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Ambers Physics 1 > Attenuation > Flashcards

Attenuation Flashcards

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1
Q

Weakening of sound beam as it travels is called

A

Attenuation

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2
Q

Attenuation is of great clinical importance in what three ways

A
  • limit imaging depth
  • has to be compensated for
  • useful in diagnosis
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3
Q

Relative units of measurement expressing loudness (intensity) of sound waves

A

Bel (B)

Decibel (dB)

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4
Q

1 bel is equal to how many decibels

A

1 Bel = 10 decibel

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5
Q

Bel or decibel is used for virtually all loudness measurements

A

Decibel

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6
Q

Instrument output uses what to determine the power of the sound leaving the transducer

A

Decibels

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7
Q

What uses decibels to express the number of shades of gray displayed on the monitor

A

Dynamic range

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8
Q

What two things use decibels to express the amount of amplification required to optimize the returning echos

A

Gains and TGC

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9
Q

What formula is used to calculate bels

A

New I
Bel = Log ( —————— )
Original I

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10
Q

What formula used to calculate decibels

A

New I
dB = 10log ( ————— )
Original I

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11
Q

Formula for power

A

New P
dB = 10log ( ————— )
Original P

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12
Q

Formula for voltage

A

New V
dB = 20Log ( ————— )
Original V

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13
Q

What are the two rules of thumbs when dealing with decibel

A
  • a 3 dB drop = half the original intensity

- a 10 dB drop = 0.1 the original intensity

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14
Q

Amount of attenuation that occurs with each cm travelled is called

A

Attenuation coefficient

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15
Q

In soft tissue the attenuation coefficient is equal to

A

Half the frequency

1/2 frequency

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16
Q

In soft tissue 0.5 dB of attenuation occurs every 1 cm per —-MHz

A

Per 1 MHz

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17
Q

What is the total attenuation formula

A

TA = (att Coeff) Times (the path length)

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18
Q

In soft tissue the formula for total attenuation is

A

TA = (1/2 frequency) times (path length)

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19
Q

The distance sound must travel in material to reduce the intensity to half the original value is called what

A

The half-value layer

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20
Q

What three things can vary attenuation

A
  • nature of tissue (dead/alive)
  • frequency of ultrasound
  • depth
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21
Q

What are the five general reasons we have attenuation

A
  • absorption
  • reflection
  • refraction
  • scatter
  • wave-front divergence
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22
Q

The conversion of sound energy into heat is called

A

Absorption

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23
Q

The dominant factor in effecting attenuation is what and it accounts for approximately how much

A

Absorption

80%

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24
Q

What are the three factors that affect absorption

A
  • viscosity
  • relaxation time on molecules
  • frequency
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25
Q

The ease in which molecules can slide past one one another is called

A

Viscosity

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26
Q

Does the viscosity increase or decrease to provide greater resistance?

A

Increase

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27
Q

Greater resistance due to increased viscosity creates more or less friction

A

More friction

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28
Q

Having increased viscosity which creates more resistance, therefore more friction, does the attenuation increase or decrease

A

Attenuation increases

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29
Q

What is it called when sound energy is converted into heat

A

Friction

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30
Q

When a mechanical force is applied to a molecule what happens to it

A

The molecule will vibrate

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31
Q

The time it takes for a molecule to come to rest is called

A

The relaxation time

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32
Q

If molecules can’t come to rest before the next compression time, is more or less energy required to reverse its direction

A

Take more energy, and it produces heat

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33
Q

Since relaxation time is pretty constant in soft tissue what has more influence in changing the amount of absorption

A

Frequency

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34
Q

If frequency increases is there more or less time for molecules to recover during relaxation process

A

Less time

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35
Q

The molecules having less time during the relaxation process, results in more or less absorption

A

Results in more absorption

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36
Q

What are the two types of reflection

A

Specular

Non-specular

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37
Q

This type of reflection occurs when a sound beam hits a large smooth surface

A

Specular

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38
Q

A large surface is relative to —— used, and is greater then what in diameter

A

Relative to the frequency used

Greater then one wavelength in diameter

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39
Q

—— from interfaces contributes to the majority of the images

A

Reflection

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40
Q

What is an example of a specular refector

A

Diaphragm

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41
Q

Intensity of the reflected sound depends on what two things

A

Angle of incidence

Acoustic impedance of two media

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42
Q

The angle of incidence is equal to what

A

The angle of reflection

43
Q

Reflections of sound that have a what way of incidence may not return to the probe

A

Non-perpendicular

44
Q

Does perpendicular or non-perpendicular incidence help improve reflection

A

Perpendicular

45
Q

How fast sound moves through a medium depends on what two things

A

Density and stiffness

46
Q

As density decreases or stiffness increased what happens to the propagation speed

A

It increases

47
Q

Acoustic impedance is also known as

A

Characteristic impedance

48
Q

Describe the relationship between acoustic pressure and the speed of the particle vibrations in a sound wave (speed propagation)

A

Acoustic impedance

49
Q

What are the units for impedance

A

Rayls (Z)

50
Q

Impedance increase if density and velocity increase or decrease

A

Increase

51
Q

Z values vary with different tissues in the body due to the different what two things

A

Density and stiffness

52
Q

Does acoustic impedance depend on frequency

A

NO

53
Q

Does there need to be a larger or smaller difference between the interface of two media to have a bigger reflection

A

Larger difference

54
Q

The amount of sound that reflects at the interface is called

A

Intensity reflection coefficient

55
Q

If we know how much sound is reflected we can then calculate what

A

The amount of sound transmitted

56
Q

The amount of sound transmitted is called the

A

Intensity transmission coefficient

57
Q

If more sound is transmitted then less sound must be reflected yes or no

A

Yes

58
Q

As the differences between two interfaces increase more or less sound will be reflected

A

More

59
Q

If the impedances between the interface is equal what happens

A

No reflection

60
Q

Reflection occurs when there is what at an interface

A

A difference in impedance (Z values)

61
Q

Is it possible for 2 tissues to have different sound velocities but have the same Z values

A

Yes it sure is

62
Q

Looking at the sound returning to the probe from an interface is what

A

Reflection

63
Q

Dealing with the sound that is transmitted across the interface

A

Refraction

64
Q

Sound obeys which law

A

Snell’s law of optics

65
Q

What happens to sound when the velocities differ across the interface and the angle of incidence is non-perpendicular

A

Refraction

66
Q

If velocities across the interface are equal then what kind of refraction occurs

A

No refraction occurs

67
Q

If velocity of first medium is greater then second what kind of refraction occurs

A

Refract (bend) towards the normal

68
Q

If velocity of first medium is less then second what kind of refraction occurs

A

Refract (bend) away from normal

69
Q

What type of reflection occurs when first medium is less then second and the angle reaches a critical value

A

Total internal reflection

70
Q

Is it possible to have different velocities but have the same Z value

A

Yes

71
Q

This only occurs when velocities are different across an interface and there in non-perpendicular incidence

A

Refraction

72
Q

Does refraction relate to Z values

A

No

73
Q

Non-specular is called

A

Scatter

74
Q

When sound interacts with interfaces that are small and rough what occurs

A

Scatter

75
Q

A small interface is small in comparison to

A

Smaller then a wavelength

76
Q

Scatter is typical from what kind of media

A

Heterogenous

77
Q

What is an example of heterogenous media

A

Cells

Suspended particles

78
Q

Is scatter dependent or independent of sound direction

A

Independent

79
Q

Is scatter responsible for external or internal texture of organs

A

Internal

80
Q

What happens to the incident beam in scatter

A

It breaks up into many different echos

81
Q

The resulting echos in scatter must equal what

A

The incident beam (100%)

82
Q

Can the echos direction be predicted

A

No impossible to predict

83
Q

The amount of scatter depends on what two things

A

Frequency

Reflector size

84
Q

If the frequency is higher what happens to scatter

A

Higher frequency greater amount of the scatter

85
Q

If the reflector is smaller what happens to the scatter

A

Smaller reflector greater amount of scatter

86
Q

When the sound is directed back to its origin it is called

A

Backscatter

87
Q

Is the backscatter responsible for the image we see

A

Yes

88
Q

Because scatter is random there is a potential for what

A

Brightness non-uniformities

Dark and bright spots

89
Q

What is the cause of brightness non-uniformities

A

The result of interference patterns from echos that have undergone multi-path scattering

90
Q

The phenomenon of brightness non-uniformities is called

A

Acoustic speckle

91
Q

To minimize acoustic speckle what techniques are used

A

Persistence also called frame averaging

92
Q

What is the specific type of scatter that occurs when sound interacts with red blood cells called

A

Rayleigh scatter

93
Q

Why do we not see blood flow at faster velocities

A

Red blood cells dimensions are much smaller then wavelengths so the scatter is weak

94
Q

The spreading of sound beam as it travels is called

A

Divergence

95
Q

As area increases what happens to intensity

A

Intensity decreases

96
Q

As sound beam diverges the intensity of the beam weakens which adds to the overall attenuation of the beam but is called what

A

Wave front divergence

97
Q

Define attenuation

A

The weakening of sound as it travels

98
Q

How many decibels of sound are required to reduce the initial intensity by half

A

3 dB

99
Q

Define half value layer

A

How many centimetres it takes to cut the intensity in half

100
Q

List the five means that sound is attenuated in soft tissue

A 
Scatter 
Wave front divergence
Absorption
Refraction
Reflection
101
Q

What happens to attenuation when viscosity of a medium is increased

A

Attenuation increases, because there is more friction meanin more absorption

102
Q

What happens to attenuation if a mediums relaxation time is increased

A

Attenuation increases because more energy is needed to move the vibrating particle in the direction of the sound beam

103
Q

What is required for reflection of sound to occur at an interface

A

A difference in Z values (acoustic impedance)

Ambers Physics 1 (7 decks)

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