physics ch 6 Ex 3

sound waves weaken or ______ as they travel in the body

attenuate

the signal is sent to the ultrasound system where it is strengthened or _____

amplified

decibels are based on a mathematical construct called ______

logarithms

the _____ or logarithm of a number represents that number of 10s that are multiplied to create the original number

log

decibel notation does not measure ______ rather decibels report _____

absolute numbers

relative changes

decibels require _____ intensities

two

______ decibels report signals that are increasing in strength

positive

_______ decibels describe signals that are decreasing in strength

negative

the decrease in intensity power and amplitude as sound travels in called

attenuation

attenuation is determined by two factors

path length

frequency of sound

the farther the sound travels the _______ the attenuation

stronger

frequency and attenuation are _____ related

directly

3 processes contribute to attenuation

reflection

scattering

absorption

what 2 forms of reflection are created in soft tissues

specular

diffuse

when the boundary is smooth the sound is reflected in only one direciton in an organized manner this is called ______ reflection

light wave strikes a

specular

mirror

when a wave reflects off an irregular surface it radiates in more than one direction this is called _____ reflection or ________

diffuse

backscatter

______ of ultrasound is the random redirection of sound in many directions

sound scatters when tissue interface is ______ , that is ____ or ____ than the wavelength of the incident sound beam

scattering

small

equal to

less than

__________ is a special form of scattering that occurs when the structures dimensions are much smaller than the beams wavelength

rayleighs scattering

_________ is the number of decibels of attenuation that occurs when sound travels one centimeter

attenuation coefficient

the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value is the

half-value layer thickness

3 types of angles

acute

right

obtuse

angles with a measure other than 90 are also called

oblique

________ means that the incident sound beam strikes the boundary at exactly 90 degrees

normal incidence

__________ is the sound waves intensity immediately before it strikes a boundary

incident intensity

_________ is the intensity of the portion of the incident sound beam that, after striking a boundary returns back from where it came

reflected intensity

_________ is the intensity of the portion of the incident beam that after striking a boundary continues forward in the same general direction that it was traveling

transmitted intensity

equation for incident intensity

incident (starting intensity) = reflected intensity + transmitted intensity

the _____________ is the percentage of the intensity that bounces back when a sound beam strikes the boundary between 2 media

intensity reflection coefficient

the ___________is the percentage of intensity that passes in the forward direction when the beam strikes an interface between 2 media

intensity transmission coefficient

the coefficients IRC and ITC are both reported as ________ and are dimensionless

percentages

when a sound beam strikes a tissue boundary at 90 degrees normal incidence reflection only occurs if the media on either side of the boundary have _________

different impedances

2 physical principles always apply to reflection with oblique incidence

conservation of energy

reflection angle= incident angle

the sound beam might bend or change direction. the bending phenomenon is called _________

refraction

refraction occurs is two conditions are satisfied

oblique incidence (not normal)

different propagation speeds of the two media

____________ quantifies the physics of refraction

snells law

reflection with normal incidence =

different impedances required

reflection and/or transmission with oblique incidence =

cant predict too complex

refraction =

oblique incidence and different speeds required

transmission =

derived from reflection information use of law of conservation of energy

the initial sound beams intensity is 10 w/cm² the final intensity of sound beam is 100 w/cm² what is the change in intensities in decibels?

+10 db

the initial sound beams intensity is 100 w/cm² the final intensity of sound beam is 50 w/cm² what is the change in intensities in decibels?

-3 db

the initial sound beams intensity is 48 w/cm² the final intensity of sound beam is 12 w/cm² what is the change in intensities in decibels?

-6 db

the initial sound beams intensity is 6 w/cm² the final intensity of sound beam is 24 w/cm² what is the change in intensities in decibels?

+6 db

the initial sound beams intensity is 10 w/cm² the final intensity of sound beam is 200 w/cm² what is the change in intensities in decibels?

13 db

what is the attenuation coefficient of a 12 MHz sound wave in soft tissue?

6 db/cm

what is the attenuation coefficient of a 5 MHz sound wave in soft tissue?

2.5 db/cm

you have an attenuation coefficient of 12 db/cm what is the frequency of your sound wave?

24 MHz

decibel notation is ________

logarithmic

attenuation is determined by 2 factors

path length

frequency of sound

frequency and attenuation are _____ related

directly

distance and attenuation are ______ related

directly

attenuation is measured in ____ and reported as a _________

decibels

relative change

more attenuation

longer distances

higher frequencies

less attenuation

shorter distances

lower frequencies

3 db

double

10 db

10x larger

-3 dB

½

-10 dB

1/10

increased by a factor of 100, decibel notation

20 dB

decreased 1/100 of its initial intensity , decibel notation

-20 dB

6 dB change

4x bigger

20 dB change

100x bigger

another way to say -10dB

10dB of attenuation

as sound strikes a boundary a portion of the wave’s energy may be redirected or ______ back to the sound source

reflected

reflection is likely to occur when the

dimension of the boundary is large , more than a few wavelengths of sound

limitation of specular reflectors

once wave is slightly off axis reflection does not return to transducer - mirror certain angles cant see reflection anymore

advantage of diffuse reflection

interfaces at suboptimal angles to sound beam can still produce reflections that will return to transducer

disadvantage of diffuse reflection

backscattered signals have a lower strength than specular reflections

higher frequency sound beams ______ than lower frequency beams

scatter much more

scattering is _____ related to frequency

directly

rayleigh scattering increases with

frequency

rayleigh scattering formula

rayleigh scattering = frequency ^ 4

frequency doubles rayleigh

16x greater

sound goes back to transducer organized =

specular

sound goes back to transducer disorganized =

diffuse of backscatter

sound in all directions organized =

rayleigh scattering

sound in all directions disorganized =

scattering

absorption is ____ related to frequency

directly

attentuation coefficient is the

number of decibels of attenuation that occurs when sound travels 1 cm

attenuation coefficient units

dB/cm

attentuation coefficient formula

total attentuation = attenuation coefficient * distance

attenuation coefficient in soft tissue formula

attenuation coefficient = frequency/2

water attenuation

extremely low

blood urin biologic fluids attenuation

low

fat attenuation

low

soft tissue attenuation

intermediate

muscle attenuation

higher

bone and lung attenuation

even higher

air attenuation

extremely high

half-value layer thickness units

cm

other names for half value layer thickness

penetration depth

depth of penetration

half-boundary layer

half-value layer thickness depends on 2 factors

medium

frequency of sound

thin half value

high frequency sound

media with high attenuation rate

thick half value

low frequency sound

media with low attenuation rate

impedance

acoustic resistance to sound traveling in a medium

reflection of an ultrasound wave depends on

the difference in acoustic impedance of 2 media at a boundary

impedance is

calculated, only medium

impedance formula

impedance (rayls) = density * propagation speed

impedance also known as

characteristic impedance

acute

less than 90