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Ultrasound Physics > Ch 6: Sound and Media > Flashcards

Ch 6: Sound and Media Flashcards

1
Q

What is it called when sound waves weaken as they travel in the body?

A

attenuation

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2
Q

What is it called when the u/s system strengthens the electrical signal?

A

amplification

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3
Q

Define ‘decibel notation’.

A

A standard measurement tool to report changes in the degree of attenuation or the extent of amplification.

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4
Q

Logarithms are…

A

the mathematical construct on which decibels are based; a novel method of rating numbers.

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5
Q

What is the log of 1,000?

A
  1. (10 x 10 x10)
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6
Q

What is the log of 10,000?

A
  1. (10 x 10 x 10 x 10)
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7
Q

What is an everyday example of a logarithmic scale?

A

The Richter scale, which measures the strength of earthquakes.

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8
Q

Decibel notation does not measure absolute numbers; rather decibels report…

A

relative changes.

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9
Q

What are the two intensities that decibels require?

A

a starting level and an actual (ending) level

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10
Q

In decibels, the measured level is divided by the starting level, which means that decibels are a…

A

ratio.

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11
Q

What is a relative measurement, a comparison, a ratio, and logarithmic?

A

decibel notation

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12
Q

Positive decibels report signals that are…

A

increasing in strength.

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13
Q

Negative decibels report signals that are…

A

decreasing in strenth.

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14
Q

Attenuation is determined by which two factors?

A

path length and frequency of sound

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15
Q

Distance and attenuation are ___ related.

A

directly. The farther sound travels, the more it attenuates.

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16
Q

Frequency and attenuation are ___ related.

A

directly. The higher the frequency, the more it attenuates.

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17
Q

What three processes contribute to attenuation?

A

reflection
scattering
absorption

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18
Q

What is it called when a portion of the wave’s energy is redirected after striking a boundary?

A

reflection

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19
Q

What two forms of reflection are created in soft tissue?

A

specular and diffuse/backscatter

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20
Q

Define ‘specular reflection’.

A

Sound that is reflected in only one direction in an organized manner, as when a boundary is smooth.

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21
Q

Define ‘diffuse/backscatter reflection’.

A

Sound that is reflected in more than one direction in a disorganized manner, as when a boundary is irregular.

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22
Q

An advantage of diffuse reflections is that…

A

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

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23
Q

A disadvantage of diffuse reflections is that…

A

backscattered signals have a lower strength than specular reflections.

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24
Q

What is it called when u/s signals are redirected in many directions?

A

scattering

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25
Q

Scattering and frequency are ___ related.

A

directly. The higher the frequency, the more the signal will scatter.

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26
Q

Sound scatters when the tissure interface is ___ the wavelength of the sound beam.

A

smaller than

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27
Q

This special form of scattering occurs when the structure’s dimensions are much smaller than the beam’s wavelength.

A

Rayleigh scattering

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28
Q

Rayleigh scattering redirects the sound wave equally in ___ directions.

A

all

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29
Q

Mathematically, scattering is related frequency raised to the ___ power.

A

fourth

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30
Q

When frequency doubles, Rayleigh scattering is ___ times greater.

A
  1. (2 x 2 x 2 x 2)
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31
Q

What is the significance of attenuation in diagnostic sonography?

A

More attenuation means less accurate images.

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32
Q

What occurs when u/s energy is converted into another energy form?

A

absorption

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33
Q

Absorption and frequency are ___ related.

A

directly. The higher the frequency, the more it absorbs.

34
Q

This is the number of decibels at attenuation that occurs when sound travels one centimeter.

A

attenuation coefficient

35
Q

Attenuation coefficient is reported in units of…

36
Q

The value of the attenuation coefficient remains ___ regardless of how far the sound travels.

37
Q

In soft tissue, the attenuation coefficient (dB/cm) and the frequency (MHz) are ___ related.

A

directly. As one increases, so does the other.

38
Q

The attenuation coefficient is ___ the frequency.

A

one half. dB/cm = MHz/2

39
Q

T/F? In air, sound waves attenuate due to absorption.

40
Q

T/F? In the lungs, sound waves attenuate due to absorption and scattering.

41
Q

T/F? In bone, sound waves attenuate due to scattering.

A

false; *due to absorption

42
Q

T/F? In biologic fluids (blood, urine, amniotic fluid), sound waves attenuate far more than in soft tissue.

A

false; *far less

43
Q

T/F? In water, there is no noticable attenuation with frequencies less than 10 MHz.

44
Q

T/F? In muscle, the attenuation properties vary.

45
Q

What is the distance sound travels that reduces the intensity of sound to one half its orginal value?

A

half-value layer thickness

46
Q

Half-value layer thickness is reported in units of…

47
Q

In clinical imaging, typical ranges of half-value layer thickness are…

A

.25 to 1 cm.

48
Q

Half-value layer thickness is also known as…

A

penetration depth, depth of penetration, half-boundary layer.

49
Q

Half-value layer thickness depends on what two factors?

A

the medium and the frequency of sound

50
Q

The half-value layer is ___ for tissues that attenuate sound a great deal, such as ___.

A

thin, lung or bone

51
Q

The half-value layer is ___ for tissues that attenuate sound just a little, such as ___.

A

thicker, fluids

52
Q

Higher frequencies result in a ___ half-value layer.

53
Q

Lower frequencies result in a ___ thick half-value layer.

54
Q

What is the acoustic resistance to sound traveling in a medium?

55
Q

Impedence is calculated by multiplying the ___ of a medium by the ___ at which sound travels in the medium.

A

density, speed.

Impedence (rayls) = density (kg/m^3) x speed (m/s)

56
Q

Impedence is reported in units of…

57
Q

In biologic media, typical values for impedence range from…

A

1.25 to 1.75 Mrayls

58
Q

Impedence is determined by the…

59
Q

___ depends upon the difference in impedences of the two media at a boundary.

A

Reflections

60
Q

What are the three types of angles?

A

acute - less than 90
right - exactly 90
obtuse - greater than 90

61
Q

Angles with a measure other than 90 are also called…

A

oblique angles, whether they’re acute or obtuse.

62
Q

Normal incidence means that the beam strikes the boundary at…

A

exactly 90 degrees.

63
Q

Normal incidence is also called…

A

perpendicular
orthogonal
right angle
90 degrees

64
Q

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

A

Incident intensity

65
Q

___ is the intensity of the portion of the beam that returns back after striking the boundary.

A

Reflected intensity

66
Q

___ is the intensity of the portion of the beam that continues forward after striking the boundary.

A

Transmitted intensity

67
Q

Since there is a conservation of energy, the incident intensity equals the ___ plus the ___.

A

reflected intensity, transmitted intensity

68
Q

What is the percentage of the intensity that bounces back when a sound beam strikes the boundary called?

A

intensity reflection coefficient (IRC)

69
Q

In clinical imaging, ___ of a sound wave’s intensity is reflected back.

A

very little (1% or less)

70
Q

What is the percentage of intensity that passes forward when a sound beam strikes the boundary called?

A

intensity transmission coefficient (ITC)

71
Q

In clinical imaging, ___ of a sound wave’s intensity transmits forward.

A

most (99% or more)

72
Q

Since there is a conservation of energy, the ITC and the IRC equal ___.

73
Q

At normal incidence, how much reflection will occur if the two media identical impedences?

74
Q

At normal incidence, how much reflection will occur if the two media have slightly different impedences?

75
Q

At normal incidence, how much reflection will occur if the two media have substantially different impedences?

76
Q

What two physical principles always apply to reflection with oblique incidence?

A

conservation of energy

reflection angle = incidenct angle

77
Q

At oblique incidence, the directionof the reflected echo is ___ and ___ to the direction of the incident beam.

A

equal and opposite

78
Q

Define ‘refraction’.

A

a change in direction of wave propagation when traveling from one medium to another; the bending of the sound beam

79
Q

Refraction only occurs if what two conditions are satisfied?

A
  • oblique incidence

* different propagation speeds of the two media

80
Q

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

A

When the speed of the two media are equal.

81
Q

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

A

When the speed of the second medium is greater than the speed of the first medium.

82
Q

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

A

When the speed of the second medium in less than the speed of the first medium.

Ultrasound Physics (23 decks)

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