Lesson 22 (Part 3) Flashcards

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1
Q

What does aliasing show?

A

Wrap around colour

- top of the graph gets cut off

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2
Q

What does aliasing depend on?

A

Nyquist limit

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3
Q

What is the Nyquist limit?

A

1/2 PRF

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4
Q

When are you more likely to get alias?

A

At higher velocities

- eg) 8kHz PRF and fD over 4 kHz would alias

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5
Q

What should doppler shift be in order to not alias?

A

lower than the nyquist limit

- lower than 1/2 the PRF

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6
Q

When will aliasing often show?

A

Within a stenosis if PRF is set too low

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7
Q

How can we use aliasing for good? (2)

A
  1. Can adjust the PRF for the normal flow
  2. Areas of stenosis will show aliasing
    - highlights areas of highest velocity
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8
Q

How does gain benefit us in colour doppler?

A

We can amplify the returning echo voltages

- similar with 2D gain

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9
Q

What do we want the gain to demonstrate in colour doppler?

A

Wall to wall filling

- within the vessel

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10
Q

What is priority also known as?

A

Echo-Write Priority

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11
Q

What will echoes below the colour cut off amplitude be shown as?

A

Colour

- anything above will be apart of the grey scale

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12
Q

What do stronger echoes show as in priority?

A

Grayscale

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13
Q

What happens if you decrease priority?

A

You will see less colour

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14
Q

What happens if you increase priority?

A

You will see more colour

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15
Q

Wall filter

A

Any movement in the tissues will produce Doppler shifts

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16
Q

What can produce a doppler shifted colour?

A
  1. Tissue vibrations
  2. Vessel movement
  3. Clutter
17
Q

What kind of velocities does clutter have?

A

Lower

18
Q

How does wall filters fix clutters?

A

Sets a cut off point

19
Q

What do wall filters do? (2)

A
  1. Filters out the noise and other non useful information

2. Keeps the useful signal

20
Q

What vessels is speed normally faster in?

A

Arteries

- compared to veins

21
Q

If you are trying to show colour in an artery, but the nearby venous flow shows up in your image, how would you get rid of it?

A

Use a wall filter

22
Q

How can you help aliasing?

A

By increasing the PRF

23
Q

What happens if you increase PRF too much?

A

You get an echo misplacement

24
Q

What could tortuous vessels look like? (2)

A
  1. Flow reversal

2. Aliasing

25
Q

What kind of frequencies are used in doppler?

A

Higher frequencies

26
Q

What do you get with higher frequencies?

A

Higher attenuation

27
Q

Angle dependence

A

An increase in Doppler angle can reduce the Doppler shift and the corresponding flow

28
Q

What will hitting a vessel close to 90 deg look like?

A

Occlusion

29
Q

Occlusion

A

No flow

30
Q

What is power doppler determined by?

A

The concentration of moving RBCs

31
Q

What does power doppler show?

A

Only the presence of flow

- nothing about velocity/direction/flow character information

32
Q

What are 3 pros to power doppler?

A
  1. Increased sensitivity
    - slow flow
    - small or deep vessels
  2. Angle independent
  3. No Aliasing
33
Q

What are 3 cons to power doppler?

A
  1. No speed / cannot quantify
  2. No direction
  3. No flow character
    - to assess stenosis
34
Q

What makes up duplex scanning? (2)

A
  1. Doppler
  2. Gray-scale imaging
  • simultaneous rapidly switches between the two