flow from turbulent flow
Variance Mode provides color change side-to-side as well as up and down.
What are Doppler Packets?
Groups of multiple ultrasound pulses used to determine blood velocities
Larger packets or longer ensemble lengths result in more accurate velocity measurements.
What is Power Doppler?
Non-directional color Doppler that detects flow without velocity or direction information
The amplitude of the reflection is directly related to the number of moving blood cells.
What does Spectral Analysis involve?
Breaking complex signals into basic building blocks to identify velocities
It can be performed using methods like Fast Fourier Transform (FFT) and Autocorrelation.
Fill in the blank: The digital technique used to process both PW and CW Doppler signals is _______.
[Fast Fourier Transform (FFT)]
What is the purpose of the Spectral Window in Doppler analysis?
To indicate the range of velocities in a spectral trace
A clear spectral window suggests laminar flow, while spectral broadening indicates turbulent flow.
What are the advantages of using Fast Fourier Transform (FFT)?
- Exceedingly accurate
- Displays all individual velocity components
- Distinguishes laminar from turbulent flow
What are the disadvantages of using larger Doppler packets?
- More time needed to acquire data
- Reduced frame rate
- Decreased temporal resolution
- Subject to aliasing
True or False: Color change in Velocity Mode is always up and down.
True
What is Autocorrelation?
A digital technique used to analyze color flow Doppler signals
It is substantially faster than FFT but less accurate.
What does a filled spectral window indicate?
Turbulent flow
It represents a wider range of velocities and Doppler shifts within the sample volume.
Fill in the blank: The packet size should be selected to balance accurate velocity measurements and adequate _______.
[temporal resolution]
What is the primary concern with conventional color flow Doppler?
Prone to aliasing and lower frame rates
This reduces temporal resolution.
What is the effect of slight motion of the transducer or patient on Power Doppler?
Prone to ‘lash artifact’
This can affect the accuracy of the flow detection.
What is Doppler Shift measured in?
Hertz
What factors are directly related to Doppler Shift?
- Velocity
- Transducer frequency
- Cosine of the angle between flow direction and sound direction
What is the effect of an angle of 30 degrees on aliasing?
Less aliasing
What angle results in maximum aliasing?
180 degrees
What factors affect aliasing?
- Slower blood velocity
- Lower frequency transducer
- Shallow gate (high PRF)
What is the first technique to eliminate aliasing?
Adjust the scale (PRF) to its maximum
What is the second technique to eliminate aliasing?
New, shallower view (sample volume)
What is the third technique to eliminate aliasing?
Lower transducer frequency
What is the fourth technique to eliminate aliasing?
Use baseline shift (down/zero)
What does range resolution affect?
The ability to distinguish between two closely spaced objects
What is the maximum velocity limitation related to?
Nyquist limit
What is the result of increasing the Nyquist limit?
Increases the ability to measure maximum velocity with Doppler
What is the disadvantage of using a higher frequency transducer?
Less sensitive to very small Doppler shifts/low velocities
What is a characteristic of Continuous Wave (CW) Doppler?
No aliasing
What is a consequence of using a deep gate in Doppler?
Increases Nyquist limit
Fill in the blank: Aliasing remains, but display is more ______.
appealing
True or False: Aliasing is ineffective if the Doppler shift wraps around itself completely.
True
What happens when the sample volume is shallower?
Less aliasing
What is the effect of a lower frequency transducer on aliasing?
Reduces aliasing
What is a key component of a Pulsed Doppler Transducer?
At least one crystal
The crystal is essential for converting electrical signals into ultrasound waves.
What type of backing material is used in a Pulsed Doppler Transducer?
Dampened PZT
PZT stands for lead zirconate titanate, which is commonly used in ultrasound transducers.
What is the Q-factor of a Pulsed Doppler Transducer?
Low Q-factor
A low Q-factor contributes to a wider bandwidth in the transducer’s operation.
What is one advantage of using higher frequency in Doppler imaging?
Improves resolution
Higher frequency ultrasound waves provide better detail in imaging.
What is the incidence angle for normal Doppler imaging?
90 degrees
Normal incidence is crucial for accurate Doppler measurements.
What phenomenon is characterized by the appearance of a second vessel color flow below the anatomical region of interest?
Mirror image
This artifact occurs in color flow Doppler imaging due to reflection.
What is the primary purpose of Color Flow Doppler?
Provides 2D flow information directly on anatomic image
This allows visualization of blood flow patterns in relation to structures.
What type of Doppler modality is most sensitive?
Continuous Wave
Continuous Wave Doppler is designed to detect high-velocity blood flow without aliasing.
What is a disadvantage of using Pulsed Wave Doppler?
Subject to aliasing
Aliasing occurs when the velocity exceeds the Nyquist limit.
Fill in the blank: The __________ mode is used with low velocity or small volume blood flow.
Power Mode
Power Mode is particularly effective for detecting low-velocity flows.
What is the effect of increasing the Wall Filter in Doppler imaging?
Rejects low-frequency Doppler signals
This helps to eliminate clutter and improve the quality of the Doppler signal.
What is the minimum number of crystals required for a Continuous Wave Doppler Transducer?
At least 2 crystals
Continuous Wave requires two crystals for simultaneous transmission and reception.
True or False: Pulsed Wave Doppler provides good range resolution.
True
Pulsed Wave Doppler is known for its ability to accurately identify the location of flow.
What happens to the temporal resolution in Color Flow Doppler due to multiple packets?
Reduced temporal resolution
Using multiple packets can slow down the acquisition of frames, affecting temporal resolution.
What is a common artifact seen in Color Flow Doppler imaging?
Clutter
Clutter can obscure true blood flow signals, making interpretation difficult.
What does the term ‘range ambiguity’ refer to in Doppler modalities?
Ambiguity in determining the exact location of flow
This can occur in Continuous Wave Doppler due to its inability to provide depth information.
What is the primary measurement outcome of Doppler modalities?
Peak velocity measurements
Doppler imaging primarily measures the maximum velocity of blood flow.
Fill in the blank: The __________ spectrum appears both above and below the baseline in Pulsed Wave Doppler.
Mirror image
This occurs due to reflections and can complicate the interpretation of flow.
