Chapter 6 Flashcards
Every 3 dB change means that the intensity will _________.
Double
Every 10 dB change means that the intensity will _________.
Increase 10 times
A reduction in the intensity of a sound beam to one-half of its original value is _____ dB.
-3 dB
A reduction in the intensity of a sound beam to one-quarter of its original value is _____ dB.
-6 dB
-10 dB means that the intensity is reduced to _______ of its original value.
One-tenth
dB is a mathematical representation with a ______ scale.
Logarithmic and relative
True or false. We need one intensity to calculate decibels.
False
A wave’s intensity is 2 mW/cm squared. There is a change of +9 dB. What is the final intensity?
16mW/cm squared
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
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
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 increase, 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/m3)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
Impedance is important in ________ at boundaries.
Reflection
Which is better to use while examining a carotid artery, a 7.5 or 3.0 MHz transducer?
7.5 MHz transducer because carotid artery is a superficial structure
A sound wave with an intensity of 50 W/cm squared strikes a boundary and is totally reflected. What is the intensity reflection coefficient?
100%
Since the wave is totally reflected, the intensity reflection coefficient is 100%
A sound wave with an intensity of 50 W/cm squared strikes a boundary and is totally reflected. What is the reflected intensity?
50 w/cm squared.
Since the wave is totally reflected.
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. Very little reflection occurs when the impedance 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%
100%. 63% + 37% = 100%
A pulse of ultrasound is propagating in bone and strikes an interface with soft tissue at 90 degrees. A giant reflection is created. From these facts alone, what can be said about the impedance? What can be said about the impedance of soft tissue? What can be said about the differences between the impedance of bone and soft tissue?
Nothing can be stated about the impedance of bone or soft tissue based on the given information. However, because a large reflection was created, the impedance of these two media must be dissimilar. Reflections with normal incidence are created based on the difference in the impedances, and when that happens, no reflection will be created.
Sound strikes a boundary between two media orthogonally. Although the media are very different, no reflection is created. How can this be?
Two different media can have the same impedance, 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
Oblique
Sound is traveling from bone to soft tissue. The impedance of the media differs 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?
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 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. From these facts alone, what can be said about the speed of sound in Medium 1? The speed of sound in medium 2? And 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 and Medium 2. However, because the beam refracted significantly, the speeds of these two media are very different.
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?
Medium 2
A sound pulse travels in Medium 1 and strikes an interference with another tissue, Medium 2, at 30 degrees. The angle of transmission is 10 degrees. In which medium is the impedance higher?
Cannot be determined
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.
A sound wave strikes a boundary with normal incidence. The impedance of the two media are identical. What percentage of the sound wave is refracted?
0%
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. The angle of reflection equals the angle of incidence.
What does the 100 mW/cm squared 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
What property has units of rayls? How is it determined?
Impedance is calculated, not measured
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 impedance.
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 impedance of the media is 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 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 (dB)
