Doppler Artifacts (aliasing, calcification) Flashcards
What type of signal is given with a PW?
Doppler beam is sent and the stopped. (pulse-listen)
What does the PW “pulse-listen” signal result in?
It results in frequent samples of the returning soundwaves rather than continuously monitoring the reflected waves.
What does PRF mean?
Pulse Repetition Frequency
For deeper depths it takes _____ for all echoes to be collected before the next pulse.
Longer.
What does the period of wait time affect?
It affects the number of pulses that can be sent to a certain depth per second. (PRF)
ANALOGY: if the strobe light turns on frequently enough, the motion seen is smooth and easy to follow. High or low PRF?
High PRF.
ANALOGY: if the strobe light is not turned on frequently enough, the motion seen is jerky and hard to follow. is it high or low PRF?
Low PRF.
In doppler, the more frequently the returning echo wave is sampled, the more _____ the display.
Accurate.
What does low PRF indicate?
Number of samples taken per second is too low so waveform seen is unrealistic.
What does PRF mean?
The number of pulses sent per second.
For doppler, what is the usual pulses sent?
4000-12,000/sec.
If less than two samples are taken per cycle, the result is?
Aliasing.
What us aliasing known as?
An artifact.
What is aliasing seen as?
It is seen as a wraparound effect.
What is aliasing?
It is falsely low frequency components in the doppler waveform.
What causes aliasing?
Too low PRF for the highest velocities.
What is the nyquist limit?
It is the PRF that will give you less than 2 samples per cycle.
what is the formula for the nyquist limit?
2 x peak Δf (or peak psv)
If the PRF is LESS THAN 2 x Δf, what will occur?
Aliasing will occur.
if the PRF is EQUAL TO or OVER 2 x peak Δf, what will occur?
Aliasing will NOT occur.
is 2 x peak velocity or 2Δf usually used?
2 x peak velocity.
What do you want the PRF to be?
Atleast 2x the highest F shift or psv.
what does the term “wraparound” in aliasing mean?
If the scale is not high enough to show the highest velocities, the equipment will show it as a negative velocity placed from the bottom up.
what are 6 ways to eliminate aliasing?
- Increase scale→also increases PRF
- Decrease baseline, if scale is maxed.
- Use lower F. transducer→lower f0 yields lower Δf.
- Find higher angle (but keep it under 60)→increase angle yields decreased cosine and decreased Δf, so lowers PRF needed.
- Change to a CW doppler→ no pulse so no aliasing, but also no velocity can be measured.
- Use high PRF mode→ only available on some machines & risk of range ambiguity.
What is high PRF mode?
It is a feature on some machines that allow multiple sample volumes (gates) in an image along the doppler beam lines.
What does high PRF mode do?
It allows for measuring high velocities at depths that go beyond the limits set by pulsed systems.
What must you be aware of when using high PRF mode?
Need to be aware of location of each SV to avoid sampling multiple vessels.
how many times must each cycle be sampled to demostrate a truer pricture of the frequency?
Must be sampled at least two times (2x).
So, the PRF must be at least____ maximum frequency shift to avoid aliasing.
2 x
Is there a maximum velocity detectable by pw doppler?
Yes.
What is the maximum velocity equation?
Vmax=C2/8 f0 d
Lower frequency transducers are better to detect what?
better to detect higher velocities due to lower frequency shifts.
What does the doppler equation mean?
Lower transducers yield lower frequency shifts.
