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Ultrasound Physics > Ch 6: Sound and Media > Flashcards

Ch 6: Sound and Media Flashcards

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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
Scattering and frequency are ___ related.
directly. The higher the frequency, the more the signal will scatter.
26
Sound scatters when the tissure interface is ___ the wavelength of the sound beam.
smaller than
27
This special form of scattering occurs when the structure's dimensions are much smaller than the beam's wavelength.
Rayleigh scattering
28
Rayleigh scattering redirects the sound wave equally in ___ directions.
all
29
Mathematically, scattering is related frequency raised to the ___ power.
fourth
30
When frequency doubles, Rayleigh scattering is ___ times greater.
16. (2 x 2 x 2 x 2)
31
What is the significance of attenuation in diagnostic sonography?
More attenuation means less accurate images.
32
What occurs when u/s energy is converted into another energy form?
absorption
33
Absorption and frequency are ___ related.
directly. The higher the frequency, the more it absorbs.
34
This is the number of decibels at attenuation that occurs when sound travels one centimeter.
attenuation coefficient
35
Attenuation coefficient is reported in units of...
dB/cm.
36
The value of the attenuation coefficient remains ___ regardless of how far the sound travels.
constant
37
In soft tissue, the attenuation coefficient (dB/cm) and the frequency (MHz) are ___ related.
directly. As one increases, so does the other.
38
The attenuation coefficient is ___ the frequency.
one half. dB/cm = MHz/2
39
T/F? In air, sound waves attenuate due to absorption.
true
40
T/F? In the lungs, sound waves attenuate due to absorption and scattering.
true
41
T/F? In bone, sound waves attenuate due to scattering.
false; *due to absorption
42
T/F? In biologic fluids (blood, urine, amniotic fluid), sound waves attenuate far more than in soft tissue.
false; *far less
43
T/F? In water, there is no noticable attenuation with frequencies less than 10 MHz.
true
44
T/F? In muscle, the attenuation properties vary.
true
45
What is the distance sound travels that reduces the intensity of sound to one half its orginal value?
half-value layer thickness
46
Half-value layer thickness is reported in units of...
cm.
47
In clinical imaging, typical ranges of half-value layer thickness are...
.25 to 1 cm.
48
Half-value layer thickness is also known as...
penetration depth, depth of penetration, half-boundary layer.
49
Half-value layer thickness depends on what two factors?
the medium and the frequency of sound
50
The half-value layer is ___ for tissues that attenuate sound a great deal, such as ___.
thin, lung or bone
51
The half-value layer is ___ for tissues that attenuate sound just a little, such as ___.
thicker, fluids
52
Higher frequencies result in a ___ half-value layer.
thin
53
Lower frequencies result in a ___ thick half-value layer.
thick
54
What is the acoustic resistance to sound traveling in a medium?
impedence
55
Impedence is calculated by multiplying the ___ of a medium by the ___ at which sound travels in the medium.
density, speed. | Impedence (rayls) = density (kg/m^3) x speed (m/s)
56
Impedence is reported in units of...
rayls (Z)
57
In biologic media, typical values for impedence range from...
1.25 to 1.75 Mrayls
58
Impedence is determined by the...
medium.
59
___ depends upon the difference in impedences of the two media at a boundary.
Reflections
60
What are the three types of angles?
acute - less than 90 right - exactly 90 obtuse - greater than 90
61
Angles with a measure other than 90 are also called...
oblique angles, whether they're acute or obtuse.
62
Normal incidence means that the beam strikes the boundary at...
exactly 90 degrees.
63
Normal incidence is also called...
perpendicular orthogonal right angle 90 degrees
64
___ is the sound wave's intensity immediately before it strikes a boundary.
Incident intensity
65
___ is the intensity of the portion of the beam that returns back after striking the boundary.
Reflected intensity
66
___ is the intensity of the portion of the beam that continues forward after striking the boundary.
Transmitted intensity
67
Since there is a conservation of energy, the incident intensity equals the ___ plus the ___.
reflected intensity, transmitted intensity
68
What is the percentage of the intensity that bounces back when a sound beam strikes the boundary called?
intensity reflection coefficient (IRC)
69
In clinical imaging, ___ of a sound wave's intensity is reflected back.
very little (1% or less)
70
What is the percentage of intensity that passes forward when a sound beam strikes the boundary called?
intensity transmission coefficient (ITC)
71
In clinical imaging, ___ of a sound wave's intensity transmits forward.
most (99% or more)
72
Since there is a conservation of energy, the ITC and the IRC equal ___.
100%
73
At normal incidence, how much reflection will occur if the two media identical impedences?
none
74
At normal incidence, how much reflection will occur if the two media have slightly different impedences?
a little
75
At normal incidence, how much reflection will occur if the two media have substantially different impedences?
a large
76
What two physical principles always apply to reflection with oblique incidence?
conservation of energy | reflection angle = incidenct angle
77
At oblique incidence, the directionof the reflected echo is ___ and ___ to the direction of the incident beam.
equal and opposite
78
Define 'refraction'.
a change in direction of wave propagation when traveling from one medium to another; the bending of the sound beam
79
Refraction only occurs if what two conditions are satisfied?
* oblique incidence | * different propagation speeds of the two media
80
Under what conditions will the transmission angle equal the incident angle?
When the speed of the two media are equal.
81
Under what conditions will the transmission angle be greater than the incident angle?
When the speed of the second medium is greater than the speed of the first medium.
82
Under what conditions will the transmission angle be less than the incident angle?
When the speed of the second medium in less than the speed of the first medium.

Ultrasound Physics (23 decks)

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