Ch 6 Interaction of Sound & Media

sound waves weaken, or _______, as they travel in the body

attenuate

the sound that comes back to the transducer is converted to an electrical signal, this signal is sent to the US system where it is strengthened, or _________

amplified

weakened =

attenuated

strengthened =

amplified

in dx US, we are interested in the degree of ______ and the extent of ______

attenuation

amplification

if the logarithm increases by 1, the actual number increases by

10

unit used for measuring the signal strength in diagnostic US

decibels

decibels does not measure absolute numbers, but rather reports relative _______

changes

two intensities required for decibels:

1. the reference/starting level

2. the actual level at the time of measurement

the reference/starting level is when the signal ____ the US machine

leaves

the actual level at the time of measurement is when the signal ______ to the US machine

comes back

if asked what the relative measurement of something is, you will use

decibels

when a waves intensity doubles, the relative change is

+ 3 dB

when a waves intensity increases ten-fold, the relative change is

+ 10 dB

when a waves intensity is reduced to 1/2 its original value, the relative change is

-3 dB

when a waves intensity is reduced to 1/10 its original value, the relative change is

- 10 dB

+ 3 dB =

doubled

+ 10 dB =

ten times larger

- 3 dB =

1/2

- 10 dB =

1/10

the decrease in intensity, power, and amplitude as sound travels through a medium

attenuation

attenuation is determined by 2 factors :

- path length

- frequency of sound

distance and attenuation are ______ related

directly

frequency and attenuation are ______ related

directly

if distance increases, attenuation will

increase

if frequency gets higher, attenuation will

increase

- reflection

- scattering

- absorption

3 processes that contribute to attenuation

as sound strikes a boundary and a portion of the waves energy may reflect back to the sound source

reflection

there are ____ forms of reflection in soft tissue

2

- specular

- diffuse

the two forms of reflection

type of reflection that occurs when sound strikes a smooth boundary and the sound is only reflected in one direction, in an organized manner

specular reflection

type of reflection that occurs when a wave hits an irregular surface and sound radiates in more than one direction

diffuse reflection

diffuse reflection, AKA

backscatter

backscattered signals have _______ strength than specular

lower

sound scatters when the tissue interface is ______ or equal to/less than the wavelength

small

______ frequency sound beams scatter much more

higher

scattering is ______ related to frequency

directly

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

Rayleigh scattering

rayleigh scattering redirects the sound wave equally in _____ directions

all

type of scattering seen exclusively with red blood cells

Rayleigh scattering

rayleigh scattering is proportional to the frequency ____

to the 4th power

rayleigh scattering increases ______ with increasing frequency

dramatically

most sizeable component of attenuation

absorption

occurs when US energy is converted into another form of energy like heat

absorption

absorption is _____ related to frequency

directly

the number of decibels of attenuation that occurs when the sound travels one cm

attenuation coefficient

the value of attenuation coefficient remains _______ regardless of how far the sound travels

constant

total attenuation (dB) =

attenuation coefficient (dB/cm) x distance (cm)

if sound travels to a depth of 5 cm and the coefficient is 2 dB/cm, what is the total attenuation?

10 dB

in soft tissue, the attenuation coefficient and the frequency are _______ related

directly

attenuation coefficient is _____ the frequency

one half

attenuation coefficient (dB/cm) =

frequency (MHz)/2

bone absorbs US energy to a ______ extent

large

______ attenuates dramatically due to scattering and absorption

lung

main mechanism of attenuation in air is

absorption

sound with frequencies above 1 MHz attenuate _______ in air

entirely

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

half-value layer thickness

units for half-value layer thickness

cm, or unit of length

typical values of half-value layer thickness

0.25 - 1 cm

- penetration depth

- depth of penetration

- half-boundary layer

AKAs for half-value layer thickness

half-value layer thickness depends on 2 factors :

the medium

the frequency of sound

half-value layer thickness is ______ proportional to frequency

inversely

a thin HVLT will have a ______ frequency of sound

high

as attenuation increases, propagation speed will

remain unchanged

the ________ produced as sound moves from one medium to another forms the basis for US imaging

reflection

________ is critical to US's ability to image structures located deep in the body

transmission

the acoustic resistance to sound traveling in a medium

acoustic impedance

acoustic impedance is calculated by multiplying the _______ of a medium by the _____ at which sound travels in the medium

density

speed of sound

impedance (Z) =

density x propagation speed (m/s)

units for acoustic impedance

Rayls (Z)

typical values for acoustic impedance

1.25-1.75 Mrayls

acoustic impedance is determined by the _______ only

medium

acoustic impedance, AKA

characteristic impedance

the angle at which sound wave strikes a tissue boundary determines the _______ of the pulse

behavior

angle less than 90 degrees

acute angle

angle exactly 90 degrees

right angle

angle greater than 90 degrees

obtuse angle

when the incident sound beam strikes the boundary at exactly 90 degrees

normal incidence

- perpendicular

- orthogonal

- right angle

- 90 degrees

AKAs for normal incidence

(PORN)

occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees

oblique incidence

oblique incidence, AKA

non-perpendicular

the sound waves intensity immediately before it strikes a boundary

incident intensity

the intensity of the portion of incident sound beam that returns to the machine after striking a boundary

reflected intensity

the intensity of the portion of the incident sound beam that continues forward after striking a boundary

transmitted sound beam

incident intensity =

reflected intensity + transmitted intensity

the percentage of the intensity that bounces back when a sound beam strikes the boundary between two media

intensity reflection coefficient

in clinical imaging, _____ of intensity is reflected back

1% or less

a ________ percentage is reflected when sound strikes a boundary between soft tissue and bone or soft tissue and air

greater

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

intensity transmission coefficient

in clinical imaging, ______ is transmitted between tissues

99%+

a _______ percentage of the wave is transmitted when sound strikes a boundary between bone and soft tissue

smaller

IRC = ITC =

100

when a sound beam strikes a tissue boundary at a 90 degree angle, ______ will occur only if the media in either side of the boundary have different impedances

reflection

two mediums with identical impedances will have ____ reflection

0

two media with slightly different impedances will have _____ amounts of reflection

small

two media with substantially different impedances will have ____ amounts of reflection

large

IRC =

{Z2 - Z1 / Z2 + Z1}^2 x 100

if two media have the same impedance, ___ of the sound is transmitted at the boundary

all

ITC (%) =

(transmitted intensity/ incident intensity) x 100

or

100 - intensity reflection coefficient

we are unable to predict whether sound will reflect or transmit with _______ incidence

oblique