Scattering and Diffraction Flashcards

1
Q

What are we trying to image?

A

The scattering distribution

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Which two properties of a wave scatter differently?

A

Sound speed and density heterogeneities

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

How does a point scatterer that is a sound speed heterogeneity scatter?

A

like a monopole source

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

How does a point scatterer that is a density heterogeneity scatter?

A

like a dipole source (with no scattering perpendicular to incident wave)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is Class 0?

A

Molecular: absorption

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is Class 1?

A

Diffusive: λ&raquo_space; scatter size so weak scattering (Rayleigh scattering)

spherical field produced by point scatterers

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is Class 2?

A

Diffractive: λ ≈ scatterer size (0.1-1mm)

Variable wavelength dependence

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is Class 3?

A

Specular: λ &laquo_space;scatterer size

straightforward reflection

no wavelength dependence

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is Class 4?

A

Moving: Red blood cells

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Which type of scattering obeys Snell’s law?

A

Specular

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What happens to sound waves at boundaries?

A

They are partially reflected

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is a boundary to a sound wave?

A

A change in density or sound speed

A change in characteristic acoustic impedance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the impedance equation?

A

Z = λc_0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What are the boundary conditions?

A

Continuity of pressure (infinite force not allowed)

p_i + p_r = p_t

Continuity of normal particle velocity (fluid must stay in contact)

u_i + u_r = u_t

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What gives the amplitude of the reflected and transmitted waves?

A

Reflection coefficient

Transmission coefficient

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is the reflection coefficient equation?

A

R = p_r / p_i

17
Q

What is the transmission coefficient equation?

A

T = p_t / p_i

18
Q

What happens if the first impedance is smaller than second (Z_1 < Z_2) ?

A

R is positive

19
Q

What happens if the second impedance is smaller than first (Z_1 > Z_2) ?

A

R is negative

20
Q

What is the conservation of energy?

A

Fraction of reflected energy (Re) + fraction of transmitted energy (Te) = 1

21
Q

What is the intensity equation for a plane wave?

A

I = pu = p^2 / Z

22
Q

Why is coupling gel needed?

A

Large impedance mismatch between transducer/body and air which means no sound is transmitted

Gel is used to couple sound into the body

23
Q

What is refraction?

A

The change in direction of a wave direction when entering a medium with a different sound speed (a change in sound speed)

(understood using Huygen’s principle)

24
Q

What does each spherical wave do in a medium?

A

It propagates at the local sound speed c and after time t, the radius is ct

25
What is Snell's law?
The refraction for a plane wave incident on a flat boundary
26
What is the equation for Snell's law?
sin θ_i / c_1 = sin θ_t / c_2
27
What happens when c_2 < c_1 ?
Direction of propagation moves towards the normal (opposite for c_2 > c_1)
28
What happens to the pressure reflection and transmission coefficient?
As it also depends on angle The Z_1 parameters are multiplied by cos θ at the incident at the normal The Z_2 parameters are multiplied by cos θ transmitted at the normal
29
What is equal to the critical angle?
θ_t = π/2 There is total reflection so R = 1
30
What is diffraction?
The leakage of sound into shadow zones It occurs around objects and corners If sound travelled in straightly lines only, no sound would propagate into the shadow zones