Ch.6 Interaction of Sound & Media Flashcards
what standard measurement tool is used to report changes such as degree of attenuation and the extent of amplification?
Decibel notation
Decibels are based on what mathematical construct?
logarithms
Decibel notation is:
(4)
a relative measurement
a comparison
a ratio
logarithmic
Decibel notation does NOT measure absolute numbers; rather
relative changes
Ex: signal strength doubled
signal is now 1/10th are large as it used to
Decibels require how many intensities?
two
1) the reference (starting level)
2) the actual level at the time of measurement
Positive decibels report what?
signals that are increasing in strength, or getting larger
What do negative decibels report?
signals that are decreasing in strength, or getting smaller
Decibels: Meaning:
3 dB
10 dB
-3 dB
-10 dB
double
ten times larger
half
one-tenth
A sound beam has increased from its initial intensity by a factor of 100. How is this described in decibel notation?
100 = 10 x 10
10 times in dB is +10 dB
= +10 , +10
= + 20
What is the meaning of a 6 dB change?
3 dB = 2x bigger
6 dB = 3 dB , 3 dB
= 2 x 2 = 4 times bigger
A sound beam has decreased to 1/100 of its initial intensity. how is this described in decibel notation?
1/100 = 1/10 x 1/10
1/10 = -10 dB
= -10 dB , -10dB
= -20 dB
What is the meaning of 20 dB?
10 dB means 10x bigger
20 dB = 10 dB , 10 dB
= 10 x 10 = 100x bigger
what is attenuation?
a decrease in intensity, power, and amplitude as sound waves travel in a medium
What two factors is attenuation determined by?
path length
frequency of sound
what is the relationship between distance and attenuation?
directly related
the farther the sound travels, the greater the attenuation
what is the relationship between frequency and attenuation?
directly related
attenuation is greater in higher frequencies
longer distances and higher frequencies means
MORE attenuation
shorter distances and lower frequencies means
LESS attenuation
What three processes contribute to attenuation?
Reflection
Scattering
Absorption
Reflection is likely to occur when the dimension of the boundary is _____
large, that is more than a few wavelengths of the sound
What are the two forms of reflection that are created in soft tissue?
Specular
Diffuse
Specular Reflection occurs
when the boundary is smooth, the sound reflects in only one direction in an organized manner
One limitation to Specular Reflection is
that once the wave is slightly off-axis, the reflection does not return to the transducer
Diffuse Reflection (backscatter) occurs
when a wave reflects off an irregular surface, it radiates in more than one direction
What is the advantage of diffuse reflections?
the interfaces at suboptimal angles to the sound beam can still produce reflections that will return to the transducer
What is the disadvantage of diffuse reflections?
is that backscattered signals have a lower strength than specular reflections
Scattering
what is it directly related to?
is the random redirection of sound in many directions.
frequency
Sound scatters when the tissue interface is ______; that is equal to or less than the _________ of the incident sound beam.
small
wavelength
What scatters more higher or lower frequency beams?
Higher frequency
Rayleigh Scattering
is 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 _______ in __ _________.
equally in all directions
What is an example of a Rayleigh scattering interaction?
red blood cells and sound waves
Lung tissue scatters sound because
the alveoli are filled with air
What is the relationship between Rayleigh scattering and frequency?
Rayleigh scattering increases dramatically with frequency (directly)
R. Scattering = Frequency^4
What reflection type does the sound wave return back to the transducer?
Specular -Organized
Diffuse/Backscatter-disorganized
What reflection type does the sound wave diffuse in all directions?
Rayleigh Scattering- organized
Scattering- disorganized
What is the conflict between frequency and attenuation?
Higher frequency sound waves produce shorter pulse, which usually create more accurate images. But higher frequency sound attenuates more and is less capable of traveling to substantial depths
Absorption occurs
when ultrasonic energy is converted into another energy form, such as heat
What is the relationship between frequency and absorption?
directly related
bc of absorption, higher frequency waves attenuate more than lower
Attenuation is a process whereby
sound energy is extracted from a wave by absorption, scattering, and reflection
attenuation coefficient
is the number of decibels of attenuation that occurs when sound travels one centimeter
units: dB/cm
what is the advantage of describing sound weakening with an attenuation coefficient ?
is that its value remains constant, regardless of how far the sound travels
If sound travels to a depth of 5 cm and the attenuation coefficient remains at 2 dB/cm, what is the total attenuation of the beam?
10 dB
what is the total attenuation (dB) formula?
total att. (dB) = atten. coefficient (dB/cm) X distance (cm)
In soft tissue, the attenuation coefficient (dB/cm) is related directly to what?
frequency
the attenuation coefficient is one-half the frequency
What is the attenuation coefficient formula in soft tissue?
atten. coeff. (dB/cm) = frequency (MHz)/2
Medium: Attenuation:
Water
Biologic fluids (blood, urine, amniotic fluid)
Fat
Soft Tissue
Muscle
Bone and Lung
Air
Extremely low
low
low
intermediate
higher
even higher
extremely high
Half-Value Layer Thickness
is the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value
–> or is the depth of tissue that results in 3 dB of attenuation to the intensity
Half-Value Layer Thickness
Units
Typical Values
Synonyms (3)
cm
0.25 to 1.0 cm
penetration depth, depth of penetration, half-boundary layer
What are the two factors that half-value thickness depends on?
the medium
the frequency of sound
Thin half value layer:
_____ frequency
media with ____ attenuation rate
Ex tissues:
high
high
lung or bone
Thick half value layer:
____ frequency
media with ____ attenuation rate
Ex tissues:
Low
Low
Fluids
Impedance
units
typical values
determined by
synonym
is the acoustic resistance to sound traveling in a medium (Z)
rayls
1.25 to 1.75 Mrayls
Medium ONLY; calculated NOT measured
characteristics impedance
Impedance formula
impedance (rayls) = density (kg/m^3) X prop. speed (m/s)
Reflection of an ultrasound wave depends upon what?
the difference
Normal incidence
synonyms (3)
means that the incident sound beam strikes at EXACTLY 90 degrees
-Perpendicular
-orthogonal
-right angle
Oblique incidence
synonyms
means that when the incident sound beam strikes the boundary at any angle other than 90 degrees
-Non-perpendicular
incident intensity
is the sound wave’s intensity immediately before it strikes a boundary
reflected intensity
is the intensity of the portion of the incident sound beam that, after striking a boundary, returns back from where it came
Transmitted intensity
is the intensity of the portion of the incident beam that, after striking a boundary, continues forward in the same general direction that it was traveling
Intensity conservation of energy at the boundary formula
incident (starting) incident = reflected intensity + transmitted intensity
Intensity Reflection Coefficient (IRC)
is the percentage of the intensity that bounces back when a sound beam strikes the boundary between two media
What is the IRC when the sound wave is reflected between two soft tissues? vs when it strikes btw soft tissue and bone or st and air?
1% or less
greater percentage of the wave is reflected
Intensity Transmission Coefficient (ITC)
is the percentage of intensity that passes in the forward direction when the beam strikes an interface between two media
ITC formula during transmission with normal incidence
ITC %= transmitted intensity/incident intensity X 100
=1 - IRC
With normal incidence, same impedance, what occurs between the two media ?
ALL of the sound is transmitted forward
What % of a sound wave’s intensity is transmitted when the beam strikes an interface btw two soft tissues? btw bone and soft tissue?
most (99% or more)
smaller %
IRC and ITC conservation of energy formula
100%= IRC + ITC
unitless
IRC formula during reflection with normal incidence
IRC %= [ Z2-Z1 / Z2 + Z1]^2 X 100
Z2: transmitted intensity
Z1: reflected intensity
When a sound beam strikes a tissue boundary at a 90 angle (normal incidence), REFLECTION occurs ONLY if the media on either side of the boundary have _______ ______
different impedances
NO reflection will occur if the two media have _____ _____
identical impedance
Which two physical principles ALWAYS apply to reflection with oblique incidence ?
Conservation of energy (100% = reflection coeff. + transmission coeff. )
Reflection angle=incident angle
Refraction
is a change in direction of wave propagation when traveling from one medium to another.
-Transmission with Oblique incidence
-transmission with a bend
Refraction occurs ONLY with which two conditions?
oblique incidence
different propagation speeds of the two media
Snell’s Law
quantifies the physics of refraction:
sine (transmission angle) / sin (incidence angle) = speed of Medium 2 / speed of Medium 1
Under what conditions will the transmission angle = the incident angle?
Refraction will NOT occur if the speeds of the two media are identical. Will move straight ahead.
Refraction requires:
oblique angles
different propagation speeds
Under what conditions will the transmission angle be greater than the incident angle?
When the speed of Medium 2 is > Medium 1
Under what conditions will the transmission angle be less than the incident angle?
When the speed of Medium 2 is < Medium 1
Angle of Transmission?
Speed 2 = Speed 1
Speed 2 > Speed 1
Speed 2 < Speed 1
no refraction; Trans = Inc
transmission angle > incident angle
transmission angle < incident angle
Requirement?
Event:
1) Reflection with normal incidence
2) Reflection with oblique incidence
3) Transmission
4) Refraction
1) different impedances required
2) can NOT predict
3) derived from reflection info, use law of conservation of energy
4) oblique incidence and different speeds required
