Doppler Artifacts (aliasing, calcification)

Question 1

What type of signal is given with a PW?

Answer 1

Doppler beam is sent and the stopped. (pulse-listen)

Question 2

What does the PW “pulse-listen” signal result in?

Answer 2

It results in frequent samples of the returning soundwaves rather than continuously monitoring the reflected waves.

Question 3

What does PRF mean?

Answer 3

Pulse Repetition Frequency

Question 4

For deeper depths it takes _____ for all echoes to be collected before the next pulse.

Answer 4

Longer.

Question 5

What does the period of wait time affect?

Answer 5

It affects the number of pulses that can be sent to a certain depth per second. (PRF)

Question 6

ANALOGY: if the strobe light turns on frequently enough, the motion seen is smooth and easy to follow. High or low PRF?

Answer 6

High PRF.

Question 7

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?

Answer 7

Low PRF.

Question 8

In doppler, the more frequently the returning echo wave is sampled, the more _____ the display.

Answer 8

Accurate.

Question 9

What does low PRF indicate?

Answer 9

Number of samples taken per second is too low so waveform seen is unrealistic.

Question 10

What does PRF mean?

Answer 10

The number of pulses sent per second.

Question 11

For doppler, what is the usual pulses sent?

Answer 11

4000-12,000/sec.

Question 12

If less than two samples are taken per cycle, the result is?

Answer 12

Aliasing.

Question 13

What us aliasing known as?

Answer 13

An artifact.

Question 14

What is aliasing seen as?

Answer 14

It is seen as a wraparound effect.

Question 15

What is aliasing?

Answer 15

It is falsely low frequency components in the doppler waveform.

Question 16

What causes aliasing?

Answer 16

Too low PRF for the highest velocities.

Question 17

What is the nyquist limit?

Answer 17

It is the PRF that will give you less than 2 samples per cycle.

Question 18

what is the formula for the nyquist limit?

Answer 18

2 x peak Δf (or peak psv)

Question 19

If the PRF is LESS THAN 2 x Δf, what will occur?

Answer 19

Aliasing will occur.

Question 20

if the PRF is EQUAL TO or OVER 2 x peak Δf, what will occur?

Answer 20

Aliasing will NOT occur.

Question 21

is 2 x peak velocity or 2Δf usually used?

Answer 21

2 x peak velocity.

Question 22

What do you want the PRF to be?

Answer 22

Atleast 2x the highest F shift or psv.

Question 23

what does the term “wraparound” in aliasing mean?

Answer 23

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.

Question 24

what are 6 ways to eliminate aliasing?

Answer 24

1. Increase scale→also increases PRF
2. Decrease baseline, if scale is maxed.
3. Use lower F. transducer→lower f₀ yields lower Δf.
4. Find higher angle (but keep it under 60)→increase angle yields decreased cosine and decreased Δf, so lowers PRF needed.
5. Change to a CW doppler→ no pulse so no aliasing, but also no velocity can be measured.
6. Use high PRF mode→ only available on some machines & risk of range ambiguity.

Question 25

What is high PRF mode?

Answer 25

It is a feature on some machines that allow multiple sample volumes (gates) in an image along the doppler beam lines.

Question 26

What does high PRF mode do?

Answer 26

It allows for measuring high velocities at depths that go beyond the limits set by pulsed systems.

Question 27

What must you be aware of when using high PRF mode?

Answer 27

Need to be aware of location of each SV to avoid sampling multiple vessels.

Question 28

how many times must each cycle be sampled to demostrate a truer pricture of the frequency?

Answer 28

Must be sampled at least two times (2x).

Question 29

So, the PRF must be at least____ maximum frequency shift to avoid aliasing.

Answer 29

2 x

Question 30

Is there a maximum velocity detectable by pw doppler?

Answer 30

Yes.

Question 31

What is the maximum velocity equation?

Answer 31

V_max=C²/8 f₀ d

Question 32

Lower frequency transducers are better to detect what?

Answer 32

better to detect higher velocities due to lower frequency shifts.

Question 33

What does the doppler equation mean?

Answer 33

Lower transducers yield lower frequency shifts.