Diagnostic Ultrasound

Question 1

What are the 3 main equations for acoustic impedance? (z)

Answer 1

z = P/v (P = local pressure, v = local particle velocity)
z = pc (pressure x velocity)
z = sqrt(pk) (sqrt(pressure x stiffness))

Question 2

What are the symbols for density, stiffness and acoustic impedance?

Answer 2

p, K, Z

Question 3

Generally, what is the factor of difference between the strongest and weakest echoes returning?

Answer 3

10 **5

Question 4

What is the Doppler Equation?

Answer 4

fr = 2 ft v/c cos(theta)D

fr = frequency received
ft = frequency transmitted
v = target velocity
c = speed of sound (1540)
fD = Doppler frequency of Doppler shift

Question 5

What is the technical name for a Doppler Waveform Display?

Answer 5

Sonogram

Question 6

What is the mathematical algorithm that converts the time domain of a sonogram to frequency?

Answer 6

Fourier transform

Question 7

How would you describe slow moving blood flow in a normal vessel?

Answer 7

Parabolic flow

Question 8

What does the amplitude of the Doppler signal depend on?

Answer 8

The number of RBCs reflecting back the signal

Question 9

What is the greyscale spectrum divided into?

Answer 9

Frequency bins

Question 10

What is a disadvantage of CW Doppler?

Answer 10

Multiple vessels within the relatively large sample volume will be detected together and superimposed

Question 11

What is an advantage of CW Doppler?

Answer 11

It has no velocity limit - can detect very high velocities

Question 12

What is range gating?

Answer 12

Only detecting signals from a particular depth

Question 13

What does sample volume depend on?

Answer 13

Pulse length and range gate length

Question 14

What condition needs to be met to detect the Doppler frequency?

Answer 14

The wave must be sampled at least twice in each period - this is known as the Nyquist Limit
- PRF = 2.fmax

Question 15

What happens when a wave is sampled at less than twice per time period?

Answer 15

Aliasing

Question 16

How can the maximum velocity of particles detected without aliasing be increased?

Answer 16

Decrease transmit frequency or depth

Question 17

What happens to maximum particle velocity detected without aliasing as depth increases?

Answer 17

This decreases

Question 18

How many cycles do PW Doppler pulses usually consist of?

Answer 18

6 - 7 cycles per pulse

Question 19

What happens to bandwidth as pulse length increases?

Answer 19

It becomes narrower

Question 20

What phenomena describes how a shorter pulse length (e.g. to look at narrow region of vessel) causes Doppler frequencies to become less well defined?

Answer 20

ISB - Intrinsic Spectral Broadening

Question 21

What is ISB?

Answer 21

A single target moving at velocity v will produce a range of Doppler frequencies, not just 1. This is related to the US scanner’s construction and cannot be avoided

Question 22

What are the 2 explanations of ISB?

Answer 22

1.Transmitted pulses have a finite bandwidth and produce a range of velocities.
The shorter the pulse, the greater the pulse bandwidth, hence the greater the ISB.

2. The beam is produced by a transducer of finite width. There will be a range of Doppler angles - producing a range of Doppler frequencies

Question 23

What factors increase ISB?

Answer 23

1. A very short range gate
2. A short pulse length
3. A very narrow beam
4. Larger Doppler angles of Ionisation i.e. closer to 90 degrees

Question 24

Why do we not use angles of <45 degrees for PW Doppler?

Answer 24

At shallow angles, there is poor penetration of ultrasound into the vessel lumen - can cause errors in velocity measurement

Question 25

What is wall thump?

Answer 25

Echoes from vessel wall can reflect much stronger than from the blood in the lumen

Question 26

How is wall thump reduced?

Answer 26

Using a high-pass filter to remove the low frequencies of the wall signal

Question 27

Why do we reduce the steer of the colour box or PW direction at depth?

Answer 27

To improve penetration of the signal