Chapter 6 Review Questions
Every 3 dB change means that the intensity will_____.
Double
Every 10 B change means that the intensity will____.
Increase ten times
A reduction in the intensity of a sound beam to one-half of its original value is ____ dB.
-3
A reduction in the intensity of a sound beam to one-quarter of its original value is ____dB.
-6
-10 dB means that the intensity is reduced to _____ of its original value.
one-tenth
dB is a mathematical representation with a
scale.
A. logarithmic and relative
B. division and relative
C. longitudinal and relative
D. logarithmic and absolute
logarithmic and relative
True or False. We need one intensity to calculate decibels.
False
A wave's intensity is 2 mW/cm?. 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 microW/cm^2
16 mW/cm^2
If the final intensity of a sound beam is more than the initial intensity, then the gain in dB is _____(+ or -).
Positive. The beam's intensity is increasing.
If the initial intensity of a sound beam is less than the final intensity, then the gain in dB is_____ (+ or -).
Positive. The beam's intensity is increasing.
Name the three components of attenuation.
absorption, reflection, and scattering
As the path length increases, the attenuation of ultrasound in soft tissue_____.
increases
Attenuation in lung tissue is [less than, greater than, the same as] attenuation in soft tissue.
greater than
Attenuation in bone is _____attenuation in soft tissue.
greater than
Attenuation in air is _____attenuation in soft tissue.
greater than
What are the units of attenuation?
Decibels (dB)
T or F. In a given medium, attenuation is unrelated to the speed of sound.
True. Attenuation and propagation speed are unrelated.
What is the relationship between ultrasound frequency and the attenuation coefficient in soft tissue?
In soft tissue, the attenuation coefficient in dB per centimeter is approximately one half of the ultrasonic frequency in MHz.
What are the units of the half-value layer thickness?
distance: centimeters
As frequency decreases, depth of penetration_____.
increases
As path length increases, the half boundary layer____.
remains the same
Impedance is associated with____.
only the medium
As the path length increases, the attenuation coefficient of ultrasound in soft tissue [decreases, remains the same, increases].
remains the same
Acoustic impedance =______ x ______
impedance = density (kg/m^3) x propagation speed (m/s)
Two media A and B have the same densities. The speed of sound in medium A is 10% higher than in medium
B. Which medium 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.
Impedance is important in ____
at boundaries.
reflections
Which is better to use while examining a carotid artery, a 7.5 or 3.0 MHz transducer?
The carotid artery is a superficial structure.
A 7.5 MHz transducer is better because the higher frequency transducer produces the better image. We can use the higher frequency transducer in this example because the structure is superficial and attenuation is of little concern.
A sound wave with an intensity of 50 W/cm? strikes a boundary and is totally reflected.
What is the intensity reflection coefficient?
A. 50 w/cm^2
B. 25 w/cm^2
C. 0 w/cm^2
D. 100%
E. 0
D. Since the wave is totally reflected, the intensity reflection coefficient is
100%.
A sound wave with an intensity of 50 W/cm? strikes a boundary and is totally reflected.
What is the reflected intensity?
A. 50 w/cm^2
B. 25 w/cm^2
C. 0 w/cm^2
D. 100%
E. 0
A. Since the wave is totally reflected, intensity is 50 w/cm^2
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 two soft tissues. The impedances of two soft tissues are similar, and the difference in impedance directly determines the intensity reflection coefficient. Very little retaion occurs when the impedances have similar, but not identical, values.
Sound is traveling in a medium and strikes a boundary with normal incidence.
If 63% of the wave's intensity is reflected back toward the transducer, what percentage is transmitted?
37% of the intensity will on transmitted.
Conservation of energy ocons at a boundary, and as a result, the sum of the reflected and transmitted intersities must
equal 100%. 63% + 37% = 100%.
A pulse of ultrasound is propagating in bone and strikes an interface with soft tissue at 90°. A giant reflection is created.
• From these facts alone, what can be said about the impedance of bone?
• What can be said about the impedance of soft tissue?
• What can be said about the differences between the impedances of bone and soft tissue?
Nothing can be stated about the impedance of bone or soft tissue based on the information given. However, because a large reflection was created, the impedances of these two media must be dissimilar. Reflections with normal incidence are created based on the difference in the impedances, not on the actual values of the impedances.
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 that happens, no reflection will be created.
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 a meaning of "equal to 90 degrees."
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 accounted for. If 90% is reflected, 10% must be transmitted.
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 beam's intensity will reflect. The remainder, of course, will transmit.
A pulse of ultrasound propagates in soft tissue, such as liver. The pulse strikes a soft tissue-soft tissue interface with oblique incidence. Some of the sound energy is transmitted. To what extent is the transmitted beam refracted?
The transmitted beam undergoes little to no refraction. A transmitted beam is refracted when the incidence is oblique and the propagation speeds are different. Because the tissues on either side of the boundary are both "soft tissues," their speeds are nearly identical and little or no refraction occurs.
A sound pulse travels in Medium 1 and strikes an interface with another tissue, Medium 2, at 30°. The angle of transmission is 10°. From these facts
alone, what can be said about
• the speed of sound in Medium 1?
• the speed of sound in Medium 2?
• the difference between the speeds of Media 1 and 2?
Given only this information, we can say nothing about the speed of sound in Medium 1 or Medium 2. However, because the beam refracted significantly (there was a 20° change in direction), the speeds of these two media are very different.
Refraction depends on the difference in the speeds of sound in the two media, not the actual speeds.
A sound pulse travels in Medium 1 and strikes an interface with another tissue, Medium 2, at 30°. The angle of transmission is 10°. 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 sound pulse travels in Medium 1 and strikes an interface with another tissue, Medium 2, at 30°. The angle of transmission is 10°. 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. " cannot be determined which material has the greater impedance.
Refraction is affected by the speed of sound in the media.
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 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.
The impedance of Medium 1 is 8 rayls.
The propagation speed is 1,450 mS.
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°. What is the reflection angle?
30°. This question is quite simple. The angle of reflection equals the angle of incidence.
What does the 100 mW/cm? represent?
The incident intensity of the sound beam.
What does the 3.5 dB/cm represent?
The attenuation coefficient of the sound in the media.
If the media are soft tissue, what is an estimate of the ultrasound frequency?
About 7 MHz; the attenuation coefficient multiplied by 2 approximates the frequency (3.5 × 2 = 7).
What property has units of rayls? How is it determined?
Impedance is calculated, not measured.
Impedance = density x speed.
The incidence between the sound wave and the boundary between Media 1 and 2 is normal. What happens at the boundary between Media 1 and 2? Why?
Both reflection and transmission occur.
There are normal incidence and different acoustic impedances.
The incidence between the sound wave and the boundary between Media 2 and 3 is normal. What happens at the boundary between Media 2 and 3? Why?
Transmission only. The impedances of the media are the same.
What type of incidence is there between
Media 3 and 4?
Oblique
What happens at the boundary between
Media 3 and 4? Why?
Reflection may occur. If transmission does occur, the sound beam will refract because there are different propagation speeds and oblique incidence.
What processes occur as the ultrasound passes through all media? What are the units of this process?
Attenuation (scattering, absorption, and reflection). Units: dB.