Chapter 6

Lung tissue scatters sound because the alveoli are filled with air.

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Rayleigh Scattering increase dramatically with increasing frequency.

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Recall. The higher frequency sound waves produces shorter pulses which gives more accurate images. However higher frequency sound attenuates more and is less capable of traveling to substantial depths.

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Scattering is directly related to frequency.

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The percentage of the incident beam that is reflected is related to the difference in the impendances of the tissues:

1. No reflection will occur if the two media have identical impedances. 2. A small reflection will occur if the impedances are slightly different. 3. A large reflection will occur if the impedances are substantially different.

Three process contribute to attenuation:

1. Reflection 2. Scattering 3. Absorption

At the boundary between two media:

1. The principle of "conservation of energy" applies. 2. If the reflected and transmitted intensities are added, the result must be the incident intensity. 3. If the IRC and ITC are added, the result equals 100%.

Incidence

A sound pulse strikes many tissue interfaces as it propagates through soft tissue. The angle at which the wave strikes the boundary determines the behavior of the pulse.

Attenuation

A sound wave weakens as it propagates in a medium. This decrease in intensity, power, and amplitude as sound travels.

Decibel notation

A standard measurement tool

Angles

Acute---less than 90 degrees Right---exactly 90 degrees Obtuse ---greater than 90 degrees

Amplified

After a sound wave is received by the transducer, converted into electricity and returned to the ultrasound system, it is strengthened.

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As a sound beam strikes a bounder. energy is conserved, and the following relationship applies: 100%=intens. reflewction coef.(IFC)(%) + intensity trans. coef. (ITC)(%).

Reflection

As sound strikes a boundary, a portion of the wave's energy may be redirected or reflected back to the sound source.

Attenuates

As sound travels in the body, it weakens.

What are the units of attenuation

Decibels

Logarithms

Decibels are based on a mathematical construct

Negative Decibels

Describe signals that are decreasing in strength or getting smaller

Attenuation

Higher Frequency & longer distances=More attenuation Lower Frequency & shorter distances = Less attenuation

Attenuation

In soft tissue depends upon the wave's frequency. Attenuation is greater in higher frequency sound than in lower frequency sound. FREQUENCY AND ATTENUATIONS ARE DIRECTLY RELATED.

What is the relationship between ultrasound frequency and the attenuation coefficient in soft tissue?

In soft tissue, the attenuation coefficient in dB per centimeter is approximately one half of the ultrasonic frequency in MHz.

Incident Intensity

Incident intensity is the sound wave's intensity immediately before it strikes a boundary.

Logarithm

Is a novel method of rating numbers. Specifically the logarithm of a number represents the number of 10's that are multiplied to create the original number 100,

Rayleigh Scattering

Is a special form of scattering that occurs when the structure's dimensions are much smaller than the beam's wavelength.

Attenuation

Is determined by two factors: 1. path length 2. frequency of sound

Impedance

Is the acoustic resistance to sound traveling in a medium. Acoustic impedance is calculated by multiplying the density of a medium by the speed that sound travels in the medium.

Reflected intensity

Is the intensity of the portion of the incident sound beam that, after striking a boundary, returns back in the direction from which it came.

Attenuation Coefficient

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

Diffuse Reflection

Most interfaces in the body are not smooth, but have some irregularities. When a wave reflects off an irregular surface, it radiates in more than one direction. This form of reflection is called diffuse reflection or backscatter.

Normal Incidence

Normal incidence means that the incident sound beam strikes the boundary at exactly 90 degrees.

Oblique Incidence

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

Absorption

Occurs when ultrasonic energy is converted into another energy form, such as heat, Like scattering, absorption is directly related to frequency.

Positive Decibels

Report signals that are increasing in strength, or getting larger.

Scattering

Scattering of ultrasound is the random redirection of sound in many directions. Sound scatters when the tissue interface is small; that is , equal to or less than the wave length of the incident sound beam.

Two Forms of reflection

Specular & Diffuse, are created in soft tissue, depending on the nature of the interface that a sound wave strikes.

Decibels

Strength of sound beams as they are created by transducers and travel through the body

IRC=Intensity Reflection coefficient

The percentage of the intensity that bounces back when a sound beam strikes the boundary between two media is the intensity reflection coefficient.

Intensity Transmission Coefficient

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

T or F. In a given medium, attenuation is unrelated to speed of sound.

True, Attenuation and propagation speed are unrelated.

Reflection with Normal Incidence

When a sound beam strikes a tissue boundary at a 90degree angle (normal incidence), reflection occurs only if the media on either side of the boundary have different impedances.

Specular Reflection

When the boundary is smooth, the sound is reflected in only one direction in an organized manner. This reflection is called Specular.

Name the three components of attenuation

absorption, reflection and scattering

Oblique angles

angles with a measure other than 90 degrees

What the units of the half-value layer thickness?

distance: centimeters

Attenuation in air is __________________ attenuation in soft tissue.

greater than

Attenuation in bone is _____________ attenuation in soft tissue.

greater than

Attenuation in lunt tissue is (less than, greater than, the same as) attenuation in soft tissue.

greater than

Acoustic impedance=______x________

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

As frequency decreases, depth of penetration_____.

increases

As the path length increase, the attenuation of ultrasound in soft tissue____________.

increases

Transmitted Intensity

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

Impedance is a characteristics of_______/

only the medium

As path length increases, the half boundary layer____________.

remains the same.

As the path length increases, the attenuation coefficient of ultrasound in soft tissue(decreases, remains the same, increases)

remains the same.