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CMI 350 > Ch.6 Interaction of Sound & Media > Flashcards

Ch.6 Interaction of Sound & Media Flashcards

1
Q

what standard measurement tool is used to report changes such as degree of attenuation and the extent of amplification?

A

Decibel notation

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2
Q

Decibels are based on what mathematical construct?

A

logarithms

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3
Q

Decibel notation is:
(4)

A

a relative measurement
a comparison
a ratio
logarithmic

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4
Q

Decibel notation does NOT measure absolute numbers; rather

A

relative changes
Ex: signal strength doubled
signal is now 1/10th are large as it used to

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5
Q

Decibels require how many intensities?

A

two
1) the reference (starting level)
2) the actual level at the time of measurement

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6
Q

Positive decibels report what?

A

signals that are increasing in strength, or getting larger

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7
Q

What do negative decibels report?

A

signals that are decreasing in strength, or getting smaller

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8
Q

Decibels: Meaning:
3 dB
10 dB
-3 dB
-10 dB

A

double
ten times larger
half
one-tenth

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9
Q

A sound beam has increased from its initial intensity by a factor of 100. How is this described in decibel notation?

A

100 = 10 x 10
10 times in dB is +10 dB
= +10 , +10
= + 20

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10
Q

What is the meaning of a 6 dB change?

A

3 dB = 2x bigger
6 dB = 3 dB , 3 dB
= 2 x 2 = 4 times bigger

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11
Q

A sound beam has decreased to 1/100 of its initial intensity. how is this described in decibel notation?

A

1/100 = 1/10 x 1/10
1/10 = -10 dB
= -10 dB , -10dB
= -20 dB

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12
Q

What is the meaning of 20 dB?

A

10 dB means 10x bigger
20 dB = 10 dB , 10 dB
= 10 x 10 = 100x bigger

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13
Q

what is attenuation?

A

a decrease in intensity, power, and amplitude as sound waves travel in a medium

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14
Q

What two factors is attenuation determined by?

A

path length
frequency of sound

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15
Q

what is the relationship between distance and attenuation?

A

directly related
the farther the sound travels, the greater the attenuation

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16
Q

what is the relationship between frequency and attenuation?

A

directly related
attenuation is greater in higher frequencies

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17
Q

longer distances and higher frequencies means

A

MORE attenuation

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18
Q

shorter distances and lower frequencies means

A

LESS attenuation

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19
Q

What three processes contribute to attenuation?

A

Reflection
Scattering
Absorption

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20
Q

Reflection is likely to occur when the dimension of the boundary is _____

A

large, that is more than a few wavelengths of the sound

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21
Q

What are the two forms of reflection that are created in soft tissue?

A

Specular
Diffuse

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22
Q

Specular Reflection occurs

A

when the boundary is smooth, the sound reflects in only one direction in an organized manner

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23
Q

One limitation to Specular Reflection is

A

that once the wave is slightly off-axis, the reflection does not return to the transducer

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24
Q

Diffuse Reflection (backscatter) occurs

A

when a wave reflects off an irregular surface, it radiates in more than one direction

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25
Q

What is the advantage of diffuse reflections?

A

the interfaces at suboptimal angles to the sound beam can still produce reflections that will return to the transducer

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26
Q

What is the disadvantage of diffuse reflections?

A

is that backscattered signals have a lower strength than specular reflections

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27
Q

Scattering
what is it directly related to?

A

is the random redirection of sound in many directions.
frequency

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28
Q

Sound scatters when the tissue interface is ______; that is equal to or less than the _________ of the incident sound beam.

A

small
wavelength

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29
Q

What scatters more higher or lower frequency beams?

A

Higher frequency

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30
Q

Rayleigh Scattering

A

is a special form of scattering that occurs when the structure’s dimensions are much smaller than the beam’s wavelength

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31
Q

Rayleigh scattering redirects the sound wave _______ in __ _________.

A

equally in all directions

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32
Q

What is an example of a Rayleigh scattering interaction?

A

red blood cells and sound waves

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33
Q

Lung tissue scatters sound because

A

the alveoli are filled with air

34
Q

What is the relationship between Rayleigh scattering and frequency?

A

Rayleigh scattering increases dramatically with frequency (directly)
R. Scattering = Frequency^4

35
Q

What reflection type does the sound wave return back to the transducer?

A

Specular -Organized
Diffuse/Backscatter-disorganized

36
Q

What reflection type does the sound wave diffuse in all directions?

A

Rayleigh Scattering- organized
Scattering- disorganized

37
Q

What is the conflict between frequency and attenuation?

A

Higher frequency sound waves produce shorter pulse, which usually create more accurate images. But higher frequency sound attenuates more and is less capable of traveling to substantial depths

38
Q

Absorption occurs

A

when ultrasonic energy is converted into another energy form, such as heat

39
Q

What is the relationship between frequency and absorption?

A

directly related
bc of absorption, higher frequency waves attenuate more than lower

40
Q

Attenuation is a process whereby

A

sound energy is extracted from a wave by absorption, scattering, and reflection

41
Q

attenuation coefficient

A

is the number of decibels of attenuation that occurs when sound travels one centimeter
units: dB/cm

42
Q

what is the advantage of describing sound weakening with an attenuation coefficient ?

A

is that its value remains constant, regardless of how far the sound travels

43
Q

If sound travels to a depth of 5 cm and the attenuation coefficient remains at 2 dB/cm, what is the total attenuation of the beam?

A

10 dB

44
Q

what is the total attenuation (dB) formula?

A

total att. (dB) = atten. coefficient (dB/cm) X distance (cm)

45
Q

In soft tissue, the attenuation coefficient (dB/cm) is related directly to what?

A

frequency
the attenuation coefficient is one-half the frequency

46
Q

What is the attenuation coefficient formula in soft tissue?

A

atten. coeff. (dB/cm) = frequency (MHz)/2

47
Q

Medium: Attenuation:
Water
Biologic fluids (blood, urine, amniotic fluid)
Fat
Soft Tissue
Muscle
Bone and Lung
Air

A

Extremely low
low
low
intermediate
higher
even higher
extremely high

48
Q

Half-Value Layer Thickness

A

is the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value
–> or is the depth of tissue that results in 3 dB of attenuation to the intensity

49
Q

Half-Value Layer Thickness
Units
Typical Values
Synonyms (3)

A

cm
0.25 to 1.0 cm
penetration depth, depth of penetration, half-boundary layer

50
Q

What are the two factors that half-value thickness depends on?

A

the medium
the frequency of sound

51
Q

Thin half value layer:
_____ frequency
media with ____ attenuation rate
Ex tissues:

A

high
high
lung or bone

52
Q

Thick half value layer:
____ frequency
media with ____ attenuation rate
Ex tissues:

A

Low
Low
Fluids

53
Q

Impedance
units
typical values
determined by
synonym

A

is the acoustic resistance to sound traveling in a medium (Z)
rayls
1.25 to 1.75 Mrayls
Medium ONLY; calculated NOT measured
characteristics impedance

54
Q

Impedance formula

A

impedance (rayls) = density (kg/m^3) X prop. speed (m/s)

55
Q

Reflection of an ultrasound wave depends upon what?

A

the difference

56
Q

Normal incidence
synonyms (3)

A

means that the incident sound beam strikes at EXACTLY 90 degrees
-Perpendicular
-orthogonal
-right angle

57
Q

Oblique incidence
synonyms

A

means that when the incident sound beam strikes the boundary at any angle other than 90 degrees
-Non-perpendicular

58
Q

incident intensity

A

is the sound wave’s intensity immediately before it strikes a boundary

59
Q

reflected intensity

A

is the intensity of the portion of the incident sound beam that, after striking a boundary, returns back from where it came

60
Q

Transmitted intensity

A

is the intensity of the portion of the incident beam that, after striking a boundary, continues forward in the same general direction that it was traveling

61
Q

Intensity conservation of energy at the boundary formula

A

incident (starting) incident = reflected intensity + transmitted intensity

62
Q

Intensity Reflection Coefficient (IRC)

A

is the percentage of the intensity that bounces back when a sound beam strikes the boundary between two media

63
Q

What is the IRC when the sound wave is reflected between two soft tissues? vs when it strikes btw soft tissue and bone or st and air?

A

1% or less
greater percentage of the wave is reflected

64
Q

Intensity Transmission Coefficient (ITC)

A

is the percentage of intensity that passes in the forward direction when the beam strikes an interface between two media

65
Q

ITC formula during transmission with normal incidence

A

ITC %= transmitted intensity/incident intensity X 100
=1 - IRC

66
Q

With normal incidence, same impedance, what occurs between the two media ?

A

ALL of the sound is transmitted forward

67
Q

What % of a sound wave’s intensity is transmitted when the beam strikes an interface btw two soft tissues? btw bone and soft tissue?

A

most (99% or more)
smaller %

68
Q

IRC and ITC conservation of energy formula

A

100%= IRC + ITC
unitless

69
Q

IRC formula during reflection with normal incidence

A

IRC %= [ Z2-Z1 / Z2 + Z1]^2 X 100
Z2: transmitted intensity
Z1: reflected intensity

70
Q

When a sound beam strikes a tissue boundary at a 90 angle (normal incidence), REFLECTION occurs ONLY if the media on either side of the boundary have _______ ______

A

different impedances

71
Q

NO reflection will occur if the two media have _____ _____

A

identical impedance

72
Q

Which two physical principles ALWAYS apply to reflection with oblique incidence ?

A

Conservation of energy (100% = reflection coeff. + transmission coeff. )
Reflection angle=incident angle

73
Q

Refraction

A

is a change in direction of wave propagation when traveling from one medium to another.
-Transmission with Oblique incidence
-transmission with a bend

74
Q

Refraction occurs ONLY with which two conditions?

A

oblique incidence
different propagation speeds of the two media

75
Q

Snell’s Law

A

quantifies the physics of refraction:
sine (transmission angle) / sin (incidence angle) = speed of Medium 2 / speed of Medium 1

76
Q

Under what conditions will the transmission angle = the incident angle?

A

Refraction will NOT occur if the speeds of the two media are identical. Will move straight ahead.

Refraction requires:
oblique angles
different propagation speeds

77
Q

Under what conditions will the transmission angle be greater than the incident angle?

A

When the speed of Medium 2 is > Medium 1

78
Q

Under what conditions will the transmission angle be less than the incident angle?

A

When the speed of Medium 2 is < Medium 1

79
Q

Angle of Transmission?
Speed 2 = Speed 1
Speed 2 > Speed 1
Speed 2 < Speed 1

A

no refraction; Trans = Inc
transmission angle > incident angle
transmission angle < incident angle

80
Q

Requirement?
Event:
1) Reflection with normal incidence
2) Reflection with oblique incidence
3) Transmission
4) Refraction

A

1) different impedances required
2) can NOT predict
3) derived from reflection info, use law of conservation of energy
4) oblique incidence and different speeds required

CMI 350 (11 decks)

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