SVT Day

Question 1

What is acoustic impedance measures in?

Answer 1

Rayls or Pa per cm

Question 2

What is the equation for acoustic impedance/

Answer 2

Z = pc

Question 3

What is the intensity reflection coefficient?

Answer 3

Intensity reflected / Intensity incident
- the bigger the difference, the more reflection

Question 4

Why does bone cause dropout?

Answer 4

Bone has a very high acoustic impedance compared to soft tissue

Question 5

What is a specular reflection?

Answer 5

Reflection at a smooth, straight boundary

Question 6

What is Rayleigh scattering?

Answer 6

Occurs with reflectors much smaller than the ultrasound wavelength
- can cause speckle in B-mode

Question 7

Why is Rayleigh scattering useful for Doppler?

Answer 7

We do not normally scan at 90 degrees in Doppler

Question 8

Why is the sky blue?

Answer 8

Short wavelength blue light gets scattered more than longer wavelengths

Question 9

What is Snells law?

Answer 9

The law of refraction
sine(theta2) / sine(theta1) = c2 / c1

Question 10

What causes attenuation?

Answer 10

absorption, scattering, reflection, refraction, beam divergence

Question 11

What are the units for attenuation?

Answer 11

dB / cm / Mhz

Question 12

How is axial resolution calculated?

Answer 12

Axial resolution = 1/2 pulse length

Question 13

What is Huygen’s Principle?

Answer 13

Array transducers use multiple piezoelectric elements and combine multiple elements to form the ultrasound beam

Question 14

Why do superficial structures sometimes appear difficult to see?

Answer 14

The are too close for interference and focussing of the beam

Question 15

What is cavitation?

Answer 15

Ultrasound pressure waves can push fluid out the way - causes rarefaction and expansion.
- can cause tiny microbubbles which can then burst and release energy as heat
- can damage surrounding objects

Question 16

How can you minimize cavitation?

Answer 16

Reduce MI

Question 17

What is Ti?

Answer 17

The ratio of total acoustic power to the power needed to raise 1kg of tissue by 1 degree

Question 18

What are the different ways of measuring ultrasound beam intensity?

Answer 18

1. Spatial peak, temporal average
2. Spatial average, temporal average
3. Spatial average, temporal peak
4. Spatial peak, temporal peak

Question 19

What is the Doppler angle?

Answer 19

The angle between the direction of flow and the transducer lens

Question 20

Why does aliasing occur due to vessel tortuousity?

Answer 20

Angle becomes closer to zero, so causes a greater Doppler shift

Question 21

What can strong side lobes do?

Answer 21

Degrade lateral resolution

Question 22

What is apodization?

Answer 22

Strength of transducer signal becomes weaker towards edges of transducer - to reduce side lobes

Question 23

What are two advantages of using arrays?

Answer 23

1. They enable electronic beam steering
2. They enable electronic beam focusing

Question 24

How is electronic focussing achieved?

Answer 24

By introducing time delays in pulse excitation of different elements

Question 25

How is receive focus achieved?

Answer 25

By introducing time delays of different durations to the individual signals so the signals will be in-phase when summed

Question 26

What is frame rate dependent on?

Answer 26

Colour line density Width and depth of colour box Number of pulses per length (Ensemble length or packet size) PRF

Question 27

What is flow?

Answer 27

Q = mean velocity * Area

Question 28

How does the total XSA change as arteries branch?

Answer 28

It increases

Question 29

What is a Newtonian fluid?

Answer 29

Uniform

Question 30

What is haematocrit?

Answer 30

The percentage of blood occupied by RBCs

Question 31

What are laminae?

Answer 31

The layers of fluid in blood

Question 32

What does laminar mean?

Answer 32

Layers of blood moving over each other

Question 33

What is Pouseilles Law?

Answer 33

Q = change in pressure / Resistance Q = (change in P * Pi * r**4) / 8 u L

Question 34

How does resistance add in series?

Answer 34

Sums normally

Question 35

How does resistance add in parallel?

Answer 35

1/Rt = 1/R1 + 1/R2 ....

Question 36

What causes reflections of pulse waves?

Answer 36

Changes in impedance e.g. branches

Question 37

What happens to pulse wave if there is a distal occlusion?

Answer 37

There is a total reflection of pressure = waveforms with no net flow - High resistance waveform

Question 38

What happens to CFA flow during exercise?

Answer 38

There is higher diastolic flow as distal arteries have dilated

Question 39

What are entrance effects?

Answer 39

When there is a change in area, there is a change in flow profile e.g. more plug-like in a stenosis

Question 40

What is shear of the velocity profile?

Answer 40

The change in velocity across the velocity profile

Question 41

What does low shear cause?

Answer 41

Thrombosis

Question 42

How does curvature change flow?

Answer 42

In laminar, fastest flow will be towards outer wall of the curve In blunt flow, is opposite

Question 43

What are the 3 components of energy in blood?

Answer 43

1. Pressure energy 2. Gravitational energy 3. Kinetic energy (velocity)

Question 44

What is boundary layer circulation?

Answer 44

Recirculation of flow due to changes (increases) in area e.g. carotid bulb or post-stenotic dilatation

Question 45

When should you measure velocities in the carotid bulb?

Answer 45

Never, unless stenosis within

Question 46

What happens to mean and peak velocity in stenosis?

Answer 46

Mean velocity will increase Peak velocity might not increase until stenosis becomes more severe

Question 47

What are viscous losses?

Answer 47

Loss of energy due to friction

Question 48

What type of losses are entrance and exit losses?

Answer 48

Inertial losses

Question 49

How do plaques ulcerate?

Answer 49

Pressure differences can cause forces across the plaque

Question 50

What is pressure drop proportional to?

Answer 50

Velocity

Question 51

Is pressure loss greater in systole or diastole?

Answer 51

Systole (higher velocity)

Question 52

What can a damped waveform indicate?

Answer 52

Proximal disease