Attenuation Flashcards
(103 cards)
1
Q
Weakening of sound beam as it travels is called
A
Attenuation
2
Q
Attenuation is of great clinical importance in what three ways
A
- limit imaging depth
- has to be compensated for
- useful in diagnosis
3
Q
Relative units of measurement expressing loudness (intensity) of sound waves
A
Bel (B)
Decibel (dB)
4
Q
1 bel is equal to how many decibels
A
1 Bel = 10 decibel
5
Q
Bel or decibel is used for virtually all loudness measurements
A
Decibel
6
Q
Instrument output uses what to determine the power of the sound leaving the transducer
A
Decibels
7
Q
What uses decibels to express the number of shades of gray displayed on the monitor
A
Dynamic range
8
Q
What two things use decibels to express the amount of amplification required to optimize the returning echos
A
Gains and TGC
9
Q
What formula is used to calculate bels
A
New I
Bel = Log ( —————— )
Original I
10
Q
What formula used to calculate decibels
A
New I
dB = 10log ( ————— )
Original I
11
Q
Formula for power
A
New P
dB = 10log ( ————— )
Original P
12
Q
Formula for voltage
A
New V
dB = 20Log ( ————— )
Original V
13
Q
What are the two rules of thumbs when dealing with decibel
A
- a 3 dB drop = half the original intensity
- a 10 dB drop = 0.1 the original intensity
14
Q
Amount of attenuation that occurs with each cm travelled is called
A
Attenuation coefficient
15
Q
In soft tissue the attenuation coefficient is equal to
A
Half the frequency
1/2 frequency
16
Q
In soft tissue 0.5 dB of attenuation occurs every 1 cm per —-MHz
A
Per 1 MHz
17
Q
What is the total attenuation formula
A
TA = (att Coeff) Times (the path length)
18
Q
In soft tissue the formula for total attenuation is
A
TA = (1/2 frequency) times (path length)
19
Q
The distance sound must travel in material to reduce the intensity to half the original value is called what
A
The half-value layer
20
Q
What three things can vary attenuation
A
- nature of tissue (dead/alive)
- frequency of ultrasound
- depth
21
Q
What are the five general reasons we have attenuation
A
- absorption
- reflection
- refraction
- scatter
- wave-front divergence
22
Q
The conversion of sound energy into heat is called
A
Absorption
23
Q
The dominant factor in effecting attenuation is what and it accounts for approximately how much
A
Absorption
80%
24
Q
What are the three factors that affect absorption
A
- viscosity
- relaxation time on molecules
- frequency
25
The ease in which molecules can slide past one one another is called
Viscosity
26
Does the viscosity increase or decrease to provide greater resistance?
Increase
27
Greater resistance due to increased viscosity creates more or less friction
More friction
28
Having increased viscosity which creates more resistance, therefore more friction, does the attenuation increase or decrease
Attenuation increases
29
What is it called when sound energy is converted into heat
Friction
30
When a mechanical force is applied to a molecule what happens to it
The molecule will vibrate
31
The time it takes for a molecule to come to rest is called
The relaxation time
32
If molecules can’t come to rest before the next compression time, is more or less energy required to reverse its direction
Take more energy, and it produces heat
33
Since relaxation time is pretty constant in soft tissue what has more influence in changing the amount of absorption
Frequency
34
If frequency increases is there more or less time for molecules to recover during relaxation process
Less time
35
The molecules having less time during the relaxation process, results in more or less absorption
Results in more absorption
36
What are the two types of reflection
Specular
| Non-specular
37
This type of reflection occurs when a sound beam hits a large smooth surface
Specular
38
A large surface is relative to —— used, and is greater then what in diameter
Relative to the frequency used
| Greater then one wavelength in diameter
39
—— from interfaces contributes to the majority of the images
Reflection
40
What is an example of a specular refector
Diaphragm
41
Intensity of the reflected sound depends on what two things
Angle of incidence
| Acoustic impedance of two media
42
The angle of incidence is equal to what
The angle of reflection
43
Reflections of sound that have a what way of incidence may not return to the probe
Non-perpendicular
44
Does perpendicular or non-perpendicular incidence help improve reflection
Perpendicular
45
How fast sound moves through a medium depends on what two things
Density and stiffness
46
As density decreases or stiffness increased what happens to the propagation speed
It increases
47
Acoustic impedance is also known as
Characteristic impedance
48
Describe the relationship between acoustic pressure and the speed of the particle vibrations in a sound wave (speed propagation)
Acoustic impedance
49
What are the units for impedance
Rayls (Z)
50
Impedance increase if density and velocity increase or decrease
Increase
51
Z values vary with different tissues in the body due to the different what two things
Density and stiffness
52
Does acoustic impedance depend on frequency
NO
53
Does there need to be a larger or smaller difference between the interface of two media to have a bigger reflection
Larger difference
54
The amount of sound that reflects at the interface is called
Intensity reflection coefficient
55
If we know how much sound is reflected we can then calculate what
The amount of sound transmitted
56
The amount of sound transmitted is called the
Intensity transmission coefficient
57
If more sound is transmitted then less sound must be reflected yes or no
Yes
58
As the differences between two interfaces increase more or less sound will be reflected
More
59
If the impedances between the interface is equal what happens
No reflection
60
Reflection occurs when there is what at an interface
A difference in impedance (Z values)
61
Is it possible for 2 tissues to have different sound velocities but have the same Z values
Yes it sure is
62
Looking at the sound returning to the probe from an interface is what
Reflection
63
Dealing with the sound that is transmitted across the interface
Refraction
64
Sound obeys which law
Snell’s law of optics
65
What happens to sound when the velocities differ across the interface and the angle of incidence is non-perpendicular
Refraction
66
If velocities across the interface are equal then what kind of refraction occurs
No refraction occurs
67
If velocity of first medium is greater then second what kind of refraction occurs
Refract (bend) towards the normal
68
If velocity of first medium is less then second what kind of refraction occurs
Refract (bend) away from normal
69
What type of reflection occurs when first medium is less then second and the angle reaches a critical value
Total internal reflection
70
Is it possible to have different velocities but have the same Z value
Yes
71
This only occurs when velocities are different across an interface and there in non-perpendicular incidence
Refraction
72
Does refraction relate to Z values
No
73
Non-specular is called
Scatter
74
When sound interacts with interfaces that are small and rough what occurs
Scatter
75
A small interface is small in comparison to
Smaller then a wavelength
76
Scatter is typical from what kind of media
Heterogenous
77
What is an example of heterogenous media
Cells
| Suspended particles
78
Is scatter dependent or independent of sound direction
Independent
79
Is scatter responsible for external or internal texture of organs
Internal
80
What happens to the incident beam in scatter
It breaks up into many different echos
81
The resulting echos in scatter must equal what
The incident beam (100%)
82
Can the echos direction be predicted
No impossible to predict
83
The amount of scatter depends on what two things
Frequency
| Reflector size
84
If the frequency is higher what happens to scatter
Higher frequency greater amount of the scatter
85
If the reflector is smaller what happens to the scatter
Smaller reflector greater amount of scatter
86
When the sound is directed back to its origin it is called
Backscatter
87
Is the backscatter responsible for the image we see
Yes
88
Because scatter is random there is a potential for what
Brightness non-uniformities
| Dark and bright spots
89
What is the cause of brightness non-uniformities
The result of interference patterns from echos that have undergone multi-path scattering
90
The phenomenon of brightness non-uniformities is called
Acoustic speckle
91
To minimize acoustic speckle what techniques are used
Persistence also called frame averaging
92
What is the specific type of scatter that occurs when sound interacts with red blood cells called
Rayleigh scatter
93
Why do we not see blood flow at faster velocities
Red blood cells dimensions are much smaller then wavelengths so the scatter is weak
94
The spreading of sound beam as it travels is called
Divergence
95
As area increases what happens to intensity
Intensity decreases
96
As sound beam diverges the intensity of the beam weakens which adds to the overall attenuation of the beam but is called what
Wave front divergence
97
Define attenuation
The weakening of sound as it travels
98
How many decibels of sound are required to reduce the initial intensity by half
3 dB
99
Define half value layer
How many centimetres it takes to cut the intensity in half
100
List the five means that sound is attenuated in soft tissue
```
Scatter
Wave front divergence
Absorption
Refraction
Reflection
```
101
What happens to attenuation when viscosity of a medium is increased
Attenuation increases, because there is more friction meanin more absorption
102
What happens to attenuation if a mediums relaxation time is increased
Attenuation increases because more energy is needed to move the vibrating particle in the direction of the sound beam
103
What is required for reflection of sound to occur at an interface
A difference in Z values (acoustic impedance)
