Physics Mod 2

Question 1

Acoustic variables

Answer 1

Related to mechanical nature of wave

1) change in acoustic pressure field
2) change in particle density
3) change in temperature
4) short term changes in particle location (particle motion)

Question 2

Sound wave parameters
symbol, description and determined by?
Frequency

Answer 2

f0
Number of compressions or rarefactions per second
source

Question 3

Sound wave parameters
symbol, description and determined by?
Period

Answer 3

P or T
time between each compression or between each rarefaction
source

P = 1 / f0

Question 4

Sound wave parameters
symbol, description and determined by?
Propagation velocity

Answer 4

C
Velocity at which the wave travels through a medium
medium

c = sqrt ( Bulk modulus / p)
bulk modulus = stress/strain = change pressure / (change volume/volume)

c ~ stiffness ~ bulk modulus (1/compressibility) ~ 1 / elasticity

Soft tissue average 1540
Bone 4080
Air 347
Lung 500

Question 5

Sound wave parameters
symbol, description and determined by?
Wavelength

Answer 5

lamda
Physical distance between wave compressions or wave rarefactions
both

lamda = c / f0

Question 6

Sound wave parameters
symbol, description and determined by?
Amplitude

Answer 6

A
maximum variation of a variable from its mean
refer to size of drive voltage, acoustic pressure field etc
Initially source then medium

Question 7

Sound wave parameters
symbol, description and determined by?
Power

Answer 7

P
Measure of the ability to perform work
related amplitude squared
Initially source then medium

Question 8

Sound wave parameters
symbol, description and determined by?
Intensity

Answer 8

I
How the power is distributed, equal to the power divided by the beam area
Initially source then medium

I = Power / beam area ~ amplitude ^2

Question 9

Better resolution comes from:
wavelength
frequency
propagation velocity

Answer 9

Shorter wavelength
higher frequency
lower propagation velocity

Question 10

Three principal forms of attenuation

Answer 10

1) Absorption
2) Reflection
3) Refraction

Question 11

Absorption define

Answer 11

Type of attenuation

Conversion of acoustic energy into heat within body

Question 12

Reflection define

Answer 12

Type of attenuation

Redirection of acoustic energy back towards the source direction

Question 13

Refraction define

Answer 13

Type of attenuation
Bending/distorting of non-reflected acoustic beam

Occurs at interface between structures
Depends on change in propagation velocity and angle formed by beam and structure

Question 14

Type of reflections

Answer 14

1) Specular
2) Back scattering
3) Rayleigh scattering

Question 15

When does Rayleigh scattering occur

Answer 15

When size of RBC much less than wavelength

Question 16

What determines how much is reflected at an interface

Answer 16

Acoustic impedance mismatch

Impedance Z = p x c
%reflection = [(Z2-Z1)/(Z2+Z1)]^2

Question 17

Unit for impedance

Answer 17

Rayls

Question 18

What type of artifact can refraction cause

Answer 18

1) shadowing

2) lateral displacement

Question 19

Conditions to have refraction

Answer 19

1) specular component only
2) change in propagation velocity
3) angle other than 0 degree incidence (perpendicular)
4) Snell’s law
5) at critical angle total internal reflection occurs

Question 20

Snell’s law

Answer 20

ci / ct = sin(thetai) / sin(thetat)