Ch 6 Sound in Media Flashcards
What two things do sound waves do when traveling?
Some reflect at the boundaries of 2 different media, remainder travels deeper to next medium boundary
Sound ______ as it travels.
weakens
Propagation
sound traveling through tissue
Reflection
portion of sound that bounces off medium boundaries
Receive
reflected wave traveling back to transducer
Attenuate
sound wave weakening while traveling
absorption
Amplified
US system strengthens reflected waves received by transducer
What are the 6 steps to signals traveling?
- Sound waves created & transmitted into body by US system & transducer
- Sound waves attenuate as they propagate
- Waves are reflected at boundary of 2 media
- Reflections received by transducer
- Waves converted to electrical signal
- Signal sent to US system to be amplified
What do decibels measure?
- strength of sound beams as they are created and propagate
- strength and brightness of images created by electrical signals
What is decibel notation?
- uses logarithms
- does not use absolute numbers
- reports relative change
Decibel notation is: (4 things)
- relative measurement
- a comparison
- a ratio
- logarithmic
Positive decibels report signals that are what?
Increasing in strength or getting larger
When intensity doubles, the relative change is ______?
3 dB
Negative decibels report what kind of signals?
- signals that are decreasing or getting smaller
When intensity is reduced to half the original value, the relative change is _____?
-3 dB
What is a logarithm?
- method of rating numbers
- represents the number of 10s multiplied to create the original number
If a log increases by 1, the number increases _____
10 fold
What is the log of 1000? 10000?
3, 4
Attenuation is a decrease in what 3 parameters as sound propagates?
Power, intensity, amplitude
What two factors is attenuation determined by?
Path length and frequency of sound
How does path length affect attenuation?
- directly affects it
- increased distance = increased attenuation
- increased distance = decreased strength
How does frequency affect attenuation?
- directly affects
- increased frequency = increased attenuation
- increased frequency = decreased strength
What 3 processes contribute to attenuation?
Reflection, scattering, absorption
What does reflection do to part of the wave that keeps traveling?
Weakens it
What are the two types of reflection in soft tissue?
Specular, diffuse
What is specular reflection?
- smooth boundary
- reflected sound travels in one direction
- travels in an organized manner
- if wave is off-axis reflection doesn’t return to sound source
What is an example of specular reflection?
Mirror
What is diffuse reflection?
- most boundaries not smooth
- reflected sound travels in more than one direction
- interfaces at variable angles w sound source will still produce reflections that will travel back to sound source
“Backscattered” reflections have _______ strength than specular reflections
Weaker
What is scattering?
- random redirection of sound in diff directions
- sound scatters when tissue interface is small
Higher frequency sound scatters ____ than lower frequency.
More
How is scattering related to frequency?
Directly
What is Rayleigh scattering?
- organized form of scattering
- occurs when structure dimensions are smaller than beam’s wavelength
What kind of structures could cause Rayleigh scattering?
RBCs, tissue cells
How does Rayleigh scattering change with increased frequency?
Drastically increases, it is proportional to frequency^4
When frequency doubles Rayleigh scattering is how much greater?
16 times
What is absorption?
It occurs when sound energy is converted into heat and radiates away
How is absorption related to frequency?
Directly, increased freq = increased absorp
What is attenuation coefficient?
Process where sound energy is extracted from a wave by absorption, scattering and reflection
How is attenuation coefficient measured?
Number of decibels of attenuation that occurs when sound travels one cm
dB/cm
What does total attenuation depend on?
- Frequency of sound wave
- distance wave traveled
- tissue wave traveled through
How is total attenuation calculated?
Attenuation coefficient (dB) x distance (cm)
How are attenuation coefficient and frequency related in soft tissue?
Directly
AC is equal to
1/2 frequency, frequency/2
Do higher or lower frequency waves lose more energy per cm?
Higher
What is half-value layer thickness?
- distance sound travels in a tissue that reduces the intensity of sound to 1/2 it’s original value
- can also be described as the depth of tissue that results in 3 dB of attenuation to the intensity
How is half-value layer thickness measured? Range?
Any length, cm
0.25-1.0 cm
What does half-value layer thickness depend on?
Medium and frequency
Thin half value is characterized by
High frequency, media with high attenuation rate (muscle, fibrous, bone)
Thick half value is characterized by
Low freq wave, media with low attenuation rate (fluid, fat)
What is impedance?
Acoustic resistance to sound traveling in a medium
How is impedance calculated?
Density of a medium x speed that sound travels in the medium
Impedance (rayls) = density (kg/m^3) x propagation speed (m/s)
What letter represents impedance?
Z
What is impedance determined by?
Medium only
How does impedance affect reflections?
Reflection depends on the difference in impedance of the two media at the boundary
What is incidence?
A pulsed sound wave strikes many tissue interfaces as it propagates through the body
The angle the wave hits the boundary determines the behavior of rest of pulse & reflection angles
What is normal incidence?
Angulation of sound wave hitting boundary is 90 degrees
What is oblique incidence?
Angulation of sound wave hitting boundary is anything other than 90 degrees
What is incident intensity?
Sound wave’s intensity immediately before reaching a boundary
What is reflected intensity?
Intensity of the portion of the incident sound beam that returns to the sound source after hitting a boundary
What is transmitted intensity?
Intensity of the portion of the incident sound beam that continues traveling forward after hitting a boundary
What is intensity reflection coefficient (IRC)?
- percentage of intensity that bounces back when a sound beam hits a boundary between 2 media
How much of a sound wave’s intensity is reflected at a boundary between 2 soft tissues?
Less than 1%
A _______ % of a sound wave’s intensity is reflected between larger impedance differences
Greater
What is intensity transmission coefficient (ITC)?
- percentage of intensity that continues traveling forward when a sound beam hits a boundary between 2 media
How much of a sound wave’s intensity is transmitted at a boundary between 2 soft tissues?
99% or more
A ______ % of a sound wave’s intensity is transmitted between larger impedance differences
Smaller
When a pulsed sound wave hits a boundary at 90 degrees, reflection only occurs if
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 tissue impedances
If a sound wave hits a boundary at 90 degrees and the two tissue have the same impedances, how much of the sound wave is reflected?
None
If a sound wave hits a boundary at 90 degrees and the 2 tissues have different but close impedances, how much reflection will occur?
A small reflection
If a sound wave hits a boundary at 90 degrees and the tissues have significantly different impedances, how much reflection will occur?
Large reflection
With normal incidence, all sound is transmitted when
The tissues have the same impedances
When a sound beam hits a boundary at an oblique angle, what happens with reflection?
It may or may not occur depending on the degree of angulation at the boundary
The reflection angle is the same as
Incident angle
When a sound wave hits a boundary at an oblique angle, what happens with transmission?
It may or may not occur depending on the degree of angulation at the boundary
If transmission occurs with oblique incidence, how will the sound beam travel?
Straight or bend/change direction
Bending of a wave is
Refraction
What is conservation of energy?
Sum of reflected sound and transmitted sound = 100%
Incident intensity (w/cm^3) =
Reflected intensity + transmitted intensity
The angle of incidence equals
The angle of reflection
The direction of reflected sound is equal and opposite to
The direction of incident beam
What is refraction?
Change in direction of wave propagation when traveling from one medium to another
Refraction only occurs with
- oblique incidence
- different propagation speeds of 2 media
If transmission occurs and there is oblique incidence and different propagation speeds of the 2 media, how will the wave travel?
It will bend, never travel straight
There is a _____ degree of bending in similar propagation speeds
Smaller
What does Snell’s law do?
Quantifies the physics of retraction
Snell’s law equation
Sin(transmission angle)/sin(incident angle) = speed (med 2)/speed (med 1)
What is sine?
A unitless number with a value of 0-1
What is medium 1 in Snell’s law?
The medium in which sound is currently traveling
What is medium 2 in Snell’s law?
Medium in which sound will be entering
Decibels require _____ intensities
Two
A wave’s intensity is 2 mW/cm^2. There is a change of 9 dB. What is the final intensity?
16 mW/cm^2
Reflection is likely to occur when the dimension of the boundary is _____
Large
How does sound attenuate in muscle?
Sound attenuates twice as much when traveling across the fibers compared to when traveling along the length of the fibers
The half-value layer is _____ for lung and bone.
Thin
Typical values for impedance range from
1.25-1.75 Mrayls
T or F. In a given medium, attenuation is unrelated to the speed of sound.
True
What is normal incidence?
The sound beam strikes the boundary at exactly 90 degrees
Normal incidence is also known as
- perpendicular
- orthogonal
- right angle/90 deg
When will the transmission angle equal the incident angle?
When the speeds of the two media are identical both angles with be equal and the beam will travel straight ahead
When will the transmission angle be greater than the incident angle?
When the speed of medium 2 is greater than the speed of medium 1
When will the transmission angle be less than the incident angle?
When the speed of medium 2 is less than medium 1
