Lesson 22 (Part 3) Flashcards Preview

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Flashcards in Lesson 22 (Part 3) Deck (34)
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1

What does aliasing show?

Wrap around colour
- top of the graph gets cut off

2

What does aliasing depend on?

Nyquist limit

3

What is the Nyquist limit?

1/2 PRF

4

When are you more likely to get alias?

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

5

What should doppler shift be in order to not alias?

lower than the nyquist limit
- lower than 1/2 the PRF

6

When will aliasing often show?

Within a stenosis if PRF is set too low

7

How can we use aliasing for good? (2)

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

8

How does gain benefit us in colour doppler?

We can amplify the returning echo voltages
- similar with 2D gain

9

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

Wall to wall filling
- within the vessel

10

What is priority also known as?

Echo-Write Priority

11

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

Colour
- anything above will be apart of the grey scale

12

What do stronger echoes show as in priority?

Grayscale

13

What happens if you decrease priority?

You will see less colour

14

What happens if you increase priority?

You will see more colour

15

Wall filter

Any movement in the tissues will produce Doppler shifts

16

What can produce a doppler shifted colour?

1. Tissue vibrations
2. Vessel movement
3. Clutter

17

What kind of velocities does clutter have?

Lower

18

How does wall filters fix clutters?

Sets a cut off point

19

What do wall filters do? (2)

1. Filters out the noise and other non useful information
2. Keeps the useful signal

20

What vessels is speed normally faster in?

Arteries
- compared to veins

21

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?

Use a wall filter

22

How can you help aliasing?

By increasing the PRF

23

What happens if you increase PRF too much?

You get an echo misplacement

24

What could tortuous vessels look like? (2)

1. Flow reversal
2. Aliasing

25

What kind of frequencies are used in doppler?

Higher frequencies

26

What do you get with higher frequencies?

Higher attenuation

27

Angle dependence

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

28

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

Occlusion

29

Occlusion

No flow

30

What is power doppler determined by?

The concentration of moving RBCs