Scattering and Diffraction Flashcards
What are we trying to image?
The scattering distribution
Which two properties of a wave scatter differently?
Sound speed and density heterogeneities
How does a point scatterer that is a sound speed heterogeneity scatter?
like a monopole source
How does a point scatterer that is a density heterogeneity scatter?
like a dipole source (with no scattering perpendicular to incident wave)
What is Class 0?
Molecular: absorption
What is Class 1?
Diffusive: λ»_space; scatter size so weak scattering (Rayleigh scattering)
spherical field produced by point scatterers
What is Class 2?
Diffractive: λ ≈ scatterer size (0.1-1mm)
Variable wavelength dependence
What is Class 3?
Specular: λ «_space;scatterer size
straightforward reflection
no wavelength dependence
What is Class 4?
Moving: Red blood cells
Which type of scattering obeys Snell’s law?
Specular
What happens to sound waves at boundaries?
They are partially reflected
What is a boundary to a sound wave?
A change in density or sound speed
A change in characteristic acoustic impedance
What is the impedance equation?
Z = λc_0
What are the boundary conditions?
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
What gives the amplitude of the reflected and transmitted waves?
Reflection coefficient
Transmission coefficient
What is the reflection coefficient equation?
R = p_r / p_i
What is the transmission coefficient equation?
T = p_t / p_i
What happens if the first impedance is smaller than second (Z_1 < Z_2) ?
R is positive
What happens if the second impedance is smaller than first (Z_1 > Z_2) ?
R is negative
What is the conservation of energy?
Fraction of reflected energy (Re) + fraction of transmitted energy (Te) = 1
What is the intensity equation for a plane wave?
I = pu = p^2 / Z
Why is coupling gel needed?
Large impedance mismatch between transducer/body and air which means no sound is transmitted
Gel is used to couple sound into the body
What is refraction?
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)
What does each spherical wave do in a medium?
It propagates at the local sound speed c and after time t, the radius is ct