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Physics 1 > C6: Attenuation > Flashcards

C6: Attenuation Flashcards

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1
Q

Define attenuation

A

weakening of sound as it travels

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2
Q

1 Bel = how many decibels (dB)

A

10

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3
Q

how many dB is 100% power in US

A

0dB

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4
Q

what is the formula for dB?

A

dB = 10log (new Intensity/original Intensity)

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5
Q

can the dB formula also be used to calculate power and voltage?

A

yes… substitute intensity in the original formula for power or voltage

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6
Q

how does a 3 dB drop effect intensity?

A

a drop of 3 dB will half the original intensity

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7
Q

how does a 10 dB drop effect intensity?

A

a drop of 10 dB is 0.1 of the original intensity

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8
Q

definition of attenuation coefficient

A

the amount of attenuation that occurs with each one centimetre travelled

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9
Q

how many dB of attenuation occurs in soft tissue for every 1 cm travelled per 1 MHz

A

0.5 dB

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10
Q

what is the attenuation coefficient for soft tissue

A

1/2 frequency

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11
Q

whats the formula for total attenuation in soft tissue?

A

TA = 1/2 (f in MHz) x Path length in cm

the attenuation coefficient if 1/2 (f) in this case

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12
Q

what is the half value layer

A

the distance that sound must travel in a material to reduce the intensity to half its original value (3 dB loss in equal to 1/2 the intensity)

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13
Q

attenuation varies with which 3 things

A

nature of tissue
frequency
depth

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14
Q

what are the 5 reasons that attenuation occurs

A

WARRS:

Wave-front divergence
Absorption
Reflection
Refraction
Scatter
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15
Q

what is the most dominant factor effecting attenuation and what % of attenuation does it account for

A

absorption

80%

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16
Q

define absorption

A

the conversion of sound into heat

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17
Q

3 factors that effect absorption

which is the most dominant factor

A

viscosity
relaxation time
frequency

frequency is the most dominant

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18
Q

describe viscosity and how it effects absorption

A
  • the ease at which molecules slide past one another
  • increased viscosity provides greater resistance and more friction… this friction converts the sound waves to heat which increases attenuation
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19
Q

describe relaxation time and how it effects absorption

A
  • time it takes a molecule to come to rest after a mechanical force is applied
  • if a molecule cant come to rest before it is stimulated again, than more force is needed to stimulate this molecule again which produces heat.
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20
Q

is relaxation time relatively constant

A

yes

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21
Q

how does increasing frequency effect relaxation time

A

increased frequency means there is less time for molecules to come to rest because sound waves are being produced at a faster rate… this increases absorption and attenuation

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22
Q

what are the 2 types of reflection, describe them

A

-specular:
occurs when a sound wave hits a large, smooth surface, larger than 1 wavelength

-non-specular:
occurs when a sound wave hits a small, rough surface, smaller than 1 wavelength

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23
Q

intensity of reflected sound depends on what two things

A

angle of incidence (90 is best, sound comes straight back)

acoustic impedance of the two media

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24
Q

what is the formula for acoustic impedance

A

acoustic impedance (Z) = density (p) x velocity (c)

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25
Q

units for impedance

A

Rayls (z)

26
Q

does acoustic impedance depend on frequency?

A

no

27
Q

what effect does a larger difference in z values of the media have on reflection

A

larger Z = bigger reflection

28
Q

what is the intensity reflection coefficient

A

amount of sound that reflects at an interface

29
Q

formula for intensity reflection coefficient

A

IRC = (Z2 - Z1 / Z2 + Z1) ^2

or IRC = reflected intensity/incident intensity

  • it does not matter which Z value you choose as Z2 or Z1*
30
Q

what is the formula for intensity transmission coefficient

A

ITC = 1 - IRC… the more sound is reflected, the less is transmitted

31
Q

how does the difference b/w the impedance values of 2 interfaces effect reflection?

A

greater difference in values = more reflection of sound

32
Q

what happens to reflection if impedance is equal across interfaces

A

no reflection

33
Q

in general, why does reflection occur

A

due to difference in Z values at an interface

34
Q

can you have 2 tissues w/ difference velocities but the same z value

A

yes

35
Q

Is reverberation a type of reflection

A

yes

36
Q

what 2 criteria must be met in order for refraction to occur

A
  • different velocities of 2 media

- non-perpendicular angle of incidence

37
Q

what is the formula for Snells law

A

{sin theda (incidence) / sin theda (transmitted)} = velocity (incidence) / velocity (transmitted)

38
Q

Snells Law: if velocities across an interface are equal will there be any refraction

A

no

39
Q

Snells Law: if velocity 1 is grater than velocity 2 how will the refracted sound beam travel

A

towards the normal

40
Q

Snells Law: if velocity 2 is grater than velocity 1 how will the refracted sound beam travel

A

away from the normal

41
Q

Snells Law: when does total internal reflection occur and what is it

A

occurs when V2 is greater than V1 and the angle reaches a critical value

-no sound will be transmitted back to the probe

42
Q

in general, why does refraction occur

A

if there is a difference in velocities at an interface and there is non- perpendicular angle of incidence

43
Q

is refraction related to Z values

A

no

44
Q

can you have equal Z values and have different velocities at an interface

A

yes (because density can be different)

45
Q

what is scatter

why does it occur

A
  • scatter is the breaking up of a sound beam into many different echos, it is responsible for the internal texture of organs and causes non uniform brightness
  • it is the result of sound interacting with interfaces that are rough and smaller than 1 wavelength (e.g. non-specular reflectors)
46
Q

does scatter usually occur with heterogenous or homogenous media

A

heterogenous (cells or particles)

47
Q

does scatter depend on sound direction

A

no

48
Q

what does back scatter refer to

A

any scatter that returns back to its origin

49
Q

the amount of scatter depends on what 2 factors

A
  • frequency

- reflector size

50
Q

how do frequency and reflector size effect scatter

A

higher frequency = more scatter

smaller reflector = more scatter

51
Q

what is responsible for the parenchyma that we see?

A

backscatter

52
Q

what phenomenon is responsible for acoustic speckle

A

scatter

53
Q

what is acoustic speckle

how does we correct for it

A

areas of non-uniform brightness seen in the parenchyma

correct by using persistence

54
Q

what is Rayleigh scatter

A

a specific type of scatter that occurs when sound waves hit RBCs (b/c they are so small)

-this type of scatter is very weak and keeps us from seeing blood flow at faster velocities

55
Q

how does wavefront divergence contribute to attenuation

A

as the sound beams diverge, the intensity of the beam weakens as it covers a larger area
…this contributes to the overall attenuation of sound

56
Q

why is attenuation clinically important (3 reasons)

A

limits our imaging depth
it needs to be compensated for (TGC, gain, etc)
can be useful to diagnose (shadowing)

57
Q

what do gains and TGCs express

A

the amount of amplification thats required to optimize returning echos (in decibels)

58
Q

instrument output, dynamic range, and gain and TGCs all use which relative units

A

decibles

59
Q

why do Z values vary with different tissues in the body

A

due to differences in density and stiffness

60
Q

why does scatter create areas of non-uniform brightness

A

due to the interference patterns from echoes that have undergone multi-path scattering… resulting in some areas of increased intensity and decreased intensity

Physics 1 (7 decks)

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