Chapter 6 Q & A Flashcards by William Kerns

decibels

every 3 db change means that the intensity will

double

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every 10 dB change means the intensity will

increase 10 times

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a reduction in the intensity of a sound beam to one-half of its original value is____dB

-3dB

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a reduction in the intensity of a sound beam to one-quarter of it original value is ________db

-6 dB

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-10dB means that the intensity is reduced to _______of its orgiginal value

one tenth

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dB is a mthmatical representation with a _______scale

logarithmic and relative

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we need one intensity to calculate decibels?

T or F

False

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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

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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

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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

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name three components of attentuation

absorption

reflection

scattering

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as the path penght increases the attenuation of ultrasound in soft tissue_________

increases

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attenuation in lung tissue is (less than, greater than or same as) attentuatoin in soft tissue

greater than

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attentuation in bone is ________attenuation in sfot tissue

greater than

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attenuation in air is __________attenuation in soft tissue

greater than

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what are the units of attenuation

decibels

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in a given medium attenuation is unrelated to the speed of sound?

T or F

True

attenuation and propagation speed are unrelated

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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

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what are the units of the half value layer thickness

distance

centimeters

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as freq decreases, depth of penetration _________

increases

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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