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Physics > Module 6 : Attenuation > Flashcards

Module 6 : Attenuation Flashcards

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1
Q

what is attenuation

A
  • weakening of sound as it travels
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2
Q

what are three reasons attenuation is of great clinical importance

A
  • limits imaging depth
  • must be compensated for
  • useful in diagnosis
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3
Q

what is 1 bel equal to

A
  • 10 decibels
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4
Q

what does a 3dB drop equate to

A
  • 1/2 original intensity
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5
Q

what does a 10 dB drop equate to

A

1/10 original intensity

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6
Q

what is the attenuation coefficient

A
  • amount of attenuation that occurs with each one centimetre travels
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7
Q

what is the attenuation coefficient in soft tissue

A
  • 0.5dB drop occurs in every one centimeter travelled per 1 MHz
  • 1/2 frequency
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8
Q

how do we calculate the total attenuation

A
  • TA = attenuation coefficient x path length
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9
Q

calculating TA in soft tissue

A

TA = 1/2f x path length

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10
Q

what is the half value layer

A
  • distance sound must travel in a material to reduce the intensity to half its original value
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11
Q

what three things doe attenuation vary with

A
  • nature of tissue
  • frequency of ultrasound
  • depth
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12
Q

what are the 5 reasons attenuation occurs

A
  • absorption
  • reflection
  • refraction
  • scatter
  • wave front divergence
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13
Q

what is the number one reason attenuation occurs

A
  • absorption
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14
Q

what is absorption

A
  • conversion of sound energy to heat

- dominant factor in attenuation

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15
Q

what three factors affect absorption

A
  • viscosity
  • relaxation time
  • frequency
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16
Q

what is viscosity

A
  • ease in which molecules can slide past each other
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17
Q

through which process is sound energy converted to heat

A
  • friction
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18
Q

how does viscosity affect attenuation

A
  • increased viscosity provides greater resistance more friction and increased attenuation
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19
Q

what is the relaxation time

A
  • when a mechanical force is applied to a molecule it will vibrate
  • the time it takes to come to rest is relaxation time
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20
Q

how does relaxation time affect attenuation

A
  • if molecule can’t come to rest before the next compression phase (increased relaxation time) than more energy is required to reverse its directions leading to increased heat
21
Q

how does frequency affect attenuation

A
  • number one reason for absorption

- increased frequency then less time available for molecule to recover during relaxation process increasing absorption

22
Q

what are the two types of reflection

A
  • specular

- non specular

23
Q

what is a specular reflection

A
  • when sound hits a larger smooth surface, greater than one wavelength in diameter
24
Q

what in the body is a good specular reflector

A
  • diaphragm
25
Q

what 2 things does the intensity of the reflected sound depend on

A
  • angle of incidence

- acoustic impedance of two mediums

26
Q

how does angle of incidence affect the intensity of the reflected sound

A
  • angle of incidence = angle of reflection

- reflections that are more non perpendicular may not return to the probe

27
Q

what is acoustic impedance

A
  • aka characteristic impedance

- relationship between acoustic pressure and the speed of particle vibrations in a sound wave

28
Q

what is the impedance equation

A
  • impedance (Z) = density x velocity
29
Q

do Z values vary between tissue

A
  • yes due to different density and stiffness
30
Q

does acoustic impedance depend on frequency

A
  • no
31
Q

what difference between Z values will cause a bigger reflection

A
  • larger Z value difference
32
Q

how can we calculate the amount of sound reflected at an interface

A
  • intensity reflection coefficient
  • IRC = reflected intensity / incident intensity
  • IRC = (Z2 - Z1) / (Z2 - Z1) ^2
33
Q

how do we calculate the intensity transmission coefficient

A

ITC = 1 - IRC

34
Q

what is refraction deal with

A
  • deals with the sound transmitted across an interface

- specifically when velocities differ across the interface and angle of incidence non perpendicular

35
Q

what is snells law

A
  • sound can bend across an interface due to a difference in media velocity
36
Q

if velocities are equal across the interface what happens to the sound

A
  • no refraction occurs
37
Q

if the velocity of the first medium is greater than the second what happens to the sound

A
  • bend towards normal
38
Q

if the velocity of the first medium is less than the second what happens to the sound

A
  • bend away from the normal
39
Q

what is total internal reflection

A
  • special type of reflection when V1 is less than V2 and angle reaches a critical value
40
Q

what is scatter a result of

A
  • sound interacting with interfaces smaller than a wavelength and rough
41
Q

what is scatter responsible for

A
  • internal texture of organs
42
Q

what does scatter result in

A
  • incident sound beam breaking up into many different echoes

- these echoes will have intensities that are a fraction of the incident beam reflecting in any direction

43
Q

what 2 things is the amount of scatter dependant on

A
  • frequency
    + higher frequency increased scatter
  • reflector size
    + smaller reflector increase scatted
44
Q

what is backscatter

A
  • when sound is directed back to its origin
45
Q

what is backscatter responsible for

A
  • organ parenchyma
46
Q

what is acoustic speckle

A
  • brightness non uniformities from interference patterns from echoes that have undergone multi path scattering
47
Q

what is Rayleigh scatter

A
  • when sound interacts with RBCs that are much smaller than a wavelength that are very very weak
48
Q

how does wave front divergence affect attenuation

A
  • as sound beam diverges the intensity of the beam weakens and adds to overall attenuation of the beam

Physics (8 decks)

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