Flashcards in Lesson 22 (Part 3) Deck (34)
What does aliasing show?
Wrap around colour
- top of the graph gets cut off
What does aliasing depend on?
What is the Nyquist limit?
When are you more likely to get alias?
At higher velocities
- eg) 8kHz PRF and fD over 4 kHz would alias
What should doppler shift be in order to not alias?
lower than the nyquist limit
- lower than 1/2 the PRF
When will aliasing often show?
Within a stenosis if PRF is set too low
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
How does gain benefit us in colour doppler?
We can amplify the returning echo voltages
- similar with 2D gain
What do we want the gain to demonstrate in colour doppler?
Wall to wall filling
- within the vessel
What is priority also known as?
What will echoes below the colour cut off amplitude be shown as?
- anything above will be apart of the grey scale
What do stronger echoes show as in priority?
What happens if you decrease priority?
You will see less colour
What happens if you increase priority?
You will see more colour
Any movement in the tissues will produce Doppler shifts
What can produce a doppler shifted colour?
1. Tissue vibrations
2. Vessel movement
What kind of velocities does clutter have?
How does wall filters fix clutters?
Sets a cut off point
What do wall filters do? (2)
1. Filters out the noise and other non useful information
2. Keeps the useful signal
What vessels is speed normally faster in?
- compared to veins
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
How can you help aliasing?
By increasing the PRF
What happens if you increase PRF too much?
You get an echo misplacement
What could tortuous vessels look like? (2)
1. Flow reversal
What kind of frequencies are used in doppler?
What do you get with higher frequencies?
An increase in Doppler angle can reduce the Doppler shift and the corresponding flow
What will hitting a vessel close to 90 deg look like?