Module 5 continued Flashcards
Spectral doppler
What 8 things can be optimised for spectral doppler
SBWSSGC
Scale
Baseline
Wall filter (rejection)
Sweep speed
Sample volume (PW doppler) position/size
Gain
Colour maps
When does spectral broadening occur with relation to the sample volume?
When the sample volume is too big
The maximum detectable velocity
The maximum doppler shift that can be unambiguously displayed on pulsed wave doppler
What is the Nyquist limit? (by definition)
What is the equation for the Nyquist limit
The maximum doppler shift that can be unambiguously displayed.
What is aliasing? When does it occur?
Aliasing occurs when the nyquist limit has been exceeded
How to overcome aliasing
Adjust baseline
Increase scale
Reduce frequency
Reduce sample volume depth
High PRF mode
Use CW doppler
Or
Wall filter
Sample volume size
What is a wall filter
A wall filter in ultrasound, particularly in Doppler ultrasound, is a signal processing tool used to remove low-frequency noise from the Doppler signal. This noise typically arises from the motion of vessel walls, the heart, or other slow-moving tissues. The wall filter is crucial for enhancing the clarity and accuracy of blood flow measurements by filtering out these unwanted signals.
Cardiac Imaging: In echocardiography, wall filters help in distinguishing between blood flow signals and cardiac tissue movements, such as valve motion.
Why cant we detect high velocities with Pulsed wave doppler?
Because its limited by the Nyquist limit
What are some factors leading to low PRF and therefore low Nyquist limit ( because half the PRF is the Nyquist limit)
Nyquist limit
Low Pulse Repetition Frequency (PRF) in Doppler ultrasound can lead to a low Nyquist limit, which can cause aliasing artifacts. Here are some factors that contribute to a low PRF and subsequently a low Nyquist limit:
Factors Leading to Low PRF
Depth of Imaging: The deeper the area being imaged, the longer it takes for the ultrasound waves to travel to the target and back. This requires a lower PRF to allow enough time for the echoes to return before the next pulse is emitted.
Transducer Frequency: Higher frequency transducers provide better resolution but have shallower penetration. To image deeper structures, a lower frequency (and thus lower PRF) may be needed.
Machine Settings: The operator’s settings can affect PRF. When imaging deep vessels or structures, the ultrasound machine automatically lowers the PRF to prevent range ambiguity (confusion about where returning echoes are coming from).
Vessel Size and Location: Larger vessels or those located deeper within the body (like abdominal aorta) require lower PRF settings to capture the Doppler signal effectively.
Impact on Nyquist Limit
The Nyquist limit is directly related to the PRF. It is defined as half the PRF:
PRF
2
Nyquist limit=
2
PRF
A low PRF results in a low Nyquist limit. The Nyquist limit determines the maximum velocity that can be accurately measured without aliasing. Aliasing occurs when the measured Doppler shift exceeds the Nyquist limit, causing high velocities to be misrepresented as lower velocities in the opposite direction.
Consequences of Low Nyquist Limit
Aliasing: When blood flow velocities exceed the low Nyquist limit, they are incorrectly displayed, causing a “wrap-around” effect in the Doppler signal. This makes it difficult to interpret the true velocity and direction of flow.
Managing Low PRF and Nyquist Limit
To mitigate the issues caused by a low PRF and Nyquist limit, the following strategies can be employed:
Adjust PRF: Increase the PRF if possible, while ensuring that the target area is still within the range of detection. This raises the Nyquist limit and reduces aliasing.
Change Angle of Insonation: Adjust the angle of the ultrasound beam relative to the blood flow to optimize the Doppler shift and minimize aliasing.
Use Baseline Shift: Shift the baseline of the Doppler display to accommodate higher velocities within the available range without aliasing.
Utilize High PRF Mode: Some ultrasound machines offer a high PRF mode, which allows for higher PRF settings even for deeper structures by using multiple sampling sites.
Lower Frequency Transducer: Use a lower frequency transducer for deeper structures to balance penetration depth and PRF.
What is tissue doppler imaging?
During a TDI exam, the ultrasound transducer is placed at specific locations such as the mitral annulus, the basal segments of the ventricles, or other myocardial regions. The Doppler signal is then used to measure the velocity of tissue motion at these sites. The resulting data helps clinicians evaluate:
The efficiency of myocardial contraction and relaxation.
The presence of regional wall motion abnormalities.
Overall diastolic function and filling pressures.
3 types of tissue doppler
Colour encoded M mode imaging
Spectral doppler imaging
Colour encoded 2d imaging
Tissue doppler-spectral
-Its a type of Pulsed wave doppler with wall filters reversed
-Displays lower velocity but higher amplitude signals
-Therefore it will accept signals from moving tissue but not moving blood
-Records signals from the myocardium and annuli
