DMS; Chapter 6 - Sound Tissue Interaction

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What is Logarithm?

AKA Log. The logarithm of a number is the number of times that the number 10 is multiplied by itself to create the original number

*Ex: Log of 100 is 2 (10 x 10)
*Ex: Log of 1000 is 3 (10 x 10 x 10)
*As the logarithm increases by 1, the actual number increases 10 fold

What does the Richter Scale measure?

The strength of earthquakes

What is the difference between a magnitude 5 and a magnitude 7 earthquake, in both real terms and logarithmic terms?

2 in real terms and 100 in logarithmic terms.

What is a decibel?

The method of quantifying or measuring the strength of electrical signal and the brightness of echoes.

What type of change does decibel measure?

Relative change

What are the two types of intensities related to decibel?

Initial Intensity - Intensity at the starting level
New Intensity - Intensity during measurement

What is the 3 Decibel Rule?

For every 3 dB, intensity doubles

What is the ratio for Decibel?

(New) Actual Intensity Level / (Initial) Starting Intensity Level
*Ratio is equivalent to 3 dB

What is a positive decibel?

A. It is a measurement of increasing strength
B. When a wave's intensity doubles, the relative change is +3 dB
C. When the intensity increases 10-fold, the relative change is +10 dB

What is a negative decibel?

A. The measurement of decreasing strength
B. When a wave's intensity is reduced by 1/2 its original value, the relative change is -3 dB
C. When the intensity is reduced to 1/10 its original value, the relative change is -10 dB

What is attenuation?

The weakening of a wave

Attenuation is the reduction of what three factors as sound travels?

1. Amplitude
2. Power
3. Intensity

What two factors determine attenuation?

1. Path Length - Depth to travel or imaging depth
2. Frequency of Sound

Attenuation is directly proportional to what two factors?

1. Depth the Sound Travels
2. Frequency of Sound Wave

How is attenuation reported?

In terms of decibels as a relative change, NOT as an absolute change

What three factors contribute to attenuation?

1. Reflection
2. Scattering
3. Absorption
*Attenuation is the process whereby sound energy is extracted from a wave by reflection, scattering and absorption

What is reflection?

The redirection of some of the sound energy back to where the source of the sound came from

When does reflection occur?

When the dimension of the boundary is large

What are the two types of reflections?

1. Specular & Non-Specular
2. Diffuse

What is specular reflection? (B-Mode)

A reflection that occurs on a (straight) smooth boundary in a single direction in an organized manner and back to the transducer

What is an advantage to specular reflection?

The reflections are strong

What is a disadvantage to specular reflection?

The reflections do not go back to the transducer if the wave doesn't hit at 90 degree angle

What are four descriptors of specular reflections?

1. Organized
2. Regular
3. Predictable
4. Occurs from a smooth boundary

What are four descriptions of non-specular reflections?

1. Disorganized
2. Chaotic
3. Random
4. Occurs from a rough boundary

What is diffuse Reflection aka Backscatter?

A reflection that occurs on an irregular boundary in different directions

What is an advantage of Diffuse Reflection?

Reflections may return to the transducer even if the waves are off-axis

What is a disadvantage of Diffuse Reflection?

Reflections are weaker than specular

What is scattering?

Random redirection of sound in many directions

When does scattering occur?

When the boundary (interface) is small or is equal or smaller than the wavelength

What is scattering directly related to?

Frequency

What is Rayleigh Scattering (Organized Scattering)?

It occurs when the structure's size is much smaller than the wavelength and the sounds are redirected equally in all directions in an organized manner

Ex: Rayleigh scattering from red blood cells
*Rayleigh scattering is directly related to frequency raised to the 4th power

What is Absorption?

The conversion of ultrasound energy into heat energy

What is the main reason why high frequency waves attenuate more than lower frequency sound waves?

Absorption

In diagnostic imaging, what type of frequency is used to provide images from the depth of clinical interest?

The highest frequencies

What is the attenuation coefficient?

The number of decibels of attenuation that occurs for every centimeter that a wave travels.

What is the unit used for the attenuation coefficient?

dB/cm

High or Low - The attenuation in bone and lungs?

High

High or Low - The attenuation in water?

Low

When is attenuation higher in muscle?

When travelling across the muscle fibers, versus along the length of the fiber.

Is attenuation higher in muscle or soft tissues?

Muscle

Attention Coefficient multiplied by the Distance Traveled is equal to?

The Total Attenuation

What is the Attenuation Coefficient in Soft Tissue?

It is equal to half of the frequency in MHz

What is the relationship between frequency and attenuation?

As frequency increases, scattering and absorption increases

How are frequency and attenuation related to each other?

They are directly proportional

What is Half-Value Layer Thickness?

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

AKA - Penetration depth, depth of penetration and half-boundary layer

What is the unit of measurement for Half-Value Layer Thickness?

Centimeter

What is the typical value range of Half-Value Layer Thickness?

0.25 to 1.0 cm

What is Impedance?

The acoustic resistance to sound traveling in a medium

What is the formula for Impedance?

Density (medium) x c (Kg/cubic meter) x m/s

What is the unit of measurement for Impedance?

Rayls (z)

What is the typical value range of impedance?

1,250,000 Z to 1,750,000 Z [Range of Impedance in Soft Tissues]

How is impedance determined?

By the medium ONLY

What is the relationship between reflection and impedance?

They are directly related. As impedance increases, reflection increases.

What is Incidence?

The striking of a sound wave at an interface as it propagates through soft tissues

What are the three types of angles?

Acute - Less than 90 degrees
Right - Exactly 90 degrees
Obtuse - Greater than 90 degrees

What is Normal Incidence?

A type of incidence when the incident sound beam strikes the boundary at exactly 90 degrees

AKA - Perpendicular, orthogonal, right angle and 90 degrees

What is Oblique Incidence

A type of incidence when the incident sound beam strikes the boundary at any angle other than 90 degrees

AKA - Non-perpendicular

What is Incident Intensity?

It's the intensity of the sound wave before it strikes a boundary

AKA - Starting intensity

Incident Intensity = Reflected Intensity + Transmitted Intensity

What is Reflected Intensity?

The intensity of the portion of a sound beam that returns back after striking a boundary

What is Transmitted Intensity?

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

What is the Intensity Reflection Coefficient?

The percentage of the intensity that returns back when the sound beam travels from one medium to another
1% or less in clinical imaging of soft tissues Abbreviated as IRC

What is the formula for Intensity Reflection Coefficient (IRC)?

(Z2-Z1/Z2+Z1)^2 x 100

What is the Intensity Transmission Coefficient?

The percentage of the intensity that travels forward in the same direction that it is traveling after striking an interface
*99% or more in clinical imaging of soft tissues
*Abbreviated ITC

How are intensities reported?

W/Cm^2

How are coefficients reported?

Percentages and therefore without units
100% = IRC(%) + ITC(%)

When does reflection occur in a normal incidence?

Reflection occurs when the impedance of the media on either side of the interface are different

Relationships between Reflection and Impedance - The greater the impedance difference between media...

The greater the reflection at the boundary

Relationships between Reflection and Impedance - The smaller the impedance difference between media...

The smaller the amount of reflection

Relationships between Reflection and Impedance - If the impedance between media on either side of the interface are the same

There is no reflection

What is an Oblique Incidence?

An incidence that occurs when the sound beam strikes the boundary between different media at an angle other than 90 degrees

AKA - Non-Perpendicular Incidence, Acute Incidence, Obtuse Incidence
*The incidence angle is greater than 0
*The reflected sound may not return back to the source
*The transmitted sound may travel in a direction different from the incident sound or it may travel in the same direction

When does reflection occur in an oblique incidence?

We cannot predict reflection and transmission because of the complicated physics that is involved

Law of Conservation of Energy (Sound)

Percent of Reflected Sound + Percent of Transmitted Sound = 100%

Law of Conservation of Energy (Intensity)

Incident Intensity = Reflected Intensity + Transmitted Intensity

Reflection Angle = Incident Angle

The reflection that occurs with oblique incidence does not return back to the transducer, but is reflected in a different direction

Angle of Reflection = Angle of Incidence

Angle of Incidence: The angle between the incident sound beam and an imaginary online that is perpendicular to the boundary

Angle of Reflection: The angle between the reflected sound beam and an imaginary line that is perpendicular to the boundary

What are the three factors associated to transmission with oblique incidence?

1. Cannot be predicted
2. It might travel in the same direction
3. It might change direction

What is the definition of refraction?

A sound beam that is transmitted in a direction different from the direction that the incident sound beam is traveling

Ahem... What is refraction again?

A form of transmission in a different direction

What are the two requirements for refraction?

1. Oblique Incidence
2. Propagation speeds in the media on either side of the boundary are different

Will refraction occur in an oblique incidence with the media having the same propagation speeds?

No.

*Remember that although we use a single propagation speed in soft tissues, this parameter changes in the same tissue.

What is Snell's Law?

The law that defines the physics of refraction.

1. If the propagation speeds of sound in both media are the same, there will be no refraction
2. If the propagation speed of sound in the second medium is greater than the first medium, the transmission angle is greater than the incident angle

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