DMS physics Attenuation

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99%

what percent of incident energy is reflected at an air/ soft tissue boundary

50%

what percentage of incident energy is reflected at a bone/ soft tissue boundary

transmission with normal incidence

whatever remains after transmission, must be reflected

impedance

when density or speed increases __________ increases

normal incidence

•90 degree sound wave interaction at boundary •perpendicular •orthogonal •right angle

specular reflection

•dominant cause of imaging artifacts •strongest form of reflection •highly angular dependent

refraction

-Edge shadowing -displacement of a structure in the image laterally -loss of signal intensity from dispersion of beam -degradation in lateral resolution -artificial sound image Are all effects of ?

blood attenuation rate

0.125 dB/(cm*MHz)

Soft tissue attenuation rate

0.5dB/(cm *MHz)

muscle attenuation rate

1.0dB/(cm*MHz)

acoustic impedance

A matching layer improves the transmission efficiency by reducing the ________________ of the high impedance of the crystal and the low impedance of the tissue

zero degrees

A perpendicular beam implies that the incident angle is _____________

Critical angle

Angle of incidence at which there is no transmission and 100% internal reflection

Frequency

As absorption increases, ____________ increases

directly related

Attenuation and distance(depth) are

much less than

Attenuation in water is ____________ than in soft tissue

absorption reflection refraction

Attenuation is the result of 3 wave interactions with the medium

absorption

Conversion of energy from the sound wave into heat within the medium

Attenuation

Dependent on both the characteristics of the wave and the characteristics of the medium

absorption

Energy converted to heat

density * propagation velocity

Equation for acoustic impedance

refraction

For ________ to exist there must be a change in propagation velocities between the 2 interfacing mediums

specular reflection

For ____________, The incident angle is equal to the reflected angle

refraction

For normal incidence there can be no __________

refraction

For normal incidence there can be no __________.

1

IRC + ITC = ? or intensity reflection coefficient + intensity transmission coefficient = ?

heterogenous

Implies varied signal strength of images structure

specular reflection

In ______________, the angle of incidence is always equal to the angle of reflection.

Absorption

In soft tissue, ________ is the dominant factor for creating attenuation.

Rayleigh scattering

Indicates the amount of scattering changes with wavelength

transmitted intensity

Intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling

incident angle

Measured as the angle formed between the line perpendicular to the reflecting structure (called the normal line) and the wave direction.

the incident angle

Measurement between the incident wave direction and the normal line (0 degree wave direction

Impedance mismatch

No __________ ____________ will result in no reflection of equivalently 100% transmission

specular reflection

Occurs from a surface which is large and smooth relative to the wavelength of the wave; Mirror-like reflection

reflection with normal incidence

Occurs only if the 2 media at the boundary have different acoustic impedances

(Back) scattering

Occurs when the surface is rough with respect to the wavelength

absorption

Of the 3 interactions which constitute attenuation, the dominant factor in soft tissue is ___________

total attenuation

Path length *attenuation coefficient =?

intensity transmission coefficient

Percentage of the intensity that passes through a boundary between 2 media

Normal incidence

Perpendicular to boundary

mega

Prefix for 1,000,000

conservation of energy assuming there is no energy lost to absorption

Reflection% + transmission%=100%

acoustic shadowing

Shadow by refraction instead of attenuation

back scatter (Diffuse reflection)

Sound reflecting towards transducer that is disorganized and random

reflection

Sound strikes a boundary and is returned towards the transducer

Less than

The acoustic impedance of air is __________ than the acoustic impedance of tissue.

frequency

The estimate in soft tissue is 1/2 the ___________, so if using a 5MHz transducer the attenuation coefficient would be 2.5dB/cm

the greater

The higher the frequency, _______________ the attenuation coefficient.

the critical angle

The incident angle at which total internal reflection occurs

normal incidence

The incident angle is zero degrees and the wave direction is parallel to the normal line

Reflection

The phenomenon of causing a propagating wave to change direction such that some of the wave energy Does not continue to propagate forward

refraction

The portion of the beam that does not reflect or absorb at an interface between two media, transmit through

Attenuation coefficient

The rate or amount of attenuation per centimeter of tissue; dB/cm (for each MHz of chosen frequency)

normal

The term __________ in reference to angles means the same as zero degree incidence

reflection

The type of ________ that occurs at an interface depend on the wavelength relative to the surface geometry

Units for acoustic impedance Z= kg • m = kg ____ ____ _____ m^3 sec m ^2sec

This can be determined by multiplying the units for density (mass per volume) with the units for the propagation velocity (distance per time)

refraction

This can result in a lateral displacement of strictures within an image.

Rayleigh scattering

This occurs when the reflecting structures are very small with respect to the wavelength; frequency dependent

impedance

Typical values range between 1.25 and 1.75 Mrayls

cm

Units for path length

dB/cm

Units for total attenuation

Rayleigh scattering

Very weak reflective mode and highly frequency dependent

normal line

What Is the red line representing

Attenuation increases

What happens when frequency or path length increases?

1%

What percent of incident energy is reflected at a soft tissue/ soft tissue boundary

scattering

When the surface is rough relative to the wavelength

no transmission only shadows

Why is it so important to not have too much reflection from any one interface?

reflection

Will vary depending on the surface of the reflecting surface

intensity reflection coefficient

Z2-Z1 ^2 Z2+Z1

water

________ absorbs less sound energy than soft tissue

reflection

________ of a sound wave depends upon a difference in acoustic impedances at the boundary between 2 media

refraction

_________ can cause loss of signal strength and cause acoustic shadows in a 2D image

impedance

_________ is a characteristic of the medium

Rayleigh scattering

_________ is a very weak reflection mechanism

Attenuation

_________ means a diminishing or decrease.

refraction

__________ Is considered an artifact which can cause objects to be drawn at the wrong location (primarily laterally displaced) or not even drawn at all

impedance

__________ is a characteristic of the medium

specular reflection

__________ is highly angle dependent; dependent on smoothness and size of the reflector relative to the wavelength

Rayleigh scattering

___________ indicates that the amount of scattering changes with wavelength

Attenuation

________ultimately limits the maximum depth from which images can be obtained

Attenuation

a measure of how the medium affects the wave

Specular reflection

angle of incidence equals the angle of reflection

ninety degrees

at the critical angle the transmission angle is ____________

directly related

attenuation and frequency are

a reflector

components of attenuation: Air is _____________

A reflector and absorber

components of attenuation: Bone is ___________________

scatter

components of attenuation: Lung is ____________

impedance (Rayls)

density(kg/m^3 * propagation speed = ?

Homogenous

echoes of a structure on image are relatively uniform

reflection

impedance is important in ________ at boundaries

Reflection

impedance is important in __________ at boundaries

dB = (10) log 1⬇️1 ➗ 1⬇️2

intensity change in decibels

incident intensity

intensity of sound at instant of striking a boundary

reflected intensity

intensity that, after striking a boundary, changes direction and returns back to the transducer

Total attenuation (dB)

path length(cm) * attenuation coefficient

total attenuation

path length* attenuation coefficient

intensity reflection coefficient

percentage of Ultrasound intensity that reflects when sound interacts with an interface between two media

Micro

prefix for 0.000001

Attenuation coefficient

rate of amount of attenuation per cm of tissue

incident intensity

reflected intensity+ transmitted intensity = ?

Snell's law

refraction is governed by

Reflection

sound strikes boundary and is returned toward the transducer

Attenuation per centimeter

the amount of ___________ does not change when path length changes

incident angle

the angle formed between the wavefront and the interface of the reflecting structure is called

intensity reflection coefficient

the greater the difference in impedances between 2 media at the boundary, the greater the ___________and the greater amount of reflection

reflection

the mechanism which makes diagnostic ultrasound work

Acoustic impedance

the ratio of acoustic pressure to flow. It allows us to calculate acoustic power, intensity and the reflection and transmission at boundaries.

Perpendicular

the wave direction is always __________ to the wavefront

refraction

this depends on 2 parameters 1) if the angle of incidence is other than perpendicular 2)if there is a change in propagation speeds at the boundary

refraction

this refers to the bending of a ray of beam at an interface between 2 different media

attenuation

this ultimately limits the maximum depth from which images can be obtained

Z=acoustic impedance of a material Z=p*c

velocity of the wave (c) times density of the molecules (p)

Refraction

wave changes direction across interface


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