Ch 3b PRF

Question 1

If the top of the colour scale is set to +40 cm/s, what is this representing?

Answer 1

-The highest detectable mean velocity of flow TOWARDS the transducer at 40 cm/s
-Encoded in light orange

Question 2

If the bottom of the colour scale is set to -40 cm/s, what is this representing?

Answer 2

-The highest detectable mean velocity of flow AWAY from the transducer at 40 cm/s
-Encoded in light green

Question 3

What does the black band/baseline in the middle of the colour scale represent?

Answer 3

Signals detected in this range either have no doppler shift OR a low doppler shift that is not seen due to wall filtering

Question 4

If there is too much aliasing in a vessel, what should we do to our colour scale?

Answer 4

Turn it up

(b/c PRF/scale is set too low)

Question 5

If there is a lack of colour seen in a vessel, what should we do to our colour scale?

Answer 5

Turn it down

(b/c PRF/scale is set too high it is blocking out colour)

Question 6

What should the colour scale be set at during a normal carotid u/s exam?

Answer 6

B/w 30-40 cm/s

Question 7

What should the colour scale be set at during a normal venous u/s exam?

Answer 7

Less than 20 cm/s

Question 8

What should the colour scale be set at during a normal cardiac u/s exam?

Answer 8

B/w 60-80 cm/s

Question 9

What is aliasing?

Answer 9

When the velocity or doppler shift goes beyond the maximum detectable velocity or frequency shift (nyquist limit)

(it is the “wrapping around” of colour)

Question 10

Is aliasing always considered bad?

Answer 10

No, it can be helpful and tell us information such as if there is a stenosis, if flow is moving fast when it should not be, etc.

Question 11

What is colour doppler?

Answer 11

A pulsed doppler system that acquires sampling

Question 12

What does the nyquist limit refer to?

Answer 12

The highest detectable doppler frequency shift possible w/o aliasing

Question 13

When does aliasing occur in regards to the PRF?

Answer 13

Occurs when the highest doppler shifted frequency (nyquist limit) exceeds 1/2 of the PRF

Question 14

The colour scale/PRF must be how many times the highest doppler shift (nyquist limit) to avoid aliasing?

Answer 14

2x

(PRF = 2 x Nyquist Limit)

Question 15

What is the nyquist limit formula?

Answer 15

Nyquist limit (highest DSF) = 1/2 PRF (scale)

Question 16

If a tire is making a 340 degree rotation clockwise, why does the brain assume it is going in a slow anti-clockwise rotation?

Answer 16

We see it going anti-clockwise b/c we are not sampling enough, we must increase the PRF to fix this

Question 17

Does aliasing occur from over or under sampling?

Answer 17

Under sampling

(increase PRF)