Ultrasound Physics Ch 6 (interaction of Sound and Media)

What happens to sound as it travels through the body?

It weakens, or attenuates

The sound wave a transducer receives during reception is converted into what type of signal?

electrical

What happens to the electrical signal?

It is sent to the ultrasound system where it is strengthened or amplified.

What does the following describe?

. a relative measurement / change

. a comparison

. a ratio

. logarithmic

Decibel Notation

what are the 2 intensities decibels (dB) require?

. the reference (starting level)

. the actual level at the time of measurement

Decibels are a ratio; the.....

measured level divided by the starting level

positive decibels report signals as they are.......

increasing in strength or getting larger

ex. stronger, louder sound

+ 3dB is

orig value doubled

ex. 1x2= 2

+ 6dB is

orig value doubled 2x

ex. 1x2=2 x 2 = 4

+ 9dB is

orig value doubled 3x

ex. 1x2= 2x2= 4x2= 8

+ 10dB is

orig value 10x larger

increases ten-fold

ex. 1 x 10= 10 or 4x10=40

negative decibels (dB) describe signals that are......

decreasing in strength or getting smaller

. weaker, softer

- 3dB is

orig value halved

ex. 1x 1/2 = 1/2

- 6dB is

orig value halved 2x

ex. 1x 1/2= 1/2x 1/2= 1/4

- 9dB is

orig value halved 3x

ex. 1x1/2= 1/2x1/2= 1/4x1/2= 1/8

- 10dB is

orig value 1/10x

one-tenth

ex. 1 x 1/10 = 1/10

What are the 2 factors attenuation is determined by?

. path length

. frequency of sound

Distance and attenuation are

directly related

attenuation is ______ in higher frequency sound than lower frequency sound

greater

How are frequency and attenuation related?

they are directly related

What do the following 2 factors describe?

. longer distances

. higher frequencies

more attenuation

What do the following 2 factors describe?

. shorter distances

. lower frequencies

What 3 processes contribute to attenuation?

-Refection

-Scattering

-Absorption

What is reflection?

the redirection of sound from where it came from, back to the sound source

Reflection is likely to occur when the dimension of the boundary is ______.

large, that is, more than a few wavelength of sound

What 2 forms of reflection are created in soft tissue?

specular and diffuse

What is specular reflection?

When a wave is reflected in a single direction by a smooth surface

E.g. when light is reflected by a mirror, you get a nice, clear reflection

What happens once the wave is slightly off-axis in specular reflection?

the reflection doesn't return to the transducer.

What is diffuse reflection (backscatter)?

Reflection in irregular/ more than one directions from an irregular surface

What is an advantage of diffuse reflection?

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

What is a disadvantage of diffuse reflections?

Backscattered signals have a lower strength than specular reflections

What is scattering of ultrasound?

the random redirection of sound in many directions

. disorganized

Sound scatters when tissue interface is

small; that is, equal to or less than the wavelength of the incident sound beam

Lungs are

a great scatterer, alveoli are filled with air

Which type of frequency sound beams scatter more?

higher frequency sound beams more than lower frq.

What is Rayleigh scattering?

Special form of scattering that occurs when the structure's dimensions are much smaller than the beam's wavelength

. organized

What does Rayleigh scattering do?

it redirects the sound wave equally in all directions

. omnidirectional (in all directions)

Sound scattered by red blood cells results in

rayleigh scattering

Scattering is related to

Frequency raised to the fourth power

Higher frequency sound waves produce shorter pulses resulting in

more accurate images

What is absorption?

.when ultrasonic energy is converted into another energy form, such as heat

.80% + of sound energy is absorbed and transformed into heat

As a result of absorption, higher frequency waves attenuate ______ lower frequency waves.

more

Bone is a great absorber, undergoes extensive absorption. the sound is absorbed resulting in......

little sound transmitted back to the transducer (Tx) to give an image, shadowing

What is the attenuation coefficient?

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

What are the units of attenuation coefficient?

dB/cm, decibels per centimeter

What does total attenuation (dB) equal?

atten. coefficient (dB/cm) x distance the sound wave traveled (cm)

For waves propagating in soft tissue attenuation coefficient is equal to ________.

frequency(MHz) divided by 2

what is an example of attenuation coef. equation?

ex. Frequency is 2MHz > atten. coef. is 2MHz/2> 1dB/cm

ex. Freq. is 6MHz > atten. coef. is 6MHz/2 > 3dB/cm

What is the order of attenuation in a medium from low to high?

. water = extremely low

. blood, urine, biologic fluids = low

. fat = low

. soft tissue = intermediate

. muscle = higher

. bone and lung = even higher

. air = extremely high

What is half value layer thickness?

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

. may also be described as the depth of tissue that results in 3dB of attenuation to the intensity

What are the units of half-value layer thickness?

centimeters (cm) or any other unit of length

What is the typical half-value layer range ?

From 0.25 to 1.0 cm

What are other names for half-value layer thickness?

- penetration depth

- depth of penetration

- half-boundary layer

Half-value layer thickness depends on what 2 factors

. the medium

. the frequency of sound

The following 2 factors are characteristics of what?

. high frequency sound

. media with high attenuation rate

Thin half value

The following 2 factors are characteristics of what?

. low frequency sound

. media with low attenuation rate

Thick half value

What is acoustic impedance?

is the acoustic resistance to sound traveling in a medium.

Reflection of an ultrasound wave depends upon

The difference in acoustic impedances of the two media at a boundary

Acoustic impedance is reported in units of.....?

What symbol/letter is impedance represented by?

. reported in units of rayls

. represented by "Z"

What is the typical values impedances ranges from?

. from 1,250,00 to 1,750,000 rayls or

. 1.25 to 1.75 Mrayls

Impedance is associated with/ determined by ....

.the medium only,

. it is calculated not measured

Acoustic impedance is also known as ......

Characteristic impedance

How is acoustic impedance calculated?

density of medium x speed

. m/sec over rayl

what are incidences?

the description of the angle at which the wave strikes the boundary (determines the behavior of the pulse)

What are the 3 types of angles?

Acute- less than 90 degrees

Right - exactly 90 degrees

Obtuse - greater than 90 degrees

Angles with a measure other than 90 degrees are also called ______ angles.

oblique

What is normal incidence?

the incident sound beam strikes the boundary at exactly 90 degrees

What is the typical value for incidence to be normal?

= to 90 degrees

What are the synonyms for normal incidence

P= perpendicular incidence

O= orthogonal incidence

R= right angle incidence

N= ninety degree incidence, 90 degrees

What is Oblique incidence?

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

What is the typical value for incidence to be oblique?

the angle must not be equal to 90 degrees

What are synonyms for oblique incidence

- Non-perpendicular

- Non-orthogonal

- Not at right angles

What is incident intensity?

is the sound wave's intensity immediately BEFORE it strikes a boundary

What is reflected intensity?

is the intensity of the portion of the incident sound beam that, AFTER striking a boundary, RETURNS BACK from where it came.

What is transmitted intensity?

is the intensity of the portion of the incident beam that, AFTER striking a boundary, continues FORWARD in the same general direction it was traveling.

Incident intensity equation is

reflected intensity + transmitted intensity

What is the Intensity Reflection Coefficient (IRC)?

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

How much of a sound waves intensity is reflected at a boundary between two soft tissues?

very little, (1% or less)

What is the Intensity Transmission Coefficient (ITC)?

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

How much of a sound wave's intensity is transmitted at a boundary between two soft tissues?

most, (99% or more)

IRC and ITC are both reported as

percentages, therefore dimensionless.

As a sound beam strikes a boundary, energy is conserved and 100% of the intensity must be accounted for?

What equation does 100% equal?

100% = IRC% + ITC%

With normal incidence, reflection only occurs if the media on the other side of the boundary have .....

different impedences

The greater the difference in impedances the .....

.greater the reflection will be

. the lesser the transmission will be

IRC% =

impedance 2 - impedance 1 / impedance 2 + impedance 1 > result x 100

With normal incidence, if the two media have the same impedance, _____ incident intensity is transmitted.

all of sound

The value of ITC ranges from ____ to ____.

and is defined by the equations

0% to 100%

. ITC% = transmitted intensity/ incident intensity > x100

. 1 - intensity reflection coefficient

What 2 principles always apply to reflection with oblique incidence?

- conservation of energy

- reflection angle = incident angle

With oblique incidence, the law of conservation applies, so 100% =

100% = reflection coefficient + transmission coefficient

the direction of the reflected echo is ___ and ___ of the direction of the incident beam with oblique incidence

equal and opposite to

. angle of incidence = angle of reflection

the angle between the incident sound beam and an imaginary line that is perpendicular to the boundary is called...

the angle of incidence

similarly the angle between the reflected sound beam and the line perpendicular to the boundary is called .....

angle of reflection

Regarding transmission with oblique incidence....

. Part of the wave is transmitted the wave might travel straight ahead or

. The sound beam might bend or change direction

What is refraction?

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

. bending phenomenon of sound beam

What are the 2 requirements for refraction to occur?

- oblique incidence

- different media with different propagation speeds

How will a sound beam bend in media with similar speeds of sound?

it will bend, at most, a few degrees

ex. soft tissue-fat interface

ex. muscle-blood interface

ex. soft tissue-fluid interface

How will a sound beam bend in media with a large difference in speeds of sound?

Bending is exaggerated

ex. bone-soft tissue interface

What will cause the transmission angle to be equal to the incident angle?

the speeds of two median are identical

What will cause the transmission angle to be greater than the incident angle?

When the speed of media 2 is greater than the speed of media 1

What will cause the transmission angle to be less than the incident angle?

When the speed of media 2 is less than the speed of media 1