Lesson 22 (Part 3)

Question 1

What does aliasing show?

Answer 1

Wrap around colour

- top of the graph gets cut off

Question 2

What does aliasing depend on?

Answer 2

Nyquist limit

Question 3

What is the Nyquist limit?

Answer 3

1/2 PRF

Question 4

When are you more likely to get alias?

Answer 4

At higher velocities

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

Question 5

What should doppler shift be in order to not alias?

Answer 5

lower than the nyquist limit

- lower than 1/2 the PRF

Question 6

When will aliasing often show?

Answer 6

Within a stenosis if PRF is set too low

Question 7

How can we use aliasing for good? (2)

Answer 7

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

Question 8

How does gain benefit us in colour doppler?

Answer 8

We can amplify the returning echo voltages

- similar with 2D gain

Question 9

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

Answer 9

Wall to wall filling

- within the vessel

Question 10

What is priority also known as?

Answer 10

Echo-Write Priority

Question 11

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

Answer 11

Colour

- anything above will be apart of the grey scale

Question 12

What do stronger echoes show as in priority?

Answer 12

Grayscale

Question 13

What happens if you decrease priority?

Answer 13

You will see less colour

Question 14

What happens if you increase priority?

Answer 14

You will see more colour

Question 15

Wall filter

Answer 15

Any movement in the tissues will produce Doppler shifts

Question 16

What can produce a doppler shifted colour?

Answer 16

1. Tissue vibrations
2. Vessel movement
3. Clutter

Question 17

What kind of velocities does clutter have?

Answer 17

Lower

Question 18

How does wall filters fix clutters?

Answer 18

Sets a cut off point

Question 19

What do wall filters do? (2)

Answer 19

1. Filters out the noise and other non useful information

2. Keeps the useful signal

Question 20

What vessels is speed normally faster in?

Answer 20

Arteries

- compared to veins

Question 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?

Answer 21

Use a wall filter

Question 22

How can you help aliasing?

Answer 22

By increasing the PRF

Question 23

What happens if you increase PRF too much?

Answer 23

You get an echo misplacement

Question 24

What could tortuous vessels look like? (2)

Answer 24

1. Flow reversal

2. Aliasing

Question 25

What kind of frequencies are used in doppler?

Answer 25

Higher frequencies

Question 26

What do you get with higher frequencies?

Answer 26

Higher attenuation

Question 27

Angle dependence

Answer 27

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

Question 28

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

Answer 28

Occlusion

Question 29

Occlusion

Answer 29

No flow

Question 30

What is power doppler determined by?

Answer 30

The concentration of moving RBCs

Question 31

What does power doppler show?

Answer 31

Only the presence of flow | - nothing about velocity/direction/flow character information

Question 32

What are 3 pros to power doppler?

Answer 32

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

Question 33

What are 3 cons to power doppler?

Answer 33

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

Question 34

What makes up duplex scanning? (2)

Answer 34

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