The following techniques can be used to minimise the amount of aliasing. Increase the Nyquist limit (increase velocity scale) Shift the baseline To increase the Nyquist limit in a particular direction Activate high PRF mode (separate post on HPRF coming soon…) Change Doppler angle to minimise the Doppler shift.
- Decreasing the pulse repetition period (PRP) to increase the PRF and the Nyquist limit.
- Applying a low-frequency transducer to create a small Doppler shift for blood flow velocity.
What causes aliasing in ultrasound?
Most sonographers encounter aliasing with pulse spectral Doppler or color Doppler. Pulsed ultrasound doesn’t have a particular upper limit for displaying the Doppler shift. It’s commonly known as the Nyquist limit. High-velocity blood circulation causes Doppler shifts beyond the Nyquist limit resulting in aliasing.
How can I reduce aliasing in my image?
The following techniques can be used to minimise the amount of aliasing. Increase the Nyquist limit (increase velocity scale) Shift the baseline To increase the Nyquist limit in a particular direction Activate high PRF mode (separate post on HPRF coming soon…) Change Doppler angle to minimise the Doppler shift.
How do you avoid aliasing when using a transducer?
Selecting a lower frequency transducer to avoid aliasing. Doppler shift is directly related to the frequency of the transducer. When measured in hertz, at the same velocity, lower Doppler shifts occur with lower frequency transducers (see image).
Is there an upper limit for Doppler aliasing in sonography?
Most sonographers encounter aliasing with pulse spectral Doppler or color Doppler. Pulsed ultrasound doesn’t have a particular upper limit for displaying the Doppler shift.
How can aliasing be corrected?
This can be corrected, if necessary, by oversampling the data. In the frequency direction, this is accomplished by sampling the signal twice as fast. In the phase direction, the number of phase-encoding steps must be increased with a longer study as a result.
How do you correct aliasing of the spectral Doppler waveforms?
Aliasing in spectral Doppler distorts the flow information and must be corrected by adjusting the PRF before velocities can be measured. The PRF should be adjusted so normal flow does not create aliasing.
What is aliasing effect in ultrasound?
In sonographic. Doppler, the result of aliasing is an apparent change in direction of blood flow in. high-velocity areas, producing flow that appears to be backward. Aliasing can occur in pulsed and color Doppler; continuous-wave.
What is the aliasing artifact in ultrasound?
Aliasing artifacts occur when blood flow exceeds the maximum flow speed measurable by the CDI system. For example, Figure 3 shows that flow in the upper branch is fast and exceeds the maximum measurable flow speed on the color scale (25 cm/s).
What are the ways to avoid aliasing?
The solution to prevent aliasing is to band limit the input signals—limiting all input signal components below one half of the analog to digital converter's (ADC's) sampling frequency. Band limiting is accomplished by using analog low-pass filters that are called anti-aliasing filters.
How do you identify aliasing?
You can detect aliasing by running a horizontal test on your oscilloscope. If the shape of the waveform changes drastically, you may have aliasing. You can also perform a peak detect test and if the waveform still changes drastically, aliasing may be an issue.
Which of the following will always eliminate aliasing?
Increasing the Doppler angle may eliminate aliasing. Enhancement weakens the amplitude of structures distally.
What causes aliasing?
Aliasing is Caused by Poor Sampling A bandlimited signal is one with a highest frequency. The highest frequency is called the bandwidth ωb . If sample spacing is T, then sampling frequency is ωs =2π/T.
What is the definition of aliasing?
noun. ali·as·ing ˈā-lē-ə-siŋ ˈāl-yə- : an error or distortion created in a digital image that usually appears as a jagged outline. We commonly observe aliasing on television.
What is the aliasing artifact?
Aliasing artifact, otherwise known as undersampling, in CT refers to an error in the accuracy proponent of analog to digital converter (ADC) during image digitization. Image digitization has three distinct steps: scanning, sampling, and quantization.
How do you get rid of refraction artifacts?
Refraction artifact should resolve if the transducer is moved such that the incident pulse is perpendicular to the interface.
How can true flow reversal be distinguished from aliasing?
How can true flow reversal be distinguished from aliasing? demonstrates "black" between the forward and away flow while aliasing does not.
What is aliasing explain in detail with spectral details of a sampled data?
Aliasing is the effect of new frequencies appearing in the sampled signal after reconstruction, that were not present in the original signal. It is caused by too low sample rate for sampling a particular signal or too high frequencies present in the signal for a particular sample rate.
Which of the following will always eliminate aliasing?
Increasing the Doppler angle may eliminate aliasing. Enhancement weakens the amplitude of structures distally.
How can true flow reversal be distinguished from aliasing?
How can true flow reversal be distinguished from aliasing? demonstrates "black" between the forward and away flow while aliasing does not.
What is the aliasing artifact?
Aliasing artifact, otherwise known as undersampling, in CT refers to an error in the accuracy proponent of analog to digital converter (ADC) during image digitization. Image digitization has three distinct steps: scanning, sampling, and quantization.
How to avoid aliasing artifacts?
You can avoid aliasing artifact through several ways. In this blog post, we will focus on two techniques: 1 Decreasing the pulse repetition period (PRP) to increase the PRF and the Nyquist limit 2 Applying a low-frequency transducer to create a small Doppler shift for blood flow velocity
How is Aliasing Resolved?
You can avoid aliasing artifact through several ways. In this blog post, we will focus on two techniques:
Does lower shifts alias Doppler?
Lower shifts are less likely to exceed the Nyquist limit (or speed limit) and therefore are less likely to alias. There are not considerable disadvantages of this method during a Doppler-only examination. However, during complex real-time imaging, lower frequency sound produces a lower quality anatomic image.
Color Doppler imaging
Color Doppler imaging (CDI) is a non-invasive way to see blood flow in arteries and veins. It is used to identify a blockage, blood clot, or narrowing of the arteries that can lead to deadly clinical outcomes such as a stroke or heart attack.
Aliasing problems in CDI
One recurring issue in CDI is the presence of so-called aliasing artifacts that hinder the visualization of blood flow. Aliasing artifacts occur when blood flow exceeds the maximum flow speed measurable by the CDI system.
Novel deep learning–based solution
The LITMUS group devised a new deep learning–based solution to address these aliasing artifacts in CDI for the femoral artery bifurcation. The femoral artery bifurcation in the thigh was chosen due to its diverse flow properties, including a wide range of flow speeds and multidirectional flow.
Clara Holoscan for real-time de-aliasing in bedside applications
CDI is widely expected to be a point-of-care modality that can be used to gain quick and immediate insights into blood flow conditions in patients. Offline processing would disrupt this utility of CDI, so it is important that the aliasing removal framework be run in real time.
Conclusion
The LITMUS group’s workflow showed how the NVIDIA Clara AGX Developer Kit and the NVIDIA Clara Holoscan SDK can resolve aliasing artifacts in CDI, in real time. Removing aliasing makes image visualization and interpretation easier by removing the ambiguity about the blood flow direction.
About the Authors
Vanessa Braunstein leads healthcare and life science product marketing at NVIDIA for our Clara products in drug discovery, genomics, medical imaging, medical devices, NLP, and smart hospitals.
What is aliasing in ultrasound?
Aliasing is an imaging error which occurs due to under-sampling. The ultrasound system is trying to image an event that is occurring faster than the rate we are sampling it, and as a result the system is uncertain about the direction of the signal and displays this as heading in the opposite direction. We see examples of aliasing in both colour and spectral Doppler, where the velocity exceeds the Nyquist Limit and the image displayed would suggest flow is heading in the opposite direction.
What is the aliasing phenomenon?
Aliasing phenomenon is a concept that causes a lot of uncertainty for many of us. We all have a rough idea of what it is, until we have to explain it to a trainee sonographer doing their best impression of a 3 year old [insert annoying voice here] “… But why???”.
What does aliasing phenomenon mean in Speedracer?
Does understanding the physics detract from the magic of Speedracer? Absolutely not! In both SpeedRacer and during our echo examinations, aliasing phenomenon simply means that something is going fast. This is exciting in the cartoon because you knew that SpeedRacer wasn’t just going for a Sunday drive to the shops, and it is really useful data in our echo exams, alerting us to the higher velocity blood flow present (and possible pathology).
What direction does the line go in continuous imaging?
In the first window (continuous imaging) we see the line heading in a clockwise direction. At a slower sampling frequency (1 frame/revolution) the line appears to be stationary. At an even slower sampling frequency, the line appears to be heading anti-clockwise (opposite direction to reality).
Can you display information on screen without aliasing?
It may be important to display the information on screen without aliasing. For example, aliasing can make visual estimation of regurgitant lesions difficult (often overestimates severity) – try estimating amount of mitral regurgitation with your colour scale set to +0.2/-0.2 m/s….
Is aliasing unique to ultrasound?
Aliasing phenomenon is not unique to ultrasound. I can remember watching the cartoon, SpeedRacer and being fascinated that the wheels of SpeedRacer’s car would start to go in the reverse direction as he sped off.
How to prevent Doppler gain?
How to prevent: Make sure your Doppler gain is not set too high and make sure your angle is not at a 90 degree angle.
What does it mean when a Doppler shift goes beyond half of the PRF?
This occurs when the Doppler shift goes beyond half of the PRF. In ultrasound this is the most common error. Aliasing is a false identity and this appears only with pulsed wave Doppler. That means you will have a very high velocity in one direction and incorrectly displayed as going in the opposite direction. Aliasing occurs with pulsed Doppler never with continuous wave and the Doppler sampling rate is too low in comparison to measure blood velocity.
Color Doppler Imaging
Aliasing Problems in CDI
- One recurring issue in CDI is the presence of so-called aliasing artifactsthat hinder the visualization of blood flow. Aliasing artifacts occur when blood flow exceeds the maximum flow speed measurable by the CDI system. For example, Figure 3 shows that flow in the upper branch is fastand exceeds the maximum measurable flow speed on the color scale (25 cm/s). The colo…
Novel Deep Learning–Based Solution
- The LITMUS group devised a new deep learning–based solution to address these aliasing artifacts in CDI for the femoral artery bifurcation. The femoral artery bifurcation in the thigh was chosen due to its diverse flow properties, including a wide range of flow speeds and multidirectional flow. The artery can be a site of blockage in conditions of peripheral artery disea…
Clara Holoscan For Real-Time De-Aliasing in Bedside Applications
- CDI is widely expected to be a point-of-care modality that can be used to gain quick and immediate insights into blood flow conditions in patients. Offline processing would disrupt this utility of CDI, so it is important that the aliasing removal framework be run in real time. NVIDIA and the LITMUS group collaborated to accelerate the de-aliasing framework to achieve real-time perf…
Conclusion
- The LITMUS group’s workflow showed how the NVIDIA Clara AGX Developer Kit and the NVIDIA Clara Holoscan SDK can resolve aliasing artifacts in CDI, in real time. Removing aliasing makes image visualization and interpretation easier by removing the ambiguity about the blood flow direction. This makes the most impact in tortuous vasculature where flow...