Wave Interactions Flashcards
What is the decible notation for RPL and RIL? (2 marks)
RIL = 10 log (I2/I1)
as I = k p^2
RPL = 20 log (p2/p1)
What two equations are there for acoustic impedance? (2 marks)
Z = p c
density and speed of sound
Z = P / U (smelly)
pressure and flow velocity
What is the equation for acoustic intensity? (1 mark)
I = P U (smelly)
What is the equation for the reflection coefficient and snells law? (2 marks)
R = (Z2-Z1)/(Z2+Z1)
n2/n1 = sin(theta2)/sin(theta1)
What effect does TIR cause in US images? (1 mark)
An acoustic shadow.
What is the difference between specular and diffuse reflection and when does it occur? (3 marks)
Specular => reflection with a reflected angle equal to the incident angle
Diffuse => reflection in all directions, due to the size of the scatterers being much smaller than the size of the wavelength, such a rough surface.
What is abberation and when does it occur? (2 marks)
Abberation = defocussing of the beam due to superfical fatty layers
How can a plane source be focussed and how is the radius of curvature related? (2 marks)
Use a lense.
R = F(c2/c1 - 1)
What is the attenuation coefficient equal to and what are the components in the equation? (6 marks)
alpha_db = af^b
The two attenuation components are scattering and absorption.
What makes up most of an US image? And what happens if a «_space;lambda, how does the intensity of this type of scattering depend on the frequency? (4 marks)
Mainly made up of scatter…
If a «_space;lamda then we get rayleigh scattering where I = k f^4
Estimate the attenuation of the liver taking a = 0.5 dB per MHz per cm and b = 1. (2 marks)
alpha_dB = aF^b = 0.5 x 6 MHz = 3 dB per cm
Estimate the depth to be 10 cm, therefore attenuation of 3 x 20 cm = 60 dB. This is equal to a reduction in RIL by 10^6!
