Chapter 6: Interaction of Sound and Media

What is the basis of creating images for ultrasound

the weak reflections created when a sound beam strikes a boundary between 2 different medias

What happens to sound waves as they travel through the body

they weaken, or attenuate

What is a logarithm

a method of rating numbers.

log of a number represents the number of 10s that are multiplied to create the original number (ex: 100=10×10, therefore the 100 is 2 (2 10s multiplied))

When the logarithmic scale increases by 1, how does the actual number increase

it increases tenfold (a logarithmic increase of 2 means the signal is 100 times larger)

What is log₁₀1000

3 (10×10×10)

What is an easy way to determine logarithm of an even power of ten

count the zeros (ex: 1,000,000 = 6)

What is decibels (dB)

the quantity of the strength of sound beams as they are created/travel through the body, and the electrical signal strength and brightness of an image

What does it mean that decibel notation is logarithmic

decibel notation does not measure absolute numbers, rather, they report relative changes (ex: strength has doubled/halved)

What 2 intensities does decibels require

the reference (starting level) and the level at the time of measurement

How are decibels measured/reported

as a ratio. measured level is divided by starting level

What do positive decibels report

signals that are increasing in strength, or getting larger

What intensity change corresponds to +3 dB

the intensity doubles

What intensity change corresponds to +10 dB

the intensity increases tenfold

What do negative decibels report

signals that are decreasing in strength, or getting smaller

What intensity change corresponds to -3 dB

the intensity is reduced to half

What intensity change corresponds to -10 dB

it is reduced to 1/10

What is attenuation

sound waves weakening as they propagate in a medium (decreases in intensity, power, and amplitude)

What is attenuation determined by and their relationship

path length: distance and attenuation are directly related

frequency of sound: frequency and attenuation are directly related

What creates more attenuation

longer distances the beam needs to travel and higher frequency

What creates less attenuation

shorter distances the beam needs to travel and lower frequency

How is attenuation measured/reported

measured in decibels and reported as relative change, not an absolute change (ex: wave propagates and intensity is reduced to 1/10 of original level, we say it attenuated 10dB because attenuate means decrease, which is why the number value isn’t negative)

What 3 processes contribute to attenuation

reflection, scattering, and absorption

What is reflection

A portion of the waves energy that is redirected, or reflected back to the sound source as sound strikes a boundary

What occurs in response to reflection

The portion of the waves energy that that continues to travel forward (transmission) is weakened

What is specular reflection

When a boundary is smooth, sound is reflected in only one direction in an organized manner

What is a limitation to specular reflection

If the wave is slightly off axis, the reflection does not return to the transducer

What is diffuse reflection or back-scatter

When a wave reflects off an irregular surface, it radiates off more than one direction (most interfaces in the body are not smooth/have some irregularities)

Advantage and disadvantage of diffuse reflection/backscatter

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

D: backscattered signals have lower strength than specular reflections

What is scattering and when does it happen

It is random redirection of sound in many directions. It happens when tissue interface is small (less than or equal to the wavelength of the sound beam)

Scattering and frequency relationship

They are directly related (high frequency sound beams scatter much more that lower frequencies)

What is Rayleigh scattering

Scattering that occurs when the structures dimensions are much smaller than the beams wavelength. It redirects the sound wave equally in all directions

Rayleigh scattering and frequency relationship

Rayleigh scattering ∝ frequency^4

What is absorption

When ultrasonic energy is converted into another energy form, such as heat

Absorption and frequency relationship

absorption is directly related to frequency

What 2 things can occur because of absorption

• higher frequency waves attenuate more than lower frequency

• Sound traveling in bone undergo extensive absorption

What is attenuation coefficient and it’s units

The number of decibels of attenuation that occur when sound travels one centimeter. Units are dB/cm

What is the advantage of describing sound weakening with attenuation coefficient, and how to calculate total attenuation

The value will always remain the same no matter how far the sound travels (if a tissue is 2dB/cm, you just multiply 2 to the depth).

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

Attenuation (and attenuation coefficient) and frequency relationship in soft tissue

They are directly related.

Attenuation coefficient is ½ the frequency (dB/cm = frequency (MHz)/2)

This makes sense because as frequency increases, scattering/absorption increases

What is the attenuation rate of air and why

it is high because of absorption

When does sound attenuate entirely

when there is a frequency above 1MHz

What is the attenuation rate of lungs and why

it is high because of scattering and absorption

What is the attenuation of bone and why

bone absorbs ultrasound energy to a large extent, resulting in substantial attenuation

What is attenuation like with biologic fluids (blood, urine, amniotic fluid)

they all attenuate sound but far less than soft tissue

What is attenuation like with frequencies <10MHz in water

there is no noticeable attenuation in this situation

What is attenuation like in muscle and the fibers

sound attenuates twice as much when traveling across fibers as when traveling along the length of the fibers

What is half-value layer thickness (aka penetration depth, depth of penetration, or half-boundary layers) and it’s units

The distance sound travels in a tissue that reduces the intensity of sound to ½ its original value. May also be described as depth of tissue that results in 3dB of attenuation to the intensity.

Units are centimeters or any units of length

Half-value layer thickness typical clinical values

Range from 0.25-1cm

what does half-value layer thickness depend on and what creates thin/thick layers

The medium and frequency of sound.

Thin/low half-value layer: tissue that attenuates sound to a great deal (lung/bone) and higher frequency

Thick/high half-value layer: tissue that attenuates sound just a little (fluids) and lower frequency

What forms the basis for the ultrasonic imaging

The reflection produced as sound moves from one medium to another

What is Transmission and what is it critical for

The process of sound energy traveling through a medium rather than being reflected/absorbed. It is critical to ultrasound ability to image structures deep in the body

What is impedance (acoustic impedance or characteristic impedance) and it’s units

The acoustic resistance to sound traveling in a medium.

Reported in units of rayls or Z

How is impedance calculated (not measured)

multiply the density of a medium by the speed that the sound travels

impedance (rayls) = density (kg/m³) x propagation speed (m/s)

What does reflection on ultrasound depend on

the difference in acoustic impedance of 2 medias at a boundary

What is the typical biological media values for impedance and what is it determined by

it ranges from 1,250,000-1,750,000rayls (1.25-1.75Mrayls).

it is determined by/associated with medium only

What is incidence

the angle at which the wave strikes the boundary

What are the 3 types/categories of incidence

acute: <90 degrees

right: 90 degrees

obtuse: > 90 degrees

oblique angles are anything other than 90 degrees

What is normal incidence (aka perpendicular, orthogonal, or right angle)

when the incidence of a sound beam strikes a boundary at exactly 90 degrees

What is oblique incidence (aka not perpendicular/right angle)

incident sound beam strikes boundary at any angle besides 90 degrees

What is incident intensity

sound wave intensity immediately before it strikes a boundary

What is reflected intensity

intensity of the portion of the incident sound beam that is reflected back after striking a boundary

What is transmitted intensity

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

What is intensity reflection coefficient (IRC)

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

IRC Clinical imaging value

very little (1% or less) sound wave intensity os reflected at a boundary between 2 soft tissues.

a greater % is reflected when sound strikes a boundary such as bone/air with soft tissue.

What is intensity transmission coefficient (ITC)

the percentage of intensity that passes forward after striking a boundary

ITC Clinical imaging value

most (99% or more) of a sound waves intensity is transmitted at a boundary between 2 soft tissues.

a smaller % is transmitted at a boundary between bone and soft tissue

What needs to be present for a reflection to occur at a 90 degree angle

the media on the other side needs to be a different impedance

What affects does impedance have on reflection

no reflection: 2 medias have identical impedance

small reflection: impedances are slightly different

large reflection: impedance are substantially different

Calculating IRC with normal incidence

this equation

When is all sound transmitted

with normal incidence and 2 media with the same impedance

calculating ITC

ITC = (transmitted intensity / incident intensity) x 100

ITC = 1 - IRC

Reflection and transmission with oblique incidence

The physics of this concept is so complex that it cannot be predicted. there could be reflections when impedances of 2 media are identical or reflection may be absent when impedance are different

2 physical principles that always apply to reflection with oblique incidence

conservation of energy

reflection angle = incidence angle

What is conservation of energy in regards to oblique incidence

100% = reflection coefficient + transmission coefficient

incident intensity = reflected intensity + transmitted energy

how is reflection angle and incident angle related with oblique incidence

direction of reflected echo is equal and opposite to direction of incident beam

What is refraction and what is it associated with

A change in direction of wave propagation when traveling from one medium to another.

it is associated with transmission

What 2 conditions need to be satisfied for refraction to occur

oblique incidence and different propagation speeds of the 2 media.

if these conditions are met, sound will never travel in a straight line, it will bend

Clinical imaging conditions of refraction

sound will bend at most a few degrees at soft tissue/fat interface, muscle/blood interface, or soft tissue/fluid interface because speed of sound in the medias are similar.

bending is exaggerated at bone/soft tissue interface because speed of sound in bone is about 3x more that in soft tissue

What is Snells Law

An equation that quantifies the physics of refraction.

medium 1 is the medium that sound is currently traveling and medium 2 is the medium that sound is entering

Under what conditions will transmission angle equal incident angle

when speed of both medias are equal.

no refraction will occur and the beam travels straight ahead

Under what conditions will transmission angle be greater than incident angle

When speed of medium 2 is greater than speed of medium 1

Under what conditions will transmission angle be less than than incident angle

When speed of medium 2 is less than speed of medium 1