PHYSICS CH 6: Interaction of sound and media
with longer distances and higher frequencies there is _____________ attenuation.
with longer distances and higher frequencies there is *more* attenuation
Sound strikes a boundary between two media orthogonally. Although the media are very different, no reflection is created. How can this be?
with normal incidence, reflections occur only when the impedances of the two media at the interface are different. Two different media can have the same impedances, and when this happens, no reflection will be created
with shorter distances and lower frequencies there is _____________ attenuation.
with shorter distances and lower frequencies there is *less* attenuation
_________________________ occurs when ultrasonic energy is converted into another energy form, such as heat.
*Absorption* occurs when ultrasonic energy is converted into another energy form, such as heat.
__________________________________ is the number of decibels of attenuation that occurs when sound travels one centimeter.
*Attenuation Coefficient* is the number of decibels of attenuation that occurs when sound travels one centimeter.
Most interfaces in the body are not smooth, but have irregularities. When a wave reflects off an irregular surface, it radiates in more than one direction. This is called?
*Diffuse Reflection or Backscatter* is when a wave reflects off an irregular surface, it radiates in more than one direction
_____________________ is the acoustic resistance to sound traveling in a medium and is calculated by multiplying the density of a medium by the speed at which sound travels in the medium.
*Impedance* is the acoustic resistance to sound traveling in a medium and is calculated by multiplying the density of a medium by the speed at which sound travels in the medium
_________________ incidence means that the incident sound beam strikes the boundary at exactly 90 degrees
*Normal Incidence* means that the incident sound beam strikes the boundary at exactly 90 degrees
_____________________ incidence occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees
*Oblique Incidence* occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees
______________________________ is a special form of scattering that occurs when the structures dimensions are much smaller than the beams wavelength.
*Rayleigh Scattering* is a special form of scattering that occurs when the structures dimensions are much smaller than the beams wavelength.
___________________________ is the intensity of the portion of the incident sound beam that, after striking a boundary, returns back where it came.
*Reflected Intensity* is the intensity of the portion of the incident sound beam that, after striking a boundary, returns back where it came.
_____________________ is a change in direction of wave propagation when traveling from one medium to another.
*Refraction* is a change in direction of wave propagation when traveling from one medium to another.
_____________________ is the random redirection of sound in many directions.
*Scattering* is the random redirection of sound in many directions.
When the boundary is smooth and the sound is reflected in only one direction in an organized manner. This is called?
*Specular Reflection* is when the boundary is smooth and the sound is reflected in only one direction in an organized manner
_____________________________ is the intensity of the portion of the incident sound beam that, after striking a boundary, continues forward in the same general direction that it was traveling.
*Transmitted Intensity* is the intensity of the portion of the incident sound beam that after striking a boundary, continues forward in the same general direction that it was traveling.
-10dB means that the intensity is reduced to __________ of its original value.
-10dB means that the intensity is reduced to *one-tenth* of its original value.
A reduction in the intensity of a sound beam to one-half of its original value is _____ dB.
-3dB
A reduction in the intensity of a sound beam to one-quarter of its original value is _____ dB.
-6dB
Name three components of attenuation:
1. Absorption 2. Reflection 3. Scattering
Attenuation is determined by TWO factors:
1. PATH LENGTH ( distance) 2. FREQUENCY ( penetration)
What three processes contribute to attenuation?
1. Reflection 2. Scattering 3. Absorption
What are the two forms of reflection?
1. Specular 2. Diffuse
The half-value layer thickness depends on two factors:
1. The medium 2. the frequency of sound
With an oblique incidence, two physical priciples always apply to reflection:
1. conservation of energy 2. reflection angle = incident angle
Refraction only occurs if two conditions are satisfied:
1. oblique incidence 2. different propagation speeds of the two media
if IRC and ITC are added the result must equal?
100%
Sound is traveling from bone to soft tissue. The impedances of the media differ significantly, and 90% of the beam's intensity is reflected. What percentage of the intensity is transmitted?
100% of the energy must be counted for. If 90% is reflected, 10% must be transmitted.
The impedance of Medium 1 is 8 rayls. The propagation speed is 1,450 m/s. The impedance of Medium 2 is 6 rayls and the speed is 1.855 km/s. A sound beam strikes the boundary between the media and is both partially transmitted and reflected. The angle of the incident beam is 30 degrees. What is the reflection angle?
30 degrees. This question is quite simple. The angle of reflection equals the angle of incidence.
Sound is traveling in a medium and strikes a boundary with normal incidence. If 63% of the waves intensity is reflected back toward the transducer, what percentage is transmitted?
37% of the intensity will be transmitted. Conservation of energy occurs at a boundary, and as a result, the sum of the reflected and transmitted intensities must equal 100%. 63% + 37% = 100%
A _________ is a number that represents the number of 10s that are multiplied to create the original number.
A *log* is a number that represents the number of 10s that are multiplied to create the original number.
A high frequency sound and media with a high attenuation rate will have a _____________ Half-Value
A high frequency sound and media with a high attenuation rate will have a *Thin* Half-Value
A low frequency sound and media with low attenuation rate will have a ___________ Half-Value
A low frequency sound and media with low attenuation rate will have a *Thick* Half-Value
What does a 3dB change in a value intensity mean? A. the value has doubles B. the value has tripled C. the value has increased 30% D. the value has increased ten times
A. A 3 decibel change in the intensity indicates that the value has doubled
An ultrasound wave strikes an interface between two media at a 90 degrees angle. The propagation speeds of the media are identical. However, the densities of the media are different. Which is true? A. reflection will definitely occur B. reflection will definitely not occur C. refraction may occur D. none of the above
A. Reflection will definitely occur. With normal incidence, reflection will occur when the impedances of the media on either side of the boundary are different from each other. In this example, the densities of the media are different while their propagation speeds are the same.
Which of the following reports the relative strength of an ultrasound wave? A. decibels B. watts C. W/cm^2 D. Pascals
A. The *relative* strength of a sound beam may be reported in decibels. The term *relative* indicates a comparison between two values, such as a beam has decreased in strength by 50% choices B, C, and D are not comparisons, but absolute measurements.
The attenuation of an acoustic wave traveling through bone is _________ its attenuation through soft tissue. A. greater than B. less than C. equal to
A. The attenuation of ultrasound in bone is greater than its attenuation in soft tissue
A sound wave reaches a rough or irregular border between two media. Under these explicit circumstances, which process is most likely to occur? A. backscatter reflection B. specular reflection C. Rayleigh scattering D. refraction
A. The smoothness or roughness of a boundary will help to determine what form of reflection takes place. Backscatter is likely to occur when the boundary has irregularities that are larger than the wavelength of the incident acoustic pulse. Diffuse reflection is another name for backscatter
Ultrasound waves traveling through lung tissue attenuate to a ______ extent than when traveling through soft tissue. A. greater B. lesser C. nearly equal
A. Ultrasound attenuates more when traveling through lung tissue than when traveling through soft tissue
A wave strikes an interface between two media and intensities are measured at the interface. What results when the reflected intensity is divided by the incident intensity? A. intensity reflection coefficient B. intensity transmission coefficient C. beam uniformity coefficient D. none of the above
A. Under these circumstances, when we divide the reflected intensity by the incident intensity, the result is the intensity reflection coefficient. The intensity reflection coefficient is the percentage of the incident intensity that is reflected as a wave strikes a boundary between two media.
What is the decibel representation when an acoustic signal is amplified? A. positive B. negative C. equals to zero
A. When a signal is amplified, its magnitude increases and it becomes stronger. In decibel notation, an increase in signal strength is described with positive decibels. Positive dB means "getting bigger"
An acoustic wave is traveling from medium X to medium Z. Medium X has a propagation speed of 1,457 m/sec and an impedance of 1.44 MRayls. Medium Z's propagation speed is 1,644 m/s and its impedance is 1.26 MRayls. The angle of incidence is 32 degrees. What is true of the angle of transmitted wave? A. it is greater than 32 degrees B. it is equal to 32 degrees C. it is less than 32 degrees D. cannot be determined
A. With refraction, the relationship between the angle of incidence and the angle of transmission is defined by Snell's Law. When the propagation speed of the medium that the sound is entering exceeds the propagation speed of the medium that the sound is currently in, the angle of transmission is greater than the incident angle
decibels are a mathematical representation with a _______________ scale. A. logarithmic & relative B. division & relative C. longitudinal & relative D. logarithmic & absolute
A. logarithmic & relative
What are the units of the intensity reflection coefficient? A. none B. W/cm squared C. watts D. decibel
A. the intensity reflection coefficient is defined as the percentage or proportion of an ultrasound beam's intensity that is reflected as the sound wave strikes a boundary between two media. The intensity reflection coefficient is reported as a percentage and therefore is unitless
Acoustic Impedance = ______________________ x __________________________
Acoustic Impedance = density (kg/m^3) multipled by propagation speed (m/s)
After a transducer receives a sound wave during reception, the sound wave is converted into electrical sound. The signal is sent to the ultrasound system where it is strengthened or _____________________.
After a transducer receives a sound wave during reception, the sound wave is converted into electrical sound. The signal is sent to the ultrasound system where it is strengthened or *amplified*
How many decibels represent a 100-fold increase in the intensity of an acoustic pulse? A. 10 B. 20 C. 100 D. 6
B. An increase of 100 times the intensity of a wave is equal to 20dB. Each 10dB indicates a tenfold increase. Therefore, 20dB means a pair of tenfold increases. 10 times 10 = 100
Which of the following terms does not belong with the others? A. orthogonal B. oblique C. normal D. perpendicular
B. Oblique means "other than 90 degrees" The other three terms all have the meaning of "equal to 90 degrees"
What event does Snell's Law govern? A. transmission B. refraction C. impedance
B. Snell's Law defines the physics of refraction
A sound pulse travels in Medium 1 and strikes an interface with another tissue, Medium 2 at 30 degrees. The angle of transmission is 10 degrees. In which medium does sound travel slowest? A. Medium 1 B. Medium 2 C. cannot be determined
B. Sound travels slowest in medium 2. When the angle of transmission is less than the angle of incidence, sound travels slower in the second medium.
A wave of acoustic energy is leaving soft tissue and transmitting into fat with an 86 degree incident angle. What is true of the angle of transmission? A. it is equal to 86 degrees B. it is less than 86 degrees C. it is greater than 86 degrees D. cannot be determined
B. The angle of transmission will be less than the angle of incidence. Fat's propagation speed is less than that of soft tissue. The angle of transmission is less than the angle of incidence when a wave travels into a second medium that reduces the wave's propagation speed. In this example, the incidence is oblique, and fat has a lower speed than soft tissue.
What is the amount of attenuation per centimeter that a sound wave undergoes called? A. beam uniformity coefficient B. attenuation coefficient C. attenuation D. duty factor
B. The attenuation coefficient reports a sound beam's decibels of attenuation per centimeter. This is a useful tool as its value remains constant, regardless of the actual path length.
As a pulse passes through soft tissue, a certain amount of acoustic energy remains in the tissue as heat. What is this constituent of attenuation called? A. scattering B. absorption C. refraction D. rarefaction
B. The conversion of acoustic energy into heat is called absorption. This process deposits a portion of the energy from the beam into the soft tissues.
An acoustic wave is traveling from medium X to medium Z. Medium X has a propagation speed of 1,457 m/sec and an impedance of 1.44 MRayls. Medium Z's propagation speed is 1,644 m/s and its impedance is 1.26 MRayls. The angle of incidence is 32 degrees. What is true of the angle of reflection? A. it is greater than 32 degrees B. it is equal to 32 degrees C. it is less than 32 degrees D. cannot be determined
B. This is a tricky question. In cases of oblique incidence, the angle of reflection is always equal to the angle of incidence Simply put, if the incident angle is 32 degrees, then the reflection angle is 32 degrees. All information regarding the propagation speeds and acoustic impedances of media X and media Z are irrelevant in this question.
Which two attributes help establish the acoustic impedance of a medium? A. density and tempature B. density and stiffness C. stiffness and elastance D. elasticity and compressibility
B. To calculate the acoustic impedance, multiply the propagation speed of the medium by the density of the medium. A medium's propagation speed is determined by the density and the stiffness of the medium. Therefore, acoustic impedance is a property of both density and stiffness.
To calculate the acoustic impedance of a medium, one should _________ the _________ by the ___________ A. divide, propagation speed, density B. multiply, density, propagation speed C. divide, density, propagation speed D. multiply, stiffness, density
B. To obtain the value of acoustic impedance, multiply the density of a medium by its propagation speed.
An acoustic wave is in medium A and traveling toward medium B. The sound beam's angle of transmission into medium B with relation to the boundary is 79 degrees. Sounds propagation speed is 1,547 m/sec in medium A and 1,745 m/sec in medium B. If reflection and transmission both occur at the boundary, what can be said of the reflection angle? A. it is greater than 79 degrees B. it is less than 79 degrees C. it is equal to 79 degrees D. cannot be determined
B. Two steps are required to determine that the angle of reflection is less than 79 degrees. 1. We know that the transmission angle is 79 degrees and that the propagation speed of medium A is less than that of medium B. From this, we can derive information about the angle of incidence. Since the speed of medium B is greater than the speed of medium A, the angle of transmission is greater than the angle of incidence. Thus, the angle of incidence must be less than 79 degrees. 2. The angle of reflection is equal to the angle of incidence. Since the angle of incidence is less than 79 degrees, the angle of reflection is also less than 79 degrees.
Sound traveling through blood attenuates to a ____ extent than when traveling through soft tissue. A. greater B. lesser C. relatively equal
B. Ultrasound waves attenuates less when traveling through blood than when traveling through soft tissue.
An ultrasound wave strikes an interface between two media. All intensities are measured directly at the interface. What results when the transmitted intensity is divided by the incident intensity? A. intensity reflection coefficient B. intensity transmission coefficient C. beam uniformity coefficient D. none of the above
B. Under these circumstances, when the transmitted intensity is divided by the incident intensity, the result is the intensity transmission coefficient. The intensity transmission coefficient is the percentage of the incident intensity that continues to propagate in the forward direction when the incident wave strikes a boundary between two media
What is the decibel notation for an acoustic signal that is attenuated? A. positive B. negative C. equal to zero
B. When a signal is attenuated, its magnitude decreases and it becomes weaker. In decibel notation, a decrease in signal strength is described with negative decibels. Negative dB means "getting smaller"
An acoustic wave in medium B is traveling toward medium A. The sound beam strikes the boundary at a 45 degree angle. The propagation speed is 1,547 m/sec for medium A and 1,745 m/sec for medium B. What is true of the angle of transmission? A. it is greater than 45 degrees B. it is less than 45 degrees C. it is equal to 45 degrees D. cannot be determined
B. With refraction, when the medium that the sound is in has a greater propagation speed than the propagation speed of the medium that the wave is entering, the angle of transmission is less than the angle of incidence. In this case, the angle of transmission is less than 45 degrees because the sound wave is entering a slower medium than the one through which it is currently propagating
Which of the following describes an angle with a measure of 45 degrees? A. orthogonal B. acute C. obtuse D. normal
B. any angle that measures less than 90 degrees is an acute angle
A wave's intensity is 2 mW/cm^2. There is a change of +9 dB. What is the final intensity? A. 6 mW/cm^3 B. 2 mW/cm^2 C. 16 mW/cm^2 D. 16 uW/cm^2
C. 16 mW/cm^2
A sound beam with an intensity of 45 W/cm squared strikes a boundary and 70% of the wave's intensity is reflected. How much is transmitted? A. 45 W/cm^2 B. 25 W/cm^2 C. 30% D 100%
C. 30% of the wave's intensity is transmitted. The law of conservation of energy states that the sum of the percentage of transmitted sound and the reflected sound equals 100%
The intensity of an ultrasound wave is changed by -6dB. This means that the current intensity is _________ as much as its original level. A. one-tenth B. four times C. one-fourth D. one-sixth
C. A change in intensity of -6 dB indicates that only one-fourth of the initial intensity remains. The minus sign indicates a decrease in signal magnitude. Each -3 dB change means that one-half of the original intensity remains. Since there are two sets of -3dB, one-half multiplied by one-half indicates that there is only one-fourth remaining.
Acoustic impedance is a property of the ______ and has units of _______. A. source of medium, Imps B. medium, dB C. medium, Rayls D. medium, Ohms
C. Acoustic impedance is a property of the medium through which sound travels. It is unaffected by the sound source or the characteristics of the wave itself. The units of impedance are Rayls
The power in a wave is increased to ten times its original value. How many decibels describe this change? A. 3 B. 6 C. 10 D. 20
C. An increase of ten times the original power of a wave is reported as +10 dB
Which of the following describes an angle with a measure of 123 degrees? A. orthogonal B. acute C. obtuse D. normal
C. Any angle that measures greater than 90 degrees is an obtuse angle
As sound propagates through a medium, the total power in the wave decreases. What is this entire process called? A. absorption B. scattering C. attenuation D. reflection
C. As a wave propagates through a medium, its power diminishes. This is attenuation. Choices A, B, and D are all components of or contributors to attenuation
The scale associated with decibel notation is ______________. A. linear B. discrete C. logarithmic D. additive
C. Decibel notation is based on the mathematical relationship of logarithms
Which of the following is true of diffuse reflections? A. they're created by smooth boundaries B. they're created by large reflectors C. sound reflects in many directions D. they do not appear in soft tissue
C. Diffuse reflections, also called backscatter, are dispersed in multiple directions
Which of the following is *not* one of the physical processes that contributes to attenuation of ultrasound waves passing through soft tissue? A. reflection B. redirection of sound in many directions C. focusing D. conversion of acoustic energy to heat
C. Focusing does not contribute to the attenuation process. In contrast, choices A,B and D reduce the intensity of a wave as it propagates
What is the uniform dispersion of a sound wave in many different directions after striking a very small particle? A. Microscattering B. Backscattering C. Rayleigh scattering D. Total absorption
C. Rayleigh scattering is the direction of an acoustic wave in many different directions as a result of striking a small particle. When Rayleigh scattering occurs, the dimension of the reflecting particle is usually less than the wavelength of the ultrasound wave. The amount of Rayleigh scattering also depends on the frequency of the ultrasound.
An acoustic pulse reflects from a very smooth boundary where the irregularities on the surface of the boundary are much smaller than the pulse's wavelength. What type of reflection is most likely to occur under these circumstances? A. partial B. Rayleigh C. specular D. total
C. Reflections from a smooth boundary are specular. A mirror is a specular reflector. The waves strike this smooth boundary and reflect in an organized, systematic manner. A boundary is considered smooth when irregularities in its surface are smaller than the wavelength of the incident ultrasonic beam.
A sound pulse travels in Medium 1 and strikes an interface with another tissue, Medium 2 at 30 degrees. The angle of transmission is 10 degrees. In which medium is the impedance higher? A. Medium 1 B. Medium 2 C. cannot be determined
C. Refraction of sound at a boundary is unrelated to the impedances of the media. therefore, with the information provided, it cannot be determined which material has the greater impedance. Refraction is affected by the speed of sound in the media.
A sound wave strikes a boundary with normal incidence. The impedances of the two media are identical. What percentage of the sound wave is refracted? A. 100% B. 75% C. 0% D. 25% E. 90%
C. Remember, refraction cannot occur with normal incidence
To which of the following is Rayleigh scattering related? A. half of the wavelength B. square root of propagation speed C. frequency to the fourth power D. PRF squared
C. The degree to which a wave exhibits Rayleigh scattering is related to frequency to the fourth power. That is, if the frequency is twice as high, Rayleigh scattering is increased 16-fold: 2 to the fourth power = 2 times 2 times 2 times 2 = 16
Which term does not belong in the group below? A. orthogonal B. at right angles C. oblique D. 90 degrees E. normal F. perpendicular
C. The five terms: orthogonal, at right angles, 90 degrees, normal, and perpendicular are synonymous. Their meanings are identical. So C, oblique is the odd one out
What is the maximum permissible value for both the intensity reflection coefficient and the intensity transmission coefficient? A. 100 B. 1% C. 1 D. Infinity
C. The maximum percentage of the incident intensity that either reflects or transmits is 1.0 or 100%. At the extremes, total reflection occurs (intensity reflection coefficient = 1.0) or complete transmission occurs (intensity transmission coefficient = 1.0) The upper limit of both of these coefficients is 1.0
What conditions are necessary for refraction to occur at a boundary between two media? A. unequal acoustic impedances and normal incidence at the boundary B. unequal densities of the media and normal incidence at the boundary C. dissimilar propagation speeds and oblique incidence at the boundary D. different elasticities of the media and oblique incidence
C. The two conditions required for refraction are: 1. a sound wave must be obliquely incident to the border between two media 2. the media on either side of the border must have dissimilar propagation speeds. Refraction occurs during transmission of a wave from one medium to another.
A sound beam travels 9cm in soft tissue. The attenuation coefficient is 3 dB/cm. What is the total attenuation that the sound beam experienced? A. 9 dB B. 3 dB C. 27 dB D. 18 dB
C. To calculate total attenuation, multiply path length by attenuation coefficient. In this example, the overall attenuation is 3 times 9 = 27 dB
If the ____ of two media are different and sound strikes a border between the media at 90 degree incidence, then reflection will occur. A. conductances B. densities C. impedances D. propagation speeds
C. Under the conditions of orthogonal or normal incidence, reflection depends on differences in the acoustic impedances of the media on either side of the boundary. With normal incidence, as long as the impedances are dissimilar, reflection will always occur
An acoustic wave is traveling through soft tissue. Its intensity undergoes six decibels of attenuation. How does the final intensity of the wave relate to the intensity of the wave when it started on its journey? A. it is now four times larger B. it is now six times larger C. it is now one-fourth as large D. it is now one-tenth as large
C. When a wave undergoes 6 decibels of attenuation, the intensity of the wave is decreased to one-fourth of its initial value. Six decibels of attenuation is made up of two groups of -3 decibels. Each -3 decibels indicates halving of intensity. One-half multiplied by one-half means that only one-fourth of the original intensity remains.
What is a decibel? A. the absolute value of a number B. a range of values C. a relationship between two numbers D. none of the above
C. a decibel represents a relationship between two numbers. A decibel is a relative measure of intensity or power. The term "relative" indicates that we are not dealing with an absolute power (such as 600 watts), but rather with how the power is related to a reference level
Which term has a meaning other than normal incidence? A. orthogonal incidence B. perpendicular incidence C. oblique incidence
C. oblique incidence has a meaning that is different than normal incidence
Which of the following is considered a Rayleigh scatterer? A. bone B. liver C. muscle D. blood cell
D. A blood cell is a Rayleigh scatterer. A red blood cell is smaller than the wavelength of the typical acoustic wave used in diagnostic imaging. When an acoustic wave strikes a blood cell, the energy within the pulse is scattered in many directions.
The intensity of a signal declines from 1.5 mW/cm^2 to 0.75 mW/cm^2 How many decibels is this change in intensity? A. 3 dB B. 0.75 dB C. -0.75 dB D. -3 dB
D. A decline in intensity to one-half of the original level is a change of -3 dB. When the new level is less than the reference level, the reported decibels must be negative. When the new intensity is greater than the reference level, the change in decibel is positive.
What remains when the reflected intensity is subtracted from the incident intensity? A. 1.0 B. incident intensity C. transmitted intensity coefficient D. transmitted intensity
D. At any point in time and space, there must be conservation of energy. In other words, all energy must be accounted for. The intensity in a wave as it reaches a border between two media is called the incident intensity. As the wave strikes the boundary, a portion of the wave's intensity is redirected toward the sound source. This is called the reflected intensity. The remainder of the energy continues to propagate and is called the transmitted intensity.
What units are used to describe attenuation? A. watts B. watts/cm^2 C. macro D. decibels
D. Attenuation is measured in decibels. Attenuation is reported as a relative change in strength, not an absolute change.
Decibel notation is a ___________________ between two numbers. A. difference B. sum C. product D. ratio
D. Decibels are calculated by dividing the final strength of a signal by the starting strength. Thus, decibel notation is a ratio between two numbers.
A sound wave strikes a boundary between two media at a 60 degree angle. This is called _____ incidence. A. orthogonal B. angular C. obtuse D. oblique
D. Oblique incidence is always present when the angle between the direction of a wave's propagation and the boundary between two media is different than 90 degrees. Oblique incidence is a definition of exclusion. That is, if the incidence is not perpendicular, it is oblique
The angle between the direction of propagation and the boundary between two media is 90 degrees. What term describes the form of incidence of the wave? A. not normal B. direct C. oblique D. orthogonal
D. Orthogonal incidence is attained when a sound wave strikes a boundary between media at exactly 90 degrees
Which of the following best describes the process described by Snell's Law? A. reflection with normal incidence B. reflection with oblique incidence C. transmission with normal incidence D. transmission with oblique incidence
D. Snell's Law defines the physics of refraction. Refraction describes the behavior of sound beams that transmit after striking a boundary obliquely. Refraction is "transmission with a bend."
Sound that is traveling in Jell-O passes through an interface at 90 degrees and continues to travel in whipped cream. The impedance of Jell-O and whipped cream are nearly identical. What percentage of the intensity is transmitted? A. 2% B. 25% C. 78% D. 99%
D. The best choice is 99%. If the impedances of Jell-O and whipped cream are nearly identical, only a very small percentage of the sound beams intensity will reflect. The remainder of course will transmit.
An ultrasound wave strikes a boundary between two media. All intensities are measured directly at the boundary. What results when the transmitted intensity is divided by the reflected intensity? A. intensity reflection coefficient B. intensity transmission coefficient C. beam uniformity coefficient D. none of the above
D. The choices A, B and C are all incorrect. When the transmitted intensity is divided by the reflected intensity, a number is obtained. This number has no special meaning in ultrasound physics, and is not identified by a special name
Attenuation is determined by which of the following factors? A. density and stiffness of the medium B. frequency of sound and propagation speed C. PRF of sound and path length D. path length and frequency of sound
D. The factors that determine the degree to which a sound beam weakens are: 1. the distance that the sound travels 2. the frequency of the sound
An ultrasound wave approaches an interface between two media at a 90 degree angle. The propagation speeds and the densities of the two media are different. Which is correct? A. reflection will definitely occur B. reflection will definitely not occur C. refraction may occur ' D. none of the above
D. The facts presented indicate that the propagation speeds and the densities of the two media are different, but this information is insufficient to determine whether reflection will take place. The important fact is whether the impedances of the media are different. If the impedances are the same, then reflection will not occur
Which value is closest to the impedance of soft tissue? A. 1.5 kiloRayls B. 2.5 deciRayls C. 19 megaRayls D. 2,000,000 Rayls
D. The impedance of soft tissue is in the range of 1.5 to 2.5 million Rayls Choices A, and B are far too low and choice C is too high
The logarithm of a numeral is defined as how many times ___________ must be multiplied together to get that number. A. 1 B. 2 C. 5 D. 10
D. The logarithm of a numeral is equal to the number of *tens* that are multiplied together to result in that number
What is the minimum permissible value for both the intensity reflection coefficient and the intensity transmission coefficient? A. different from each other B. -1 C. 100% D. 0
D. The minimum value for the intensity transmission and the intensity reflection coefficients is zero. At one extreme, it is possible to have total transmission and no reflection (intensity reflection coefficient =0) At the other extreme, it is possible for total reflection to occur. No transmission would exist
What results when the intensity transmission coefficient and the intensity reflection coefficient are added together? A. incident intensity coefficient B. acoustic impedance C. total intensity D. 1.0
D. The sum of the intensity reflection coefficient (IRC) and the intensity transmission coefficent (ITC) equals one or 100%. The ITC is the percentage of a sound waves intensity that is transmitted at a boundary. Since there is conservation of energy at the boundary, these percentages added together equal 100% or 1.0
A sound pulse strikes a boundary between two media with normal incidence. The speed of sound in the two media are very different; however, the impedances of the media are identical. What will happen? A. a big echo will be produced B. a medium echo will result C. a small echo will result D. there will be no echo at all
D. When a sound wave is normally incident at a boundary, reflection will occur only when the media have dissimilar impedances. In this example, the wave is normally incident but the media have identical impedances. Under tehse circumstances, no reflection will occur and 100% of the sound will be transmitted.
Two acoustic waves strike a boundary between two media. The waves are traveling in a direction 90 degrees to the boundary. Reflection of these waves depends on differences in the ___. A. frequencies of the two waves B. propagation speeds of the two media C. amplitudes of the two waves D. impedances of the two media
D. With normal incidence, reflection will occur at a boundary when the media on either side of the boundary have different acoustic impedances. Reflection is not dependent on the waves' characteristics such as an amplitude and frequency. With normal impedance, only one condition must be met for reflection to take place: the impedances of the media on either side of the boundary must be different.
Sound propagates from one medium with a density of 1.16 kg/m^3 to a second medium with a density of 1.02 kg/m^3. If the angle of transmission is 49 degrees, what can be correctly stated about the angle of incidence? A. it is less than 49 degrees B. it is equal to 49 degrees C. it is greater than 49 degrees D. it cannot be determined
D. no definite statement can be made about the angle of incidence. Since the propagation speeds of the media are unknown, we cannot make any conclusions about the incident angle. The density of a medium helps to determine the propagation speed, but knowledge of a tissues stiffness is insufficient to deduce which of the two media has a higher propagation speed.
What are the units of the intensity transmission coefficient? A. watts/square centimeter B. watts C. decibels D. none of the above
D. the intensity transmission coefficient is defined as the *percentage* of an ultrasound beam's intensity that is transmitted as the sound wave passes through a boundary between two media. The intensity transmission coefficient is a percentage, and is reported without units.
Which of these lists indicates media with increasing attenuation of ultrasound? A. water, lung, soft tissue, bone, air B. lung, air, soft tissue, bone C. lung, fat, muscle D. water, blood, fat, muscle, bone, air
D. water, blood, fat, muscle, bone, air are in order with increasing attenuation rates of ultrasound
Decibel notation does not measure absolute numbers; rather decibels report ________________ changes.
Decibel notation does not measure absolute numbers; rather decibels report *relative changes*
Decibels are based on a mathematical construct called ______.
Decibels are based on a mathematical construct called *logarithms*
True or False. We need one intensity to calculate decibels.
False.
True or False? Refraction always occurs at the interface between two media when the propagation speeds of the media are unequal
False. A sound wave refracts at the boundary between two media when *two* conditions are satisfied: 1. the sound beam must be obliquely incident 2. the propagation speeds of the two media must be different from each other.
True or False? When reflection occurs with oblique incidence, the angle of reflection equals the angle of incidence. This is known as snell's law
False. It is indeed true that the angle of reflection is equal to the angle of incidence when reflection occurs at an oblique incident angle. The falsehood in this statement is that is not Snell's Law.
True or False? The period of a sound wave determines whether refraction will occur.
False. Refraction does not depend upon the characteristics of a wave. Refraction only depends upon the incidence of the sound beam and the speeds of the media.
Scattering is related to _________.
Frequency raised to the fourth power
Half-Value Layer is also called: _______________________________, ______________________________, & _____________________________________.
Half Value Layer is also called: *penetration depth*, *depth of penetration*, *half-boundary layer*
In clinical imaging, the interaction of ultrasound and ________________________________ results in rayleigh scattering.
In clinical imaging, the interaction of ultrasound and *red blood cells* results in raylegh scattering.
In soft tissue the attenuation coefficient is _______________ the frequency.
In soft tissue the attenuation coefficient is *one-half* the frequency
What is the relationship between ultrasound frequency and the attenuation coefficient in soft tissue?
In soft tissue, the attenuation coefficient in decibels per centimeter is approximately one half the ultrasonic frequency in MHz
Incident intensity = ____________________________ + ______________________________
Incident Intensity = reflected intensity + transmitted intensity
IRC formula
Intensity reflected divided by initial intensity
Two media A and B have the same densities. The speed of sound in medium A is 10% higher than in medium B. Which mediumm has the higher acoustic impedance?
Medium A's acoustic impedance is higher than medium B's. Recall that impedance equals speed multiplied by density. Since both media have identical densities and medium A's speed is 10% higher, then medium A's impedance is 10% higher.
Negative decibels describe signals that are ______________ in strength, or getting ________________.
Negative decibels describe signals that are *decreasing* in strength, or getting *smaller.*
Normal incidence is also called ____________________________, ________________________, _________________________, & _______________________________.
Normal Incidence is also called: - Perpendicular - Orthogonal - Right Angle - 90 degrees
Positive decibels report signals that are ______ in strength or getting _______.
Positive decibels report signals that are *increasing* in strength or getting *larger*
If the final intensity of a sound beam is more than the initial intensity, then the gain in dB is ____. (+ or -)
Positive. The beams intensity is increasing.
If the initial intensity of a sound beam is less than the final intensity, then the gain in dB is _____
Positive. The beams intensity is increasing.
Impedance is reported in what units?
Rayls (Z)
Sound waves weaken, or ______ as they travel through the body.
Sound waves weaken, or *attenuate* as they travel through the body.
The ________________________________ is the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value.
The *Half-Value Layer Thickness* is the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value.
The _______________________________________ is the percentage of the intensity that bounces back when a sound beam strikes the boundary between two media
The *Intensity Reflection Coefficient* (IRC) is the percentage of the intensity that bounces back when a sound beam strikes the boundary between two media
The ______________________________________ is the percentage of the intensity that continues to move forward when the beam reaches a boundary between two media.
The *Intensity Transmission Coefficient* is the percentage of the intensity that continues to move forward when the beam reaches a boundary between two media
Which is better to use while examining a carotid artery, a 7.5MHz or 3.0MHz transducer?
The carotid artery is a superficial structure. A 7.5MHz transducer is better because the higher frequency transducer produces a better image
If the *reflected* and *transmitted* intensities are *added*, the result must be the?
The incident intensity
True or False? The proportion of the incident intensity that is reflected at the border between two media will increase as the impedances of the media become increasingly dissimilar
True. As sound waves strike the border between two media, reflection occurs if their impedances are different. Greater differences between the two impedances create stronger reflections. If the impedances are only slightly different, then a weak reflection will be produced.
True or False? Attenuation and propagation speed are unrelated
True. Attenuation and propagation speed are entirely unrelated. Attenuation is the weakening of a beam as it travels and has nothing to do with how fast it travels
True or False. In a given medium, attenuation is unrelated to the speed of sound.
True. Attenuation and propagation speed are unrelated.
True or False? Refraction occurs at the border between two media if and only if there is oblique incidence of the wave at the boundary.
True. Refraction cannot occur if a wave is normally incident to the boundary between two media. It can occur only if the incident wave is oblique to the boundary.
Attenuation for each: Water Blood, Urine, Biologic Fluids Fat Soft Tissue Muscle Bone & Lung Air
Water = extremely low Blood, Urine, Biologic Fluids = low Fat = low Soft Tissue = intermediate Muscle = higher Bone & Lung = even higher Air = extremely high
What is the log of 1,000?
We multiply 10 by itself 3 times to create the number 1,000 so the log of 1,000 is 3
When a sound beam strikes a tissue boundary at 90 degree angle, it is a ________________________ incidence
When a sound beam strikes a tissue boundary at a 90 degree angle, it is a *normal incidence*
When a waves intensity doubles, the relative change is ______.
When a waves intensity doubles, the relative change is postive *3 decibels*
When frequency doubles, Rayleigh Scattering is ________ times greater
When frequency doubles, Rayleigh Scattering is *16* times greater
When intensity increases ten-fold, the relative change is ________.
When intensity increases ten-fold, the relative change is positive *10 decibels*
When the intensity is reduced to 1/10 its original value the relative change is ______.
When the intensity is reduced to 1/10 its original value the relative change is negative *10 decibels*
When the intensity is reduced to 1/2 its original value, the relative change is _____________.
When the intensity is reduced to 1/2 its original value, the relative change is negative *3 decibels*
Sound travels in a medium and orthogonally strikes a boundary with a different medium. Although sound waves traveling in the media have vastly different speeds, there is no refraction. How can this be?
With normal incidence, refraction cannot occur. Refraction occurs only when there are different speeds and oblique incidence. Both conditions must be met. In this example, the incidence is normal - no refraction can occur
A pulse of ultrasound is propagating in soft tissue, such as liver. The pulse strikes a boundary with a different soft tissue at normal incidence. What portion of the intensity is reflected back toward the transducer? why?
a very small percentage of sound, typically less than 1% is reflected at a boundary between 2 soft tissues. The impedances of two soft tissues are similar, and the difference in impedance directly determines the intensity reflection coefficient. very little reflection occurs when the impedances have similar, but not identical values.
Total Attenuation =
attenuation coefficient (dB/cm) x distance (cm)
what are the units for attenuation coefficient?
dB/cm
What are the units of attenuation?
decibels (dB)
Distance and attenuation are ______ related.
directly
Frequency and attenuation are ______ related.
directly
Scattering is ______ related to frequency.
directly
disorganized sound back to the transducer is _____________________ reflection
disorganized sound back to the transducer is *Diffuse Reflection or Backscatter*
disorganized sound in all directions is ____________________________
disorganized sound in all directions is *Scattering*
What are the units of the half-value layer thickness?
distance: centimeters
Every 3dB change means that the intensity will ____.
double
attenuation coefficient formula:
frequency (MHz)/2
Attenuation in air is _______ attenuation in soft tissue.
greater than
Attenuation in bone is _________ attenuation in soft tissue.
greater than
Attenuation in lung tissue is [less than, greater than, the same as] attenuation in soft tissue.
greater than
Every 10dB change means that the intensity will _____.
increase ten times
As frequency decreases, depth of penetration _________________.
increases
As the path length increases, the attenuation of ultrasound in soft tissue _______.
increases
Higher frequency sound beams scatter _______________ (more or less) than lower frequency beams.
more
C2 = C1
no refraction; transmission angle = incident angle
Impedance is associated with ____________________.
only the medium
organized sound back to the transducer is ____________________ reflection
organized sound back to the transducer is *Specular Reflection*
organized sound in all directions is _______________________________
organized sound in all directions is *Rayleigh Scattering*
As sound strikes a boundary, a portion of the wave's energy may be redirected, or _____ back to the sound source
reflected
reflection occurs only if the media on either side of the boundary have _______________________ impedances
reflection occurs only if the media on either side of the boundary have *different* impedances
Impedance is important in ____ at boundaries.
reflections
Impedance is determined by
the medium, calculated not measured
ITC formula
the transmitted intensity divided by incident intensity multiplied by 100 or 1 - intensity reflection coefficient
C2 > C1 =
transmission angle greater than incident angle
C2 < C1 =
transmission angle less than incident angle
