Chapter 6 Q & A Flashcards
decibels
every 3 db change means that the intensity will
double
every 10 dB change means the intensity will
increase 10 times
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 it original value is ________db
-6 dB
-10dB means that the intensity is reduced to _______of its orgiginal value
one tenth
dB is a mthmatical representation with a _______scale
logarithmic and relative
we need one intensity to calculate decibels?
T or F
False
a wave’s intensity is 2mW/cm^2.
There is a change of +9db.
What is the final intensity?
16 mW/cm^2
+9 = 3db and 3db and 3db
so it is 2x2x2 or 8 times greater
if the final intensity of a sound beam is more than the initial intensity then the gain in dB is (+ or -)
positive
beam intensity is increasing
if the iniitial intensity of a sound beam is less than the final intensity then teh gain in dB is (+ or -)
positive
beam intensity is increasing
name three components of attentuation
absorption
reflection
scattering
as the path penght increases the attenuation of ultrasound in soft tissue_________
increases
attenuation in lung tissue is (less than, greater than or same as) attentuatoin in soft tissue
greater than
attentuation in bone is ________attenuation in sfot tissue
greater than
attenuation in air is __________attenuation in soft tissue
greater than
what are the units of attenuation
decibels
dB
in a given medium attenuation is unrelated to the speed of sound?
T or F
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 approx one half of the ultrasonic frequency in MHz
what are the units of the half value layer thickness
distance
centimeters
as freq decreases, depth of penetration _________
increases
as path lenght increases the half boundry 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___________
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?
mediums A has the higher acoustic impedance
impedance = speed x density so if one increase so does the impedance
impedance, speed and density are all directly related
impedance is important in _______at boundries
reflections
which is better to use while examing a carotid artery, a 7.5 or 3.0 MHz transduce?
7.5 Mhz
carotid is superficial and therefore does not need the deeper penetration of the lower freq
a sound wave with an intensity of 50 W/cm^2 strikes a boundry and is totally reflected.
What is the intensity reflection coefficient?
100%
the whole wave was reflected
a sound wave with an intensity of 50 W/cm^2 strikes a boundry and is totally reflected.
What is the reflected intensity?
50 w/cm^2
wave totally reflected
a pulse of ultrasound is propagating in soft tissue such as the liver. The pulse strikes a boundry with a different soft tissue at normal incidence.
What portion of the intensity is reflected back toward the transducer and why?
very small percentage of sound is reflected at a boundry of two soft tissues. IMpedances are so similar
Sound is traveling in a medium and strikes a boundry with normal incidence. If 63% of the wave’s intensity is reflected back toward the transducer what percentage is transmitted?
37%
conservation of energy must account for 100% of initial intensity
a pulse of ultrasound is propagating in bone and strikes an interface with soft tissue at 90°. A giant reflection is caused
from these facts 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 of impedance between bone and soft tissue
nothing can be stated about the impedance based on the information given
however, because a large refelction was creasted the impedances of these two media must be different.
Reflections with normal incidence are created based upon the difference in impedance, not actual values
sound strikes a boundry 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 impedance of the two media are different. tow different media can have the same impedances
which of the following terms does not belong with the others
orthogonally
oblique
normal
perpendicular
oblique
means other than 90°
the rest all mean = to 90°
sound is traveling from bone to soft tissue.
The impedances of the media differ significantl;y and 90% of the beams intensity is relfected.
What percentage of the intensity is tranmitted?
10% transmission
conservation of energy 100% must be accounted for
sound traveling in Jell-O passes through an interface at 90° and continues to travel in whipped cream. The impedance of Jell-O and whipped cream are nearly identical. What percentage of intensity is transmitted?
99%
best choice since nearly all would be transmitted since the impedances are so close
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?
little to no refraction occurs.
transmitted beam is refracted when the incidence is oblique and the propagatoins speeds are different. because soft tissue on either side of boundry are soft tissues, their speeds are nearly identical and little or no refraction ocurs
a sound pulse travels in Medium 1 and strikes an interface wtih another tissue, medium 2 at 30°. The angle of transmission is 10°. From these facts alone what can be said about
speed of sound in Medium 1
speed of sound in Medium 2
difference between the speeds of media 1 and 2
we can say nothing about the speed of sound in media 1 and 2.
however since there is a difference in the angle of transmission the speeds are different in the media
refractoin depends on the difference of speed, not the actual speed
sound pulse travfels in medium 1 and strikes an interface with medium 2 at 30°. the angle of transmission is 10°. In which medium dows the sound travel slowest
slowest in medium 2
when the angle of transmission is less that the angle of incidence the sound is slower in the 2nd medium
sound pulse travels in medium 1 and strikes an interface wtih another tissue medium 2 at a 30°. The angle of transmission is 10°.
In which medium is the impedance higher?
refraction of sound at a boundry is unrelated to the impedance of the media therefore with the info given it cannot be determined
refractoin is only affected by the speed of the sound in the media
sound travels in a medium and orthogonally strikes a boundry 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 refractoin cannot occur
refraction occurs only when there are different speeds and oblique incidence
both conditions must be met
a sound wave strikes a boundry with normal incidence. The impedances of the two media are identical.
What percentage of the sound wave is refracted?
refractoin cannot occur with normal incidence
the impedance of medium 1 is 8 rayls. The propagation speed is 1450m/s. The impedance of medium 2 is 6 rayls and the speed is 1855m/s. A sound beam strikes the boundry between the media and is both partially transmitted and reflected. The angle of incident beam is 30°.
What is the reflectoin angle
30° the angle of reflection alwasy equals the angle of incidence
What does the 100mW/cm^2 represent
incident intensity of the sound beam
what does the 3.5 dB/cm represent
attenuation coefficient of the sound in the media
if the media are sfot tissue, what is an estimate of the ultrasound frequencey?
about 7 MHz
attenuation coefficient multiplied by 2 approximates the frequency
3.5x2=7
what property hsa units of rayls?
how is it determined?
impedance
calculated not measured
impdedance= density x speed
the incidence betweenthe sound wave and the boundry between media 1 and 2 is normal.
What happens at the boundry between 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 boundry between media 2 and 3 is normal.
What happens at the boundry between media 2 and 3?
Why?
transmission only
impedances of the media are the same
what type of incidence is there between media 3 and 4?
oblique
what happens at the boundry between media 3 and 4?
Why
reflection may occur
if transmission does occur the sound beam will refract because there are different propagatoins speeds and oblique incidence
what processes occur as the ultrasound passes all media?
what are the units of this process?
attenuation occurs
dB
decibels
