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Physics > Chapter 6 > Flashcards

Chapter 6 Flashcards

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1
Q

Sound waves _ as they travel in the body

A

weaken, or attenuate

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2
Q

The sound that comes back to the transducer is converted to _. That is sent to the ultrasound system where it is _

A

an electrical signal

strengthened or amplified

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3
Q

In diagnostic ultrasound, we are often interested in _

A

the degree of attenuation or the extent of amplification

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4
Q

The logarithm or log of a number represents

A

the number of 10s that are multiplied to create the original number

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5
Q

If the logarithm increases by 1, the actual number

A

increases ten-fold

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6
Q

A logarithmic increase of 2 indicates that the actual number

A

increases by 100 times

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7
Q

10 x 10 = 100. the log of 100 =

A

2

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8
Q

10,000 = 10 x 10 x 10 x 10. the log of 10,000 =

A

4

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9
Q

Tip for logarithms

A

For even powers of 10, count the zeros!

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10
Q

The decibel is a common unit for measuring

A

the signal strength in diagnostic ultrasound

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11
Q

Decibel notation is

A

logarithmic

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12
Q

Decibels do not measure _, they report _

A

absolute numbers

relative changes

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13
Q

Decibels require what 2 intensities?

A

The reference/starting level

The actual level at the time of measurement

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14
Q

Decibels: Ratio =

A

measured level divided by starting level

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15
Q

Decibels are useful units to make

A

Comparisons

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16
Q

Decibels are commonly used to describe

A

the relationship between various measured sound levels and the threshold of human hearing

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17
Q

If asked what the relativemeasurement of something is, we will use

A

Decibels

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18
Q

_ report signals that are increasing in strength or getting larger.

A

Positive decibels

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19
Q

When a wave’s intensity doubles, the relative change is

A

+3 dB

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20
Q

When intensity increases ten-fold, the relative change is

A

+10 dB

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21
Q

_ describe signals that are decreasing in strength or getting smaller.

A

Negative decibels

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22
Q

When the intensity is reduced to ½ its original value, the relative change is

A

-3 dB

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23
Q

When the intensity is reduced to 1/10 its original value, the relative change is

A

-10 dB

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24
Q

3dB means

A

Double

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25
Q

10 dB means

A

10 times larger

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26
Q

-3 dB means

A

Half

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27
Q

-10dB means

A

1/10

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28
Q

The decrease in intensity, power, and amplitude as sound travels through a medium.

A

Attenuation

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29
Q

Attenuation is determined by two factors:

A

Path length

Frequency of sound

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30
Q

Relationship between distance and attenuation

A

Directly related

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31
Q

Relationship between frequency and attenuation

A

Directly related

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32
Q

Units for attenuation

A

measured in dB and reported as relative change, not as an absolute change.

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33
Q

More attenuation=

A

Longer distance

higher frequency

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34
Q

Less attenuation=

A

Shorter distance

Lower requency

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35
Q

Three processes contribute to attenuation:

A

Reflection
Scattering
Absorption

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36
Q

As sound strikes a boundary, a portion of the wave’s energy may

A

be reflected back to the sound source

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37
Q

Reflection _ the portion of the sound wave that continues in the forward direction

A

weakens

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38
Q

There are two forms of reflection in soft tissue:

A

Specular

Diffuse

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39
Q

Specular reflection occurs when

A

sound strikes a smooth boundary and and the sound is reflected in only one direction in an organized manner

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40
Q

Specular refletion: If the wave if off-axis, the reflection _

A

Does not return to the transducer

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41
Q

Diffuse reflection

A

When a wave hits an irregular surface, it radiates in more than one direction

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42
Q

Diffuse reflection is AKA

A

backscatter

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43
Q

Backscattered signals have a _ strength than specular reflections

A

lower

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44
Q

Diffuse reflection: Interfaces at suboptimal angles to the sound beam can

A

still produce reflections that will return to the transducer.

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45
Q

Scattering of ultrasound is the

A

random redirection of sound in many directions.

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46
Q

Sound scatters when

A

the tissue interface is small (equal to or less than the wavelength of the incident sound beam)

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47
Q

Higher frequency sound beams scatter _ than lower frequency beams.

A

much more

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48
Q

Relationship between scattering and frquency

A

Directly related

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49
Q

Rayleigh scattering

A

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

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50
Q

Rayleigh scattering redirects the sound wave _

A

equally in all directions (organized and omnidirectional)

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51
Q

Rayleigh scattering: _ cells

A

Red blood cells

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52
Q

Rayleigh scattering increases dramatically with

A

increasing frequency

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53
Q

Relationship between rayleigh scatterng and frequency

A

Proportional to frequency^4

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54
Q

Most sizeable component of attenuation is _

A

Absorption

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55
Q

Absorption occurs when

A

ultrasonic energy is converted into another form of energy like heat

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56
Q

Relationship between absorption and frequency

A

Directly related

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57
Q

The number of decibels of attenuation that occurs when sound travels one centimeter.

A

Attenuation coefficient

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58
Q

The value of the attenuation coefficient remains constant regardless of _

A

how far the sound travels.

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59
Q

When the attenuation coefficient is known, it is easy to determine

A

the total attenuation of a sound wave as it travels

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60
Q

Total attenuation (dB) =

A

attenuation coefficient (dB/cm) x distance (cm)

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61
Q

Relationship between attenuation coefficient and frequency IN SOFT TISSUE

A

Directly related

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62
Q

Attenuation Coefficient is _ the frequency

A

one-half

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63
Q

Attenuation coefficient =

A

frequency/2

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64
Q

_ absorbs ultrasound energy to a large extent

A

Bone

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65
Q

Lung attenuates dramatically due to

A

Scattering and absorption

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66
Q

Main mechanism of attenuation in air is

A

Absorption

67
Q

Sound with frequencies above 1 MHz attenuate _ in air

A

entirely

68
Q

Attenuation properties in muscle _

A

Vary

69
Q

Attenuation is _ when the sound is traveling across the muscle fibers vs. traveling along the length of the muscle fibers

A

twice as high

70
Q

Medium: Water
Attenuation:

A

Extremely low

71
Q

Medium: Blood, urine, biologic fluid

Attenuation:

A

Low

72
Q

Medium: Fat
Attenuation:

A

Low

73
Q

Medium: Soft tissue
Attenuation:

A

Intermediate

74
Q

Medium: Muscle
Attenuation:

A

High

75
Q

Medium: Bone and lung Attenuation:

A

Higher than muscle

76
Q

Medium: Air
Attenuation:

A

Extremely high

77
Q

3dB of attenuation=

A

-3dB

78
Q

The distance sound travels in a tissue that reduces the intensity of sound to one-half its original value.

A

Half value layer thickness

79
Q

Units for half value layer thickness

A

Cm or any other unit of length

80
Q

Typical values for half value layer thickness

A

0.25 – 1 cm

81
Q

Half value layer thickness AKAs

A

Penetration depth

Depth of penetration Half-boundary layer

82
Q

Half value layer thickness depends on what 2 factors

A

Medium

Frequency of sound

83
Q

Thin half value: _ frequency

Media with _ attenuation rate

A

High

High

84
Q

Thick half value: _ frequency

Media with _ attenuation

A

Low

Low

85
Q

The _ produced as sound moves from one medium to another forms the basis for ultrasonic imaging

A

Reflection

86
Q

_ is critical to ultrasound’s ability to image structures located deep in the body.

A

Transmission

87
Q

The acoustic resistance to sound traveling in a medium

A

Impedence

88
Q

Impedence is calculated by

A

multiplying the density of a medium by

the speed at which sound travels in the medium.

89
Q

Reflection of a sound wave depends upon

A

the difference in acoustic impedances of the two media at a boundary.

90
Q

Equation for impedence

A

Impedance (rayls) = density (kg/m3) x prop. speed (m/s)

91
Q

Units for impedence

A

Rayls

92
Q

Impedence is often represented by

A

Z

93
Q

Typical values for impedence

A

1,250,000 to 1,750,000 rayls (1.25 to 1.75 Mrayls)

94
Q

Impedence is determined by

A

Medium only. It is calculated, not measured.

95
Q

Acoustic impedence is AKA

A

Characteristic impedence

96
Q

The angle at which a sound wave strikes a tissue boundary determines

A

the behavior of the pulse.

97
Q

The incident sound beam strikes the boundary at exactly 90 degrees

A

Normal incidence

98
Q

Normal incidence AKAs

A 
PORN
Perpendicular 
Orthogonal 
Right angle
90 degrees
99
Q

Occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees.

A

Oblique incidence

100
Q

Oblique incidence aka

A

Non-perpendicular

101
Q

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

A

Incident Intensity

102
Q

the intensity of the portion of the incident sound beam that returns to the machine after striking a boundary

A

Reflected Intensity

103
Q

the intensity of the portion of the incident beam that continues forward after striking a boundary.

A

Transmitted Intensity

104
Q

There is _ of energy at the boundary.

A

Conservation

105
Q

Equation for incident intensity

A

Incident intensity = reflected intensity + transmitted intensity

106
Q

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

A

Intensity Reflection Coefficient (IRC)

107
Q

In clinical imaging, _ of a sound wave’s intensity is reflected at a boundary between two soft tissues

A

Very little

Less than 1% or less

108
Q

IRC: _ is reflected when sound strikes a boundary between soft tissue and bone or between soft tissue and air.

A

A greater percentage

109
Q

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

A

Intensity Transmission Coefficient (ITC)

110
Q

In clinical imaging, _of a sound wave’s intensity is transmitted at a boundary between two soft tissues

A

Most

99%+

111
Q

ITC: a _ percentage of the wave is transmitted when sound strikes a boundary between bone and soft tissue.

A

Smaller

112
Q

IRC and ITC are both reported as

A

Percentages

113
Q

_ applies to IRC and IT

A

Conservation of energy

114
Q

IRC+ITC=

A

100%

115
Q

When a sound beam strikes a tissue boundary at a 90 degree angle, reflection occurs only if

A

the media on either side of the boundary have different impedances.

116
Q

The percentage of the incident beam that is reflected is related to

A

the difference in the impedances of the tissues

117
Q

Reflection with Normal Incidence: Two media with identical impedances=

A

No reflection

118
Q

Reflection with Normal Incidence: Two media with slightly different impedances =

A

Small reflection

119
Q

Reflection with Normal Incidence: Two media with substantially different impedances =

A

Large reflection

120
Q

Equation for IRC

A

IRC = [Z2-Z1/Z2+Z1]2 x 100

121
Q

Sound strikes a boundary with normal incidence, if 60% of the intensity is reflected back towards the transducer, what percentage is transmitted?

A

40%

122
Q

Equations for ITC

A

ITC (%) = (transmitted intensity/incident intensity) x 100

ITC (%) = 100 – intensity reflection coefficient

123
Q

Transmission with Normal Incidence: If two media have the same impedance, _ is transmitted at the boundary.

A

All of the sound

124
Q

The percentage of the intensity that continues to move forward when the beam reaches a boundary between two media

A

Transmission with Normal Incidence

125
Q

_ is more complex than reflection and transmission with Normal Incidence

A

Oblique incidence

126
Q

With oblique incidence we are unable to

A

predict whether sound will reflect or transmit with oblique incidence

127
Q

Reflection and Transmission with Oblique Incidence: reflections _ even with identical impedences between the tissue

A

May occur

128
Q

Reflection and Transmission with Oblique Incidence: reflections may be absent with

A

Different impedences

129
Q

Two physical principles always apply to reflection with oblique incidence:

A

Conservation of energy

Reflection angle = incident angle

130
Q

Conservation of Energy with Oblique Incidence: The sum of the percentage of the sound reflected and the percentage of the sound transmitted must equal

A

100%

131
Q

Reflection coefficient + transmission coefficient =

A

100%

132
Q

Conservation of Energy with Oblique Incidence: The sum of the reflected and transmitted intensities must equal

A

the incident intensity.

133
Q

Incident intensity =

A

reflected intensity + transmitted intensity

134
Q

Reflection Angle =

A

Incident Angle

135
Q

When reflection occurs with oblique incidence, the sound beam is

A

not directed back to the transducer.

136
Q

The direction of the reflected echo is equal and opposite to the

A

direction of the incident beam.

137
Q

The angle between the incident sound beam and an imaginary line perpendicular to the boundary is called

A

the angle of incidence

138
Q

The angle between the reflected sound beam and the line perpendicular to the boundary is called

A

the angle of reflection

139
Q

With oblique incidence, transmission of any or all of the beam is

A

Uncertain

140
Q

If transmission occurs, the wave may travel straight ahead or it may

A

bend or change direction. (refraction)

141
Q

Transmission with a bend

A

Refraction

142
Q

Change in direction of wave propagation when traveling from one medium to another.

A

Refraction

143
Q

Refraction occurs with

A

light waves as well as sound waves

144
Q

Refraction only occurs if two conditions are satisfied:

A

Oblique incidence

Different propagation speeds of the two media

145
Q

At a soft tissue-fat interface, a muscle-blood interface, or a soft tissue- fluid interface, the sound beam will

A

bend at most only a few degrees due to similar propagation speeds.

146
Q

Bending is exaggerated at a bone-soft tissue interface because

A

the speed of sound in bone is nearly 3 times greater than in soft tissue.

147
Q

Snell’s law defines

A

the physics of refraction

148
Q

Sin(transmission angle)/

sin(incident angle)=

A

speed of medium 2/

speed of medium 1

149
Q

A sine is a unitless number with a value between

A

0 and 1

150
Q

Every angle has an associated _ that can be found in a reference table

A

sine

151
Q

Medium 1 =

A

the medium in which the sound is currently traveling

152
Q

Medium 2 =

A

the medium into which the sound is entering

153
Q

Refraction will not occur when

A

the speeds of the two media are identical

154
Q

Refraction: The angle of incidence will equal

A

the angle of transmission

155
Q

Refraction: If the speed of medium 2 is greater than the speed of medium 1, the transmission angle will be

A

greater than the incident angle.

156
Q

Refraction: If the speed of medium two is less than the speed of medium 1, the transmission angle will be

A

less than the incident angle.

157
Q

Refraction:
Speed: speed 2 = speed 1
Angle of transmission: _

A

No refraction; transmission angle incident angle

158
Q

Refraction:
Speed: speed 2 greater than speed 1
Angle of transmission: _

A

Transmission angle is greater than incident angle

159
Q

Refraction:
Speed: Speed 2 less than speed 1
Angle of transmission: _

A

Transmission angle less than incident angle

160
Q

Event: Reflection w/ normal incidence
Requirement: _

A

Different impedances required

161
Q

Event: Reflection with oblique incidence
Requirement: _

A

We cannot predict, it’s too complex!

162
Q

Event: Transmission
Requirement: _

A

Derived from reflection information; use law of conservation of energy

163
Q

Event: Refraction
Requirement: _

A

Oblique incidence and different speeds required

Physics (25 decks)

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