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???”.
This week’s post was requested by Bec from South Australia. Thanks for jumping straight in with the tricky stuff!
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.
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.
This is also known as the wagon-wheel effect, as it was often seen in old western movies. The old movie cameras had a very low frame rate compared to modern movie cameras. This was not an issue for relatively slow moving events, such as people walking around, but the wagon-wheels turned a lot faster than the camera was taking the frames for the movie. At slow speeds, the wheel looked to be turning normally. As the wheel sped up, the image sped up to until it reached the same speed as the frame rate of the camera. At this point the wheel may look to be almost stopped on the film. With increasing speeds, the wheel then appears to go backwards.
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).
This optical illusion became characteristic of speed, hence the artists drawing the effect for SpeedRacer.
We also see similar optical illusions created by aliasing with airplane propeller blades and strobe water fountains.
Check out these cool examples of aliasing:
What does Speed Racer have to do with the display of our colour Doppler echo images???
The Nyquist limit is determined by half the pulse repetition frequency.
Nyquist Limit = 1/2 PRF
The PRF (and therefore Nyquist limit) become the old-style movie camera in our story. The moving blood cells are the wagon wheel spokes. Remember colour is a pulse wave Doppler technique… a pulse is sent and the system then waits for the signal to return before it sends another pulse. If a second pulse is sent before the the first one is received, the machine doesn’t know which pulse the returning echo has come from…so it waits a designated time for the echos to return based on the depth of the image. If we are pulsing something in the near field, we don’t need to wait as long for the echo to return as we do if we are pulsing something farther away.
The greater the depth, the lower the PRF (needs to wait longer for the signal to return) and therefore the lower the Nyquist limit (maximum velocity that can be displayed unambiguosly).
So if we have a high Nyquist limit, and a low blood flow velocity, the blood flow direction is displayed correctly. As the blood flow speeds up, and exceeds the velocity of the Nyquist limit, the system is not sampling often enough to be certain of which direction flow is occuring in and therefore it is displayed erroneously as “wrapping-around” the scale and appearing as if the direction has been reversed. The ultrasound system has experienced the same optical illusion that we see looking at the wagon-wheels.
We want to display direction of flow correctly… How do we avoid 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….
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. I can’t really see any point to this…we have now substituted one error (range/direction ambiguity) with a different error (underestimation of the Doppler shift therefore underestimate velocity)
Increase colour scale or shift colour baseline to minimise aliasing effect.
Shifting the scale and baseline is commonly used to optimise PW Doppler
Can aliasing be useful in ultrasound? Yes. We often use aliasing to help identify regions of flow acceleration or turbulence. This may be important to help try and locate a narrowing or obstruction. We also take advantage of how clearly aliased flow can be seen to identify ventricular septal defects by CFI. In other words the contrasting colour of the aliased flow to the normal flow draws attention to the region, making it easier for the sonographer to appreciate. We all know how difficult seeing an ASD with low velocity flow (no aliasing) can be!
Can we add the velocities to calculate the “correct velocity”? No. If you need to measure a velocity, then take measures to avoid aliasing. There is a portion of the signal “lost” as aliasing occurs. This is analogous to the wagon wheel appearing like it isn’t turning when the revolutions matched the cameras frame rate. We simply don’t know how much signal is lost as it wraps around.
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).
Hope that helps, Bec… at the least you will probably get the SpeedRacer theme stuck in your head for the rest of the week! Enjoy!!