Chapter 6 Flashcards
(163 cards)
1
Q
Sound waves _ as they travel in the body
A
weaken, or attenuate
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
3
Q
In diagnostic ultrasound, we are often interested in _
A
the degree of attenuation or the extent of amplification
4
Q
The logarithm or log of a number represents
A
the number of 10s that are multiplied to create the original number
5
Q
If the logarithm increases by 1, the actual number
A
increases ten-fold
6
Q
A logarithmic increase of 2 indicates that the actual number
A
increases by 100 times
7
Q
10 x 10 = 100. the log of 100 =
A
2
8
Q
10,000 = 10 x 10 x 10 x 10. the log of 10,000 =
A
4
9
Q
Tip for logarithms
A
For even powers of 10, count the zeros!
10
Q
The decibel is a common unit for measuring
A
the signal strength in diagnostic ultrasound
11
Q
Decibel notation is
A
logarithmic
12
Q
Decibels do not measure _, they report _
A
absolute numbers
relative changes
13
Q
Decibels require what 2 intensities?
A
The reference/starting level
The actual level at the time of measurement
14
Q
Decibels: Ratio =
A
measured level divided by starting level
15
Q
Decibels are useful units to make
A
Comparisons
16
Q
Decibels are commonly used to describe
A
the relationship between various measured sound levels and the threshold of human hearing
17
Q
If asked what the relativemeasurement of something is, we will use
A
Decibels
18
Q
_ report signals that are increasing in strength or getting larger.
A
Positive decibels
19
Q
When a wave’s intensity doubles, the relative change is
A
+3 dB
20
Q
When intensity increases ten-fold, the relative change is
A
+10 dB
21
Q
_ describe signals that are decreasing in strength or getting smaller.
A
Negative decibels
22
Q
When the intensity is reduced to ½ its original value, the relative change is
A
-3 dB
23
Q
When the intensity is reduced to 1/10 its original value, the relative change is
A
-10 dB
24
Q
3dB means
A
Double
25
10 dB means
10 times larger
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-3 dB means
Half
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-10dB means
1/10
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The decrease in intensity, power, and amplitude as sound travels through a medium.
Attenuation
29
Attenuation is determined by two factors:
Path length
| Frequency of sound
30
Relationship between distance and attenuation
Directly related
31
Relationship between frequency and attenuation
Directly related
32
Units for attenuation
measured in dB and reported as relative change, not as an absolute change.
33
More attenuation=
Longer distance
| higher frequency
34
Less attenuation=
Shorter distance
| Lower requency
35
Three processes contribute to attenuation:
Reflection
Scattering
Absorption
36
As sound strikes a boundary, a portion of the wave’s energy may
be reflected back to the sound source
37
Reflection _ the portion of the sound wave that continues in the forward direction
weakens
38
There are two forms of reflection in soft tissue:
Specular
| Diffuse
39
Specular reflection occurs when
sound strikes a smooth boundary and and the sound is reflected in only one direction in an organized manner
40
Specular refletion: If the wave if off-axis, the reflection _
Does not return to the transducer
41
Diffuse reflection
When a wave hits an irregular surface, it radiates in more than one direction
42
Diffuse reflection is AKA
backscatter
43
Backscattered signals have a _ strength than specular reflections
lower
44
Diffuse reflection: Interfaces at suboptimal angles to the sound beam can
still produce reflections that will return to the transducer.
45
Scattering of ultrasound is the
random redirection of sound in many directions.
46
Sound scatters when
the tissue interface is small (equal to or less than the wavelength of the incident sound beam)
47
Higher frequency sound beams scatter _ than lower frequency beams.
much more
48
Relationship between scattering and frquency
Directly related
49
Rayleigh scattering
A special form of scattering that occurs when the structure’s dimensions are much smaller than the beam’s wavelength.
50
Rayleigh scattering redirects the sound wave _
equally in all directions (organized and omnidirectional)
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Rayleigh scattering: _ cells
Red blood cells
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Rayleigh scattering increases dramatically with
increasing frequency
53
Relationship between rayleigh scatterng and frequency
Proportional to frequency^4
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Most sizeable component of attenuation is _
Absorption
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Absorption occurs when
ultrasonic energy is converted into another form of energy like heat
56
Relationship between absorption and frequency
Directly related
57
The number of decibels of attenuation that occurs when sound travels one centimeter.
Attenuation coefficient
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The value of the attenuation coefficient remains constant regardless of _
how far the sound travels.
59
When the attenuation coefficient is known, it is easy to determine
the total attenuation of a sound wave as it travels
60
Total attenuation (dB) =
attenuation coefficient (dB/cm) x distance (cm)
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Relationship between attenuation coefficient and frequency IN SOFT TISSUE
Directly related
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Attenuation Coefficient is _ the frequency
one-half
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Attenuation coefficient =
frequency/2
64
_ absorbs ultrasound energy to a large extent
Bone
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Lung attenuates dramatically due to
Scattering and absorption
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Main mechanism of attenuation in air is
Absorption
67
Sound with frequencies above 1 MHz attenuate _ in air
entirely
68
Attenuation properties in muscle _
Vary
69
Attenuation is _ when the sound is traveling across the muscle fibers vs. traveling along the length of the muscle fibers
twice as high
70
Medium: Water
Attenuation:
Extremely low
71
Medium: Blood, urine, biologic fluid
| Attenuation:
Low
72
Medium: Fat
Attenuation:
Low
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Medium: Soft tissue
Attenuation:
Intermediate
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Medium: Muscle
Attenuation:
High
75
Medium: Bone and lung Attenuation:
Higher than muscle
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Medium: Air
Attenuation:
Extremely high
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3dB of attenuation=
-3dB
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The distance sound travels in a tissue that reduces the intensity of sound to one-half its original value.
Half value layer thickness
79
Units for half value layer thickness
Cm or any other unit of length
80
Typical values for half value layer thickness
0.25 – 1 cm
81
Half value layer thickness AKAs
Penetration depth
| Depth of penetration Half-boundary layer
82
Half value layer thickness depends on what 2 factors
Medium
| Frequency of sound
83
Thin half value: _ frequency
| Media with _ attenuation rate
High
| High
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Thick half value: _ frequency
| Media with _ attenuation
Low
| Low
85
The _ produced as sound moves from one medium to another forms the basis for ultrasonic imaging
Reflection
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_ is critical to ultrasound’s ability to image structures located deep in the body.
Transmission
87
The acoustic resistance to sound traveling in a medium
Impedence
88
Impedence is calculated by
multiplying the density of a medium by
| the speed at which sound travels in the medium.
89
Reflection of a sound wave depends upon
the difference in acoustic impedances of the two media at a boundary.
90
Equation for impedence
Impedance (rayls) = density (kg/m3) x prop. speed (m/s)
91
Units for impedence
Rayls
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Impedence is often represented by
Z
93
Typical values for impedence
1,250,000 to 1,750,000 rayls (1.25 to 1.75 Mrayls)
94
Impedence is determined by
Medium only. It is calculated, not measured.
95
Acoustic impedence is AKA
Characteristic impedence
96
The angle at which a sound wave strikes a tissue boundary determines
the behavior of the pulse.
97
The incident sound beam strikes the boundary at exactly 90 degrees
Normal incidence
98
Normal incidence AKAs
```
PORN
Perpendicular
Orthogonal
Right angle
90 degrees
```
99
Occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees.
Oblique incidence
100
Oblique incidence aka
Non-perpendicular
101
the sound wave’s intensity immediately before it strikes a boundary
Incident Intensity
102
the intensity of the portion of the incident sound beam that returns to the machine after striking a boundary
Reflected Intensity
103
the intensity of the portion of the incident beam that continues forward after striking a boundary.
Transmitted Intensity
104
There is _ of energy at the boundary.
Conservation
105
Equation for incident intensity
Incident intensity = reflected intensity + transmitted intensity
106
The percentage of the intensity that bounces back when a sound beam strikes the boundary between two different media.
Intensity Reflection Coefficient (IRC)
107
In clinical imaging, _ of a sound wave’s intensity is reflected at a boundary between two soft tissues
Very little
| Less than 1% or less
108
IRC: _ is reflected when sound strikes a boundary between soft tissue and bone or between soft tissue and air.
A greater percentage
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The percentage of intensity that passes in the forward direction when the beam strikes an interface between two media.
Intensity Transmission Coefficient (ITC)
110
In clinical imaging, _of a sound wave’s intensity is transmitted at a boundary between two soft tissues
Most
| 99%+
111
ITC: a _ percentage of the wave is transmitted when sound strikes a boundary between bone and soft tissue.
Smaller
112
IRC and ITC are both reported as
Percentages
113
_ applies to IRC and IT
Conservation of energy
114
IRC+ITC=
100%
115
When a sound beam strikes a tissue boundary at a 90 degree angle, reflection occurs only if
the media on either side of the boundary have different impedances.
116
The percentage of the incident beam that is reflected is related to
the difference in the impedances of the tissues
117
Reflection with Normal Incidence: Two media with identical impedances=
No reflection
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Reflection with Normal Incidence: Two media with slightly different impedances =
Small reflection
119
Reflection with Normal Incidence: Two media with substantially different impedances =
Large reflection
120
Equation for IRC
IRC = [Z2-Z1/Z2+Z1]2 x 100
121
Sound strikes a boundary with normal incidence, if 60% of the intensity is reflected back towards the transducer, what percentage is transmitted?
40%
122
Equations for ITC
ITC (%) = (transmitted intensity/incident intensity) x 100
| ITC (%) = 100 – intensity reflection coefficient
123
Transmission with Normal Incidence: If two media have the same impedance, _ is transmitted at the boundary.
All of the sound
124
The percentage of the intensity that continues to move forward when the beam reaches a boundary between two media
Transmission with Normal Incidence
125
_ is more complex than reflection and transmission with Normal Incidence
Oblique incidence
126
With oblique incidence we are unable to
predict whether sound will reflect or transmit with oblique incidence
127
Reflection and Transmission with Oblique Incidence: reflections _ even with identical impedences between the tissue
May occur
128
Reflection and Transmission with Oblique Incidence: reflections may be absent with
Different impedences
129
Two physical principles always apply to reflection with oblique incidence:
Conservation of energy
| Reflection angle = incident angle
130
Conservation of Energy with Oblique Incidence: The sum of the percentage of the sound reflected and the percentage of the sound transmitted must equal
100%
131
Reflection coefficient + transmission coefficient =
100%
132
Conservation of Energy with Oblique Incidence: The sum of the reflected and transmitted intensities must equal
the incident intensity.
133
Incident intensity =
reflected intensity + transmitted intensity
134
Reflection Angle =
Incident Angle
135
When reflection occurs with oblique incidence, the sound beam is
not directed back to the transducer.
136
The direction of the reflected echo is equal and opposite to the
direction of the incident beam.
137
The angle between the incident sound beam and an imaginary line perpendicular to the boundary is called
the angle of incidence
138
The angle between the reflected sound beam and the line perpendicular to the boundary is called
the angle of reflection
139
With oblique incidence, transmission of any or all of the beam is
Uncertain
140
If transmission occurs, the wave may travel straight ahead or it may
bend or change direction. (refraction)
141
Transmission with a bend
Refraction
142
Change in direction of wave propagation when traveling from one medium to another.
Refraction
143
Refraction occurs with
light waves as well as sound waves
144
Refraction only occurs if two conditions are satisfied:
Oblique incidence
| Different propagation speeds of the two media
145
At a soft tissue-fat interface, a muscle-blood interface, or a soft tissue- fluid interface, the sound beam will
bend at most only a few degrees due to similar propagation speeds.
146
Bending is exaggerated at a bone-soft tissue interface because
the speed of sound in bone is nearly 3 times greater than in soft tissue.
147
Snell’s law defines
the physics of refraction
148
Sin(transmission angle)/
| sin(incident angle)=
speed of medium 2/
| speed of medium 1
149
A sine is a unitless number with a value between
0 and 1
150
Every angle has an associated _ that can be found in a reference table
sine
151
Medium 1 =
the medium in which the sound is currently traveling
152
Medium 2 =
the medium into which the sound is entering
153
Refraction will not occur when
the speeds of the two media are identical
154
Refraction: The angle of incidence will equal
the angle of transmission
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Refraction: If the speed of medium 2 is greater than the speed of medium 1, the transmission angle will be
greater than the incident angle.
156
Refraction: If the speed of medium two is less than the speed of medium 1, the transmission angle will be
less than the incident angle.
157
Refraction:
Speed: speed 2 = speed 1
Angle of transmission: _
No refraction; transmission angle incident angle
158
Refraction:
Speed: speed 2 greater than speed 1
Angle of transmission: _
Transmission angle is greater than incident angle
159
Refraction:
Speed: Speed 2 less than speed 1
Angle of transmission: _
Transmission angle less than incident angle
160
Event: Reflection w/ normal incidence
Requirement: _
Different impedances required
161
Event: Reflection with oblique incidence
Requirement: _
We cannot predict, it's too complex!
162
Event: Transmission
Requirement: _
Derived from reflection information; use law of conservation of energy
163
Event: Refraction
Requirement: _
Oblique incidence and different speeds required
