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PHYS 216 > Attenuation > Flashcards

Attenuation Flashcards

1
Q

What is attenuation?

A

The weakening of sound beams as it travels

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2
Q

Why is attenuation important? 3

A
  1. Limits our imaging depth
  2. Must be compensated for
  3. Can be useful for diagnosis
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3
Q

What are the units we will use to measure loudness (intensity level) of sound waves?

A

Bels (B) and decibel (dB)

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4
Q

What is the unit conversion for Bels and decibels?

A

1 Bel = 10 decibels

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5
Q

Why do we normally use dB for virtually all loudness measurements?

A

Because dB is a rather large unit.

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6
Q

Instrument output uses decibels to determine what?

A

The power of the sound leaving the transducer

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7
Q

Dynamic range uses decibels to do what?

A

Express the number of shades of grey displayed on the monitor.

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8
Q

Gain and TGC uses decibels to express what?

A

The amount of amplification required to optimize the returning echoes

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9
Q

What is the formula used to calculate bels?

A

Bel = log (new I/ original I)

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10
Q

What is the formula to calculate decibels?

A

DB= 10log(new I/ original I)

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11
Q

The bel and decibel formula can be used to calculate what?

A

Power and voltage as well

DB= 10log ( new P/ original P)

DB = 20log( New V/ Original V)

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12
Q

What is the rule of thumb for dealing with decibel?

A
  1. A 2 dB drop = 1/2 the original intensity
  2. A 10 dB drop = 0.1 of the original intensity
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13
Q

What is the attenuation coefficient? 2

A

The amount of attenuation that occurs with each one centimetre travelled.

  1. In soft tissue 0.5 dB of attenuation occurs for ever one centimeter travelled per 1 MHz
  2. In soft tissue attenuation coefficient is equivalent to 1/2 frequency
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14
Q

What is the formula for attenuation coefficient (TAtt)?

A

TAtt = Att. Coef x path length ( CM)

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15
Q

In soft tissue what is the formula for total attenuation (TAtt)?

A

TAtt = (1/2)f x path length (cm)

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16
Q

What is half-value layer?

A

The distance sound must travel in a material to reduce the intensity to half its original value

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17
Q

What is the half value intensity numerically?

A

Equivalent to 3 dB

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18
Q

Attenuation can vary with what three things?

A
  1. Nature of the tissue
  2. Frequency of the ultrasound
  3. Depth
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19
Q

What are the five general reasons we have have attenuation?

A
  1. Absorption
  2. Reflection
  3. Refraction
  4. Scatter
  5. Wave-font divergence
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20
Q

What is absorption in attenuation?

A

Conversion of sound energy to heat and is the dominant factor of attenuation accounting for 80%

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21
Q

What is three factors that effect absorption in attenuation?

A
  1. Viscosity
  2. Relaxation time on molecules
  3. Frequency
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22
Q

What is viscosity in terms of absorption?

A

Viscosity is the ease in which molecules can slide past one another

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23
Q

What does having a increased viscosity do for attenuation?

A

Greater resistance, more friction and thus increased attenuation

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24
Q

What is friction in terms of absorption?

A

Sound energy

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25
Q

In terms of absorption when a mechanical forces is applied what happens?

A

The molecule will vibrate

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26
Q

What is relaxation time?

A

The time it takes to come to rest

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27
Q

If molecules can’t come to rest before the next compression phase, what happens?

A

More energy is required to reverse its direction. This produces heat

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28
Q

If relaxation time in soft tissue relatively constant, what influences the amount of absorption?

A

The changes in frequency

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29
Q

In terms of absorption if frequency is increased then there is less what?

A

Time available for molecules to recover during the relaxation process. This results in more absorption

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30
Q

What are the two types of reflection?

A
  1. Specular
  2. Non- Specular
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31
Q

Specular reflection occurs when?

A

A sound beam hits a large, smooth surface

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32
Q

What is a large surface relative to?

A

The frequency used and is greater than one wavelength in diameter

33
Q

Reflection from interfaces contributes to what?

A

The majority of our image

34
Q

What is a good example of a good Specular reflector?

A

The diaphragm

35
Q

The intensity of the reflected sound depends on what two things?

A
  1. Angle of incidence
  2. Acoustic impedance of two media
36
Q

For a reflection the angle of incidence will equal what?

A

The angle of reflection

37
Q

If reflections of sound that have non-perpendicular incidence what might happen?

A

The sound may not return to the probe

38
Q

Perpendicular incidence helps improve what?

A

Reflection

39
Q

Acoustic impedance is the other factor that determines what?

A

The intensity of reflected sound

40
Q

Density and stiffness determines what?

A

How fast sound moves through a medium

41
Q

As density decreases or stiffness increases what happens?

A

Speed increases

42
Q

What is the impedance (z) formula?

A

Impedance(z) = Density(P) x Velocity(c)

43
Q

Acoustic impedance describes what relationship?

A

Acoustic pressure and the speed of particle vibrations in a sound wave

44
Q

What is the units of impedance?

A

Rayls (z)

45
Q

What does z increases with what?

A

Density or velocity

46
Q

Z values vary with the different tissues in the body since what?

A

There are differences in the density and the stiffness

47
Q

Acoustic impedance does not depend on what?

A

Frequency

48
Q

The larger the Z value difference at the interface of 2 media what happens?

A

The bigger the reflection

49
Q

What is the intensity reflection coefficient?

A

The amount of sound that reflects at an surface

50
Q

What is the Intensity reflection coefficient (IRC) formula?

A

IRC= reflected intensity (Ir) / incident intensity (Ii)

Or

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

51
Q

What is the Intensity transmission coefficient (ITC)?

A

ITC = 1 - IRC

** the more sound reflected the less transmitted

52
Q

If the impedance is equal across the interface what kindof reflection happens?

A

No reflection will occur

53
Q

What does refraction deal with?

A

The sound that is transmitted across the surface

54
Q

What is refraction?

A

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

55
Q

What is the formula for snell’s law of optics?

A

(Sin0i/sin0t) = (Vi/Vt)

56
Q

What does Snell’s law of optics mean for refraction?

A

Sound can bend across an interface due to a difference in media velocity

57
Q

If we know the velocity of tissue across the interface and the angle of the sound that comes in what can we do in terms of refraction?

A

We can calculate how much it will bend

58
Q

If the velocities across the interface are equal what happens?

A

No refraction will occur

59
Q

If the velocity of the first medium is greater than the second, what happens?

A

The sound will bend towards the normal

60
Q

What happens if the velocity of the first medium is less than the second?

A

Sound will bend away from the normal

61
Q

What is total internal reflection?

A

A special type of reflection occurs when V1 is less than V2 and the angle reaches a critical value

62
Q

What is scatter?

A

The result of sound interacting with interfaces smaller than a wavelength and rough

63
Q

Where does scatter typically happen?

A

From heterogeneous media and is independent of sound direction

64
Q

Scatter is responsible for what?

A

Internal texture

65
Q

Scatter results in what changes to our sound beam?

A

The incident sound beam breaking up into many different echoes, in as many directions as possible

66
Q

Echoes during scatter have intensities that are what?

A

Intensities that are a fraction of the incident beam

67
Q

What is backscatter?

A

When sound is directed back to its origin

68
Q

The amount of scatter depends on what?

A

Frequency and reflector size

69
Q

In terms of scatter what happens if we have higher frequency?

A

The greater the amount of scatter

70
Q

What happens when the reflector is smaller in terms of scatter?

A

The greater the amount of scatter

71
Q

Backscatter is responsible for what?

A

The parenchyma we see

72
Q

Because scatter is random there is potential for what?

A

There is the potential for brightness non-uniformities

73
Q

What is acoustic speckle?

A

Interference patterns from echoes that have undergone multi-path scattering

74
Q

What does modern equipment do in terms of acoustic speckle?

A

Utilizes techniques to help minimize acoustic speckle

75
Q

What is Rayleigh scatter?

A

There is a specific type of scatter that happens when sound interacts with red blood cells

76
Q

How does RBC’s dimensions affect sounds?

A

They have dimensions that are smaller than a wavelength of sound. This type of scatter is very weak and is why we do not see blood flow at faster velocities

77
Q

What is wave front divergence?

A

The divergence spreading off the sound beam as it travels. The relationship between intensity and divergent area where the intensity decreases as the divergent area increases.

78
Q

In terms of wave font diverergence, as the sound beam diverges the intensity of the beam weakens and adds what?

A

Overall attenuation of the beam

79
Q

What happens to the amount of absorption if there is an increase in relaxation time?

A

It increases

PHYS 216 (8 decks)

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