Understanding Ultrasound Physics chapter 6 - Interaction of Sound and Media

What is the attenuation coefficient in soft tissue?

0.5 dB/cm/ MHz

Decibel Notation

A standard measurement tool used to report RELATIVE changes in attenuation or amplification

- LOGARITHMIC

-comparison

-a ratio

Logarithm

Mathematical construct for rating numbers:

-The LOG represents the # of 10's that are multiplied to create the original number

- To determine the log of powers of ten, simply count the ZEROES

Decibels are a Ratio, what is needed to solve a dB problem?

RATIO =

(Actual level / Starting level)

This means that TWO intensities are required

1. the STARTING level

2. ACTUAL level at the time of measurement

Amplified by +6 dB

The intensity has increased 4X = QUADRUPLED

Amplified by +9 dB

The intensity has increased 8X

Amplified by +20 dB

The intensity has increased 100X

- +30 dB = 1000X

- +40 dB = 10000X

etc

Attenuated by - 6dB

The intensity has decreased by a fourth (-1/4)

Attenuated by -9dB

The intensity has decreased by an eighth (-1/8)

Attenuated by -20dB

The intensity has decreased by -1/100

-30 dB = -1/1000

-40 dB = - 1/10000

etc

Attenuation is determined by TWO factors:

1. PATH LENGTH

[distance]

2. FREQUENCY

[penetration]

Relationship between FREQUENCY and PENETRATION

INVERSELY RELATED

- High frequency means LESS penetration (the sound beam can NOT travel as far into the body)

- Low frequency means MORE penetration

Attenuation is reported as:

a RELATIVE CHANGE, not absolute

3 Processes that contribute to attenuation

1. Reflection

2. Scattering

3. Absorption

Reflection

- As a sound strikes a boundary a portion of the wave's energy is redirected/or REFLECTED back to the SOUND SOURCE

- Reflection is likely to occur when the dimension of the boundary is LARGE

2 Types of Reflection

1. Specular Reflection

2. Diffuse Reflection

Specular Reflection

When the boundary is LARGE and SMOOTH (i.e. diaphragm):

The sound is reflected in ONE direction in and ORGANIZED manner.

- Must be at 90 DEGREE ANGLE

Diffuse Reflection (Backscatter)

This type of reflection occurs when the sound beam strikes an IRREGULAR boundary and radiates in MORE THAN ONE direction.

- Does not have to be 90 degree angle

- Backscatter has lower strength than specular reflection

Scattering

RANDOM redirection of sound in MANY DIRECTIONS

- Sound scatters when the tissue interface SMALL

- It is DISORGANIZED

What causes the most scattering?

AIR - Lung tissue will cause scattering because the alveoli are filled with air.

Rayleigh Scattering

Type of scattering that occurs when structure's DIMENSIONS are MUCH SMALLER than the beams wavelength

Relationship between RAYLEIGH SCATTERING and FREQUENCY

DIRECTLY RELATED:

The higher the frequency the higher the Rayleigh Scattering

Rayleigh scattering is PROPORTIONAL to FREQUENCY to the 4th POWER

How does Rayleigh Scattering redirect the sound wave?

EQUALLY in ALL DIRECTIONS

- It is ORGANIZED

What is an important tissue property that influences the amount of REFLECTION

acoustic IMPEDANCE

Impedance

Acoustic resistance to sound traveling in a medium

- Impedance DEPENDS upon the DIFFERENCE between the IMPEDANCES of the 2 media at the boundary

- If the impedances EQUAL each other, there is NO REFLECTION

How is IMPEDANCE calculated

Z = density (kg/cm3) X propagation speed (c)

UNITS: rayls

Incidence

refers to the ANGLE at which a wave strikes a boundary which will then determine the behavior of that pulse

PORNN

1. P - Perpendicular

2. O - Orthogonal

3. R - Right

4. N - Normal

5. N - Ninety

All these words mean NORMAL incidence at 90 DEGREES

Incident Intensity

The sound wave's intensity immediately BEFORE it strikes a boundary

Reflected Intensity

The intensity of the portion of the incident sound beam that AFTER striking the boundary, RETURN BACK in the direction from which it came

Transmitted Intensity

The intensity of the portion of the incident sound beam that AFTER striking the boundary, CONTINUED FORWARD in the direction it was traveling

Since there is conservation of energy at the boundary:

Incident (starting) Intensity =

Reflected Intensity + Transmitted Intensity

What UNITS is Intensity measured in?

The unit for all intensities is W/CM2

Intensity Reflection Coefficient (IRC)

Is the PERCENTAGE (%) of the intensity that BOUNCES BACK when the sound beam strikes the boundary between 2 media

What is the IRC between SOFT TISSUE and AIR?

The percent reflected back is 90-99%

What is the IRC between SOFT TISSUE and BONE?

The percent reflected back is 50%

What is the IRC between SOFT TISSUE and SOFT TISSUE?

The percent reflected back is 1%

Intensity Transmission Coefficient (ITC)

The percentage (%) of the intensity that passes in the FORWARD DIRECTION when the sound beam strikes the interface between 2 media

Since there is conservation of energy at the boundary:

Intensity REFLECTION coeffecient (IRC) (%)

+

Intensity TRANSMISSION coeffecient (ITC) (%)

=

100%

Reflection with Normal Incidence

When a sound beam strikes a tissue boundary at a 90 DEGREE ANGLE (normal incidence), reflection occurs ONLY if the media on the other side of the boundary have DIFFERENT IMPEDANCES

What would cause NO REFLECTION to occur?

If Z1 = Z2

If the impedances are IDENTICAL, no reflection will occur

TRANSMISSION = !00% because nothing is reflected back

Reflection with OBLIQUE incidence

Reflection MAY OR MAY NOT occur:

However, 2 principles ALWAYS apply

1. Conservation of energy

2. REFLECTION ANGLE = INCIDENT ANGLE

REFLECTION ANGLE = INCIDENT ANGLE

When reflection occurs with oblique incidence, the sound beam is NOT redirected back to the transducer, but rather in a DIFFERENT DIRECTION.

The direction it goes in will be EQUAL and OPPOSITE to the incident angle:

ANGLE OF INCIDENCE = ANGLE OF REFLECTION

Transmission with Oblique Incidence

similarly MAY or MAY NOT OCCUR:

If it does occur, the beam could travel in a straight line (angle of transmission)

or

the sound beam could bend/change direction (angle of refraction)

Refraction

TRANSMISSION WITH A BEND:

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

Refraction occurs only if 2 conditions are met:

1. OBLIQUE incidence (angle) - not 90 degrees

2. DIFFERENT PROPAGATION SPEEDS between the 2 media.

- If these 2 conditions are met, the sound beam WILL NOT travel in a straight line, it will BEND

Snell's Law

Snell defined the physics of refraction (LAW OF REFRACTION)

- has to do with the relationship between the incident angle and TRANSMISSION ANGLE

Half-value layer thickness units...?

Cm

Attenuation coefficient units...?

dB/cm

T/F: attenuation and prop speed are unrelated

True

T/F: if 2 media have the same prop. speed but different densities, the one with higher density will have higher impedance

True

Impedance (rayls) = density mass/volume x c

When ultrasound encounters a boundary with perpendicular incidence, the _______ must be different to produce a reflection

Impedance

What is half value layer thickness?

Distance that sound must travel in a tissue to decrease by half [ attenuate 3dB]

3 synonyms for half value layer thickness

Penetration depth, depth of penetration, half boundary layer

What is total attenuation?

Attenuation coefficient multiplied by distance traveled through the tissues

Determined by path length and frequency

Impedance is determined by...?

Medium

Half boundary layer is determined by...?

Medium and frequency

Acoustic impedance is a property of the _____, and determined by its_____ & _____

Medium, density, stiffness

A sound beam has > from its initial intensity by a factor of 100, how is this described in dB?

20dB

A wave's intensity is 2mW/cm^2,

There is a 9dB change,

What is the final intensity?

16mW/cm^2

Def of Attenuation Coefficient

# of dBs of attenuation that occurs when sound travels one cm

dB/cm

Total attenuation formula

TotalAttenu = AttenuCo [dB/cm] x distance [cm]

In air, frequencies above____ attenuate entirely

1 MHz

No noticeable attenuation occurs with frequencies less than_____ is observed in water

10 MHz

Typical values for 1/2 value layer thickness

0.25 - 1.0 cm

Tissue impedance is______, not_____

Calculated, measured

Typical values for impedance

1.25 - 1.75 Mrayls

Synonym for impedance

Characteristic impedance,

Acoustic impedance

As path length increases, the half boundary layer will...

Remain the same

As path length increases, the attenuation coefficient in soft tissue will...?

Remain the same

Impedance is important in______ at boundaries

Reflections

Intensity reflection coefficient (IRC) formula

(With regard to impedances)

Orthogonal means...?

Normal incidence

Transmission with Normal Incidence (ITC) coefficient formula

ITC (%) = transmitted intensity/incident intensity x 100

A sound pulse travels in Med1 and strikes a boundary with Med2 at 30*,

The AoT is 10*

In which medium is the impedance higher?

Cannot be determined with info provided.

Refraction is affected by SPEED OF SOUND in the media

attenuate

to reduce in force or degree; to weaken

amplified

increased strength

decibel notation

a relative measurement, a comparison, a ratio, logarithmic

Every 3 dB change means that the intensity will?

double

Every 10 dB change means that the intensity will?

increase ten times

A reduction in the intensity of a sound beam to one-half of its original value is _____ dB.

-3 db

A reduction in the intensity of a sound beam to one-quarter of its original value is _____ dB.

-6 db

-10 dB means that the intensity is reduced to _____ of its original value.

one-tenth

dB is a mathematical representation with a

logarithmic and relative scale

true or false

we need one intensity to calculate decibels.

false

a wave's intensity is 2mw/cm^2. there is a change of +9 db. what is the final intensity?

c. 16 mw/cm^2

If the final intensity of a sound beam is more than the initial intensity, then the gain in dB is ____. (+ or -)

positive.

If the initial intensity of a sound beam is less than the final intensity, then the gain in dB is _____

positive.

Distance and attenuation are ______ related.

directly

Frequency and attenuation are ______ related.

directly

More attenuation

longer distances, higher frequencies

Less attenuation

shorter distances, lower frequencies

three processes contribute to attenuation

1. Reflection

2. Scattering

3. Absorption

two forms of reflection

specular and diffuse are created in soft tissue, depending on the nature of the interface that a sound wave strikes

specular reflection

a reflection produced by a smooth surface in which parallel light rays are reflected in parallel

diffuse reflection or backscatter

Reflection that occurs when parallel rays of light hit a rough surface and all reflect at different angles

Rayleigh scattering

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

what is the significance of attenuation in diagnostic sonography?

the goal in diagnostic imaging is to use the highest frequency that still provides images to the depth of the structures of clinical interest.

Absorption

the process or action by which one thing absorbs or is absorbed by another.

Absorption and frequency are ____ related.

directly