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 hte 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/cm2. There is a change of of +9 dB. What is the final intensity?
16 mW/cm2
Every 3 dB change means that the intensity will _________.
double
Every 10 dB change means that the intensity will __________.
increase 10 times
If the final intensity of a sound beam is more than the initial intensity, then the gain in dB is __________ (+ or -).
positive
If the initial intensity of a sound beam is less than the final intensity, then the gain in dB is __________ (+ or -).
positive
Name the three components of attenuation.
reflection, scattering, and absorption
As the path length increases, the attenuation of the ultrasound in soft tissue __________.
increases
Attenuation in lung tissue is __________ attenuation in soft tissue. {greater than, less than, the same as}
greater than
Attenuation in bone is __________ attenuation in soft tissue. {greater than, less than, the same as}
greater than
Attenuation in air is __________ attenuation in soft tissue. {greater than, less than, the same as}
greater than
What are the units of attenuation?
decibels (dB)
True or False.
In a given medium, attenuation is unrelated to speed of sound.
True.
What is the relationship between ultrasound frequency and the attenuation coefficient in soft tissue?
in soft tissue, the attenuation coefficient (dB/cm) is approx one-half of the ultrasonic frequency (MHz)
What are the units of the half-value layer thickness?
length; centimeters
As frequency decreases, depth of penetration __________.
increases
As path length increases, the half boundry layer __________.
remains the same
Impedance is characteristic of __________.
only the medium
As the path length increases, the attenuation coefficient of ultrasound in soft tissue __________. {decreases, increases, reamins the same}
remains the same
Acoustic impedance = __________ x __________.
impedance = density x propagation speed
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 boundarys.
reflections
Which is better to use while examining a carotid artery, a 7.5 or 3.0 MHz transducer?
7.5 MHz
A sound wave with intensity of 50 W/cm2 strikes a boundary and is totally reflected. What is the intensity reflection coefficient?
100%
since the wave is totally reflected
A sound wave with intensity of 50 W/cm2 strikes a boundary and it totally reflected. What is the reflected intensity?
50 W/cm2
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 (less than 1%) is reflected at a boundary between two soft tissues. very little reflection 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%
Sound is traveling in a medium and orthogonally strikes a boundary with a different medium. Although the media are very different, no reflection is created. How can this be?
the two different media have the same impedance, and when this 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
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?
10% is 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?
99%
A pulse of ultrasound propagates in soft tissue, such as liver. The pulse strikes a soft tissue-soft tissue inerface with oblique incidence. Some of the sound energy is transmitted. To what extent is the transmitted beam refracted?
beam undergoes little to no refraction because the tissues are both soft tissues
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 soudn in Medium 2
-the difference between the speeds of Media 1 and 2?
nothing can be said about the speed of sound in Medium 1 and 2 but the speeds of these two mediums are very differnt because the beam is refracted significantly
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, 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 degrees. The angle of transmission is 10
degrees. In which medium is the impedance higher?
cannot be determined, impedances are unrelated to refraction
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?
it has normal incidence, refraction cannot occur
A sound wve strikes a boundary at normal incidence. The impedances of the two media are identical. What percentage of the sound wave is refracted?
0%, refraction cannot occur with normal incidence
The impedance of Medium 1 is 8 rayls and the propagation speed is 1,450 m/s. The impedance of Medium 2 is 6 rayls and the propagation speed is 1.854 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, angle of reflection equals the angle of incidence
What does the 100 mW/cm2 stand for?
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, waht is an estimate of the ultrasound frequency?
about 7 MHz, attenuation coefficient multiplied by 2 approximates frequency
What property ha units of rayls? How is it determined?
Impedance, it is calculated, not measured.
Impedance = density x speed
The incidence between the sound wave and the boundary between Media 1 and Media 2 is normal. What happens at the boundary between Media 1 and 2? Why?
Reflection and transmission occurs. There is 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 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
