Ultrasound II

Question 1

What is high quality spectral Doppler waveform?

Answer 1

Doppler angle less than 60

Question 2

What are audible frequencies?

Answer 2

Audible

0.1-15 kHz

Question 3

What does spectral Doppler cover?

Answer 3

A range of frequencies
Flowing blood cells has a range of velocities
Spectral broadening

Question 4

What does blood have?

Answer 4

Low amplitude echoes

Low high velocities/frequencies shift

Question 5

What do vessel walls have?

Answer 5

High amplitude (I.e.) strong echoes 
Low velocity/frequency shift

Question 6

How can you discriminate blood and tissue echoes on the basis of frequency?

Answer 6

Wall filter

Question 7

What is demodulation?

Answer 7

Extracting Doppler frequency shifts information a received ultrasound signal

Separation of the Doppler frequencies from underlying transmitted signals

Question 8

What is high pass filtering?

Answer 8

Removal of tissue signal

Question 9

What is the frequency estimation?

Answer 9

Where the Doppler frequencies and amplitudes are calculated

Question 10

What is continuous wave pulse?

Answer 10

Measures the velocity over a wide range of depth

Question 11

What are PW devices?

Answer 11

Gated

Only echoes revived within a particular time (depth) range are considered

Question 12

Where do PW devices transmit pulses?

Answer 12

At a rate determined by pulse repetition frequency (PRF)

~ 2-8 kHZ

Question 13

What is the region from which Doppler signals are obtained?

Answer 13

By the depth and length of gate which are both controlled by operator

Question 14

Explain how aliasing applies to PW Doppler measurements?

Answer 14

Relationship between RFP and maximum velocity

PW Doppler demodulation is essentially a sampling process

Limit on max velocity that can be measured

Question 15

What is the main difference between CW and PW?

Answer 15

Estimation of high velocity (blood)

Question 16

What happens if the pulse repetition frequency (PRF) is too low?

Answer 16

The Doppler frequency shift cannot be estimated properly - aliasing (not in CW)

Question 17

What happens when PRF is high?

Answer 17

There are a sufficient number of samples to enable the Doppler system to detect the frequency correctly

As PRF stops there comes a point at which the Doppler signal is only just sampled sufficiently

Question 18

What happens if there are 2 samples per cycle?

Answer 18

Nyquist limit
The max Doppler frequency shift which can be detected is half of PRF

If PRF drops further the Doppler system can no longer calculate the correct frequency

Question 19

What is aliasing?

Answer 19

The highest Doppler frequency shift that can be measured is equal to PRF/2

Question 20

What is the colour Doppler ultrasound?

Answer 20

Colour coded velocity information superimposed on B-mode image (received echoes are processed twice)

The colour of each pixel represents average Doppler shift

Frame rate < B-mode imaging

Question 21

What are the common artefact encountered in Doppler Imaging?

Answer 21

1. Incorrect gain settings
2. Shadowing
3. Aliasing: high velocities appear as low velocities
4. Flash artefact
5. Colour spilling outside vessel

Question 22

What are the advantages of power Doppler imaging ?

Answer 22

1. Angle independence
2. Sensitivity
3. Useful for visualising complex vascular geometries

Question 23

What are the applications of TCD?

Answer 23

1. Evaluation of cerebral blood flow
2. Access through temporal window
3. MCA > circle of Willis
4. Stenosis, aneurysm, and embolus detection
5. Useful for monitoring neuro vascular surgical procedures

Question 24

What are the carotid stenosis measurement?

Answer 24

Peak systolic velocity at site of plaque as a measure of degree of stenosis

Direct measurements from B-more also useful

Question 25

What are the basic principles of ultrasound safety?

Answer 25

1. Only perform scan if clinically required
2. Obtain all information required to make a diagnosis
3. Minimise time that probe is in contact with patient
4. Use Doppler modes sparingly, turn only once site has been found using B mode imaging, minimise size of colour box
5. Take Notice if thermal and mechanical indices displayed by scanner and change output settings or limit exposure accordingly
6. Ensure scanners are properly maintained

Question 26

What are at-risk groups and tissues ?

Answer 26

1. Embryo during first trimester
2. Foetus during second-third trimesters
3. Foetus when mother has a fever
4. Neonate
5. The eye

Question 27

What are the bio effect mechanism?

Answer 27

Thermal: temperature rise

Non thermal: mainly cavitation.

Question 28

What are risk of a bio effect indicated by?

Answer 28

Mechanical index (MI) - mechanical damage primarily due to cavitation

Thermal index (TI) - thermal damage from a sustained rise in temperature

Question 29

What is cavitation?

Answer 29

Gas bubbles in a liquid respond to ultrasound waves

MI > 0.3
- possibility of minor damage to neonatal lung

MI > 0.7
- risk of cavitation if a micro bubble contrast agent is being used. And a theoretical risk even if such contrast agents are not used

Question 30

What is the range of Doppler frequency shift?

Answer 30

Up to about 15kHz

Question 31

What is Ft and what is it’s range?

Answer 31

Frequency of transmitted ultrasound signal

Range: 2-15 MHz for medical ultrasound

Question 32

What is C?

Answer 32

Speed of sound

Range: around 1500 m/s

Question 33

What happens when PRF is too low?

Answer 33

Aliasing can occur in the blood velocity measurements

Which gives rise to a characteristic aliasing artefact

Low PRF means that is it not possible to measure high velocities

PRF = 2. Fs - the Nyquist limit

Question 34

What is the equation for maximum measurable blood velocity?

Answer 34

Vmax = c. PRF/2ft