Chapter 6 Flashcards
Sound waves _ as they travel in the body
weaken, or attenuate
The sound that comes back to the transducer is converted to _. That is sent to the ultrasound system where it is _
an electrical signal
strengthened or amplified
In diagnostic ultrasound, we are often interested in _
the degree of attenuation or the extent of amplification
The logarithm or log of a number represents
the number of 10s that are multiplied to create the original number
If the logarithm increases by 1, the actual number
increases ten-fold
A logarithmic increase of 2 indicates that the actual number
increases by 100 times
10 x 10 = 100. the log of 100 =
2
10,000 = 10 x 10 x 10 x 10. the log of 10,000 =
4
Tip for logarithms
For even powers of 10, count the zeros!
The decibel is a common unit for measuring
the signal strength in diagnostic ultrasound
Decibel notation is
logarithmic
Decibels do not measure _, they report _
absolute numbers
relative changes
Decibels require what 2 intensities?
The reference/starting level
The actual level at the time of measurement
Decibels: Ratio =
measured level divided by starting level
Decibels are useful units to make
Comparisons
Decibels are commonly used to describe
the relationship between various measured sound levels and the threshold of human hearing
If asked what the relativemeasurement of something is, we will use
Decibels
_ report signals that are increasing in strength or getting larger.
Positive decibels
When a wave’s intensity doubles, the relative change is
+3 dB
When intensity increases ten-fold, the relative change is
+10 dB
_ describe signals that are decreasing in strength or getting smaller.
Negative decibels
When the intensity is reduced to ½ its original value, the relative change is
-3 dB
When the intensity is reduced to 1/10 its original value, the relative change is
-10 dB
3dB means
Double
10 dB means
10 times larger
-3 dB means
Half
-10dB means
1/10
The decrease in intensity, power, and amplitude as sound travels through a medium.
Attenuation
Attenuation is determined by two factors:
Path length
Frequency of sound
Relationship between distance and attenuation
Directly related
Relationship between frequency and attenuation
Directly related
Units for attenuation
measured in dB and reported as relative change, not as an absolute change.
More attenuation=
Longer distance
higher frequency
Less attenuation=
Shorter distance
Lower requency
Three processes contribute to attenuation:
Reflection
Scattering
Absorption
As sound strikes a boundary, a portion of the wave’s energy may
be reflected back to the sound source
Reflection _ the portion of the sound wave that continues in the forward direction
weakens
There are two forms of reflection in soft tissue:
Specular
Diffuse
Specular reflection occurs when
sound strikes a smooth boundary and and the sound is reflected in only one direction in an organized manner
Specular refletion: If the wave if off-axis, the reflection _
Does not return to the transducer
Diffuse reflection
When a wave hits an irregular surface, it radiates in more than one direction
Diffuse reflection is AKA
backscatter
Backscattered signals have a _ strength than specular reflections
lower
Diffuse reflection: Interfaces at suboptimal angles to the sound beam can
still produce reflections that will return to the transducer.
Scattering of ultrasound is the
random redirection of sound in many directions.
Sound scatters when
the tissue interface is small (equal to or less than the wavelength of the incident sound beam)
Higher frequency sound beams scatter _ than lower frequency beams.
much more
Relationship between scattering and frquency
Directly related
Rayleigh scattering
A special form of scattering that occurs when the structure’s dimensions are much smaller than the beam’s wavelength.
Rayleigh scattering redirects the sound wave _
equally in all directions (organized and omnidirectional)
Rayleigh scattering: _ cells
Red blood cells
Rayleigh scattering increases dramatically with
increasing frequency
Relationship between rayleigh scatterng and frequency
Proportional to frequency^4
Most sizeable component of attenuation is _
Absorption
Absorption occurs when
ultrasonic energy is converted into another form of energy like heat
Relationship between absorption and frequency
Directly related
The number of decibels of attenuation that occurs when sound travels one centimeter.
Attenuation coefficient
The value of the attenuation coefficient remains constant regardless of _
how far the sound travels.
When the attenuation coefficient is known, it is easy to determine
the total attenuation of a sound wave as it travels
Total attenuation (dB) =
attenuation coefficient (dB/cm) x distance (cm)
Relationship between attenuation coefficient and frequency IN SOFT TISSUE
Directly related
Attenuation Coefficient is _ the frequency
one-half
Attenuation coefficient =
frequency/2
_ absorbs ultrasound energy to a large extent
Bone
Lung attenuates dramatically due to
Scattering and absorption
Main mechanism of attenuation in air is
Absorption
Sound with frequencies above 1 MHz attenuate _ in air
entirely
Attenuation properties in muscle _
Vary
Attenuation is _ when the sound is traveling across the muscle fibers vs. traveling along the length of the muscle fibers
twice as high
Medium: Water
Attenuation:
Extremely low
Medium: Blood, urine, biologic fluid
Attenuation:
Low
Medium: Fat
Attenuation:
Low
Medium: Soft tissue
Attenuation:
Intermediate
Medium: Muscle
Attenuation:
High
Medium: Bone and lung Attenuation:
Higher than muscle
Medium: Air
Attenuation:
Extremely high
3dB of attenuation=
-3dB
The distance sound travels in a tissue that reduces the intensity of sound to one-half its original value.
Half value layer thickness
Units for half value layer thickness
Cm or any other unit of length
Typical values for half value layer thickness
0.25 – 1 cm
Half value layer thickness AKAs
Penetration depth
Depth of penetration Half-boundary layer
Half value layer thickness depends on what 2 factors
Medium
Frequency of sound
Thin half value: _ frequency
Media with _ attenuation rate
High
High
Thick half value: _ frequency
Media with _ attenuation
Low
Low
The _ produced as sound moves from one medium to another forms the basis for ultrasonic imaging
Reflection
_ is critical to ultrasound’s ability to image structures located deep in the body.
Transmission
The acoustic resistance to sound traveling in a medium
Impedence
Impedence is calculated by
multiplying the density of a medium by
the speed at which sound travels in the medium.
Reflection of a sound wave depends upon
the difference in acoustic impedances of the two media at a boundary.
Equation for impedence
Impedance (rayls) = density (kg/m3) x prop. speed (m/s)
Units for impedence
Rayls
Impedence is often represented by
Z
Typical values for impedence
1,250,000 to 1,750,000 rayls (1.25 to 1.75 Mrayls)
Impedence is determined by
Medium only. It is calculated, not measured.
Acoustic impedence is AKA
Characteristic impedence
The angle at which a sound wave strikes a tissue boundary determines
the behavior of the pulse.
The incident sound beam strikes the boundary at exactly 90 degrees
Normal incidence
Normal incidence AKAs
PORN Perpendicular Orthogonal Right angle 90 degrees
Occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees.
Oblique incidence
Oblique incidence aka
Non-perpendicular
the sound wave’s intensity immediately before it strikes a boundary
Incident Intensity
the intensity of the portion of the incident sound beam that returns to the machine after striking a boundary
Reflected Intensity
the intensity of the portion of the incident beam that continues forward after striking a boundary.
Transmitted Intensity
There is _ of energy at the boundary.
Conservation
Equation for incident intensity
Incident intensity = reflected intensity + transmitted intensity
The percentage of the intensity that bounces back when a sound beam strikes the boundary between two different media.
Intensity Reflection Coefficient (IRC)
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
IRC: _ is reflected when sound strikes a boundary between soft tissue and bone or between soft tissue and air.
A greater percentage
The percentage of intensity that passes in the forward direction when the beam strikes an interface between two media.
Intensity Transmission Coefficient (ITC)
In clinical imaging, _of a sound wave’s intensity is transmitted at a boundary between two soft tissues
Most
99%+
ITC: a _ percentage of the wave is transmitted when sound strikes a boundary between bone and soft tissue.
Smaller
IRC and ITC are both reported as
Percentages
_ applies to IRC and IT
Conservation of energy
IRC+ITC=
100%
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.
The percentage of the incident beam that is reflected is related to
the difference in the impedances of the tissues
Reflection with Normal Incidence: Two media with identical impedances=
No reflection
Reflection with Normal Incidence: Two media with slightly different impedances =
Small reflection
Reflection with Normal Incidence: Two media with substantially different impedances =
Large reflection
Equation for IRC
IRC = [Z2-Z1/Z2+Z1]2 x 100
Sound strikes a boundary with normal incidence, if 60% of the intensity is reflected back towards the transducer, what percentage is transmitted?
40%
Equations for ITC
ITC (%) = (transmitted intensity/incident intensity) x 100
ITC (%) = 100 – intensity reflection coefficient
Transmission with Normal Incidence: If two media have the same impedance, _ is transmitted at the boundary.
All of the sound
The percentage of the intensity that continues to move forward when the beam reaches a boundary between two media
Transmission with Normal Incidence
_ is more complex than reflection and transmission with Normal Incidence
Oblique incidence
With oblique incidence we are unable to
predict whether sound will reflect or transmit with oblique incidence
Reflection and Transmission with Oblique Incidence: reflections _ even with identical impedences between the tissue
May occur
Reflection and Transmission with Oblique Incidence: reflections may be absent with
Different impedences
Two physical principles always apply to reflection with oblique incidence:
Conservation of energy
Reflection angle = incident angle
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%
Reflection coefficient + transmission coefficient =
100%
Conservation of Energy with Oblique Incidence: The sum of the reflected and transmitted intensities must equal
the incident intensity.
Incident intensity =
reflected intensity + transmitted intensity
Reflection Angle =
Incident Angle
When reflection occurs with oblique incidence, the sound beam is
not directed back to the transducer.
The direction of the reflected echo is equal and opposite to the
direction of the incident beam.
The angle between the incident sound beam and an imaginary line perpendicular to the boundary is called
the angle of incidence
The angle between the reflected sound beam and the line perpendicular to the boundary is called
the angle of reflection
With oblique incidence, transmission of any or all of the beam is
Uncertain
If transmission occurs, the wave may travel straight ahead or it may
bend or change direction. (refraction)
Transmission with a bend
Refraction
Change in direction of wave propagation when traveling from one medium to another.
Refraction
Refraction occurs with
light waves as well as sound waves
Refraction only occurs if two conditions are satisfied:
Oblique incidence
Different propagation speeds of the two media
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.
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.
Snell’s law defines
the physics of refraction
Sin(transmission angle)/
sin(incident angle)=
speed of medium 2/
speed of medium 1
A sine is a unitless number with a value between
0 and 1
Every angle has an associated _ that can be found in a reference table
sine
Medium 1 =
the medium in which the sound is currently traveling
Medium 2 =
the medium into which the sound is entering
Refraction will not occur when
the speeds of the two media are identical
Refraction: The angle of incidence will equal
the angle of transmission
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.
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.
Refraction:
Speed: speed 2 = speed 1
Angle of transmission: _
No refraction; transmission angle incident angle
Refraction:
Speed: speed 2 greater than speed 1
Angle of transmission: _
Transmission angle is greater than incident angle
Refraction:
Speed: Speed 2 less than speed 1
Angle of transmission: _
Transmission angle less than incident angle
Event: Reflection w/ normal incidence
Requirement: _
Different impedances required
Event: Reflection with oblique incidence
Requirement: _
We cannot predict, it’s too complex!
Event: Transmission
Requirement: _
Derived from reflection information; use law of conservation of energy
Event: Refraction
Requirement: _
Oblique incidence and different speeds required
