Spectral Waveforms

Question 1

How do targets moving away from the transducer impact frequency?

Answer 1

They decrease frequency

Question 2

How do targets moving towards the transducer impact frequency?

Answer 2

They increase frequency

Question 3

What is the speed of sound in blood?

Answer 3

1570m/s

Question 4

What is the Doppler Equation?

Answer 4

fd = 2 v/c * Cos(theta) ft

Question 5

Why is Doppler frequency greatest at 0 degrees?

Answer 5

Cos(theta) = 1
(note for 90 degrees, cos(theta) = 0

Question 6

What Doppler angle should be used in practice?

Answer 6

The smallest Doppler angle possible whilst maintaining the adequate signal strength
- heel-toeing can be used to improve this in practice

Question 7

What is the spectrum of Doppler frequencies?

Answer 7

The range of Doppler frequencies that arise from the varying speeds of RBCs

Question 8

What is an instantaneous Doppler Power spectrum?

Answer 8

A plot of the strength of the doppler signal strength at any one time

Question 9

What is displayed on a spectral trace when doppler angle is known and when it is not?

Answer 9

Known = velocity
Unknown = frequency shift

Question 10

What happens to EDV as resistance increases?

Answer 10

EDV decreases

Question 11

What is the Nyquist Limit?

Answer 11

The limit that at least 2 samples are required to know that the amplitude has oscillated above and below zero
- determines when aliasing will occur

Question 12

How do echoes from theta(edge) compare to those of theta(mid)

Answer 12

Theta(edge) will have a lower Doppler angle and therefore a higher Doppler frequency

Question 13

Can you change the angle the scanner uses to calculate the velocity scale i.e. theta(mid) vs theta(edge)

Answer 13

No - so it is important to know what your scanner uses

Question 14

What does wall thump filter remove?

Answer 14

High amplitude, low frequency Doppler shifts caused by specular reflection from the slowly moving wall

Question 15

What are wall thump filter frequencies typically?

Answer 15

< 150 Hz

Question 16

What does each pixel mean in a colour flow display?

Answer 16

The mean Doppler frequency of the echoes from the pixel sample volume

Question 17

What 2 reasons may cause a change of colour hue?

Answer 17

1. Change in velocity - e.g. stenosis or calibre change
2. Change in angle with respect to transducer

Question 18

Why is frame rate slower for colour compared to B-mode?

Answer 18

To estimate the mean Doppler frequency, the transducer must send 8-12 pulses for each line
- this is why is important to use small colour boxes in practice

Question 19

Why can colour NOT be used to make quantitative measurements?

Answer 19

Each pixel is proportional to mean velocity and the doppler angle is not known and is assumed to be zero
Significant errors may also be present when measuring diameter with colour

Question 20

What is power doppler?

Answer 20

Colour super-imposed on B-mode, similar to colour
However, colour is proportional to total backscattered power - does not show velocity or direction, only where flow is