DMS physics Attenuation
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
