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Attenuation is 3 factors _______
- reflection
- scattering/refraction
- absorption
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most attenuation occurs due to ____ which is converted to _____
due to absorption which is converted to heat
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attenuation is directly related to ___ and ______
directly to path length and frequency
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attenuation is measured in
decibles
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two types of reflection
specular and diffuse(non-specular)
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specular occurs when the sound wave strikes a boundary that is
smooth and large
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diffuse occurs when the sound wave strikes a boundary that is
- -rough
- -This occurrence is different from specular reflection in that scattering occurs when the interface that the waveform encounters is rough The reflected soundwave will go back in many different directions.
- -Lung tissue scatters sound because the alveoli are filled with air. When the sound beam strikes an irregular surface reflections radiate in multiple directions
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scattering (diffuse or non-specular) is when the ultrasound beam is redirected in
many directions
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the sound beam of scattering is _______ than the wavelength
equal or smaller
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Rayleigh scattering occurs when the tissue is
much smaller than the wavelength
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all scattering ______ with increasing frequency
increases
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the most common example of Rayleigh scatter is
RBC red blood cells
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Rayleigh scattering will increase exponentially to the ____ power with increases in frequency
4th power
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absorption is the dominant form of attenuation of the_______
sound beam
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the ______the frequency the more absorption
higher
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bone will have a ____ amount of absorption
large
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attenuation is related to frequency. as the frequency increase attenuation will
increase
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the rate of attenuation is called the attenuation
coefficient
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The attenuation coefficient is
0.5XmHz
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air and lung tissue have a very ___rates of attenuation
high
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reflection is produced when
sound moves from one medium to another is the basis of the image we see
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impedance is the amount of _____ a sound wave encounter as it propagates through a medium
resistance.
Impedance determines how much of the wave is reflected back and how much of the wave is transmitted.
Impedance is a characteristic of the medium and thus each medium has its own acoustic impedance.
Reflections created by two media at a boundary depends on their impedance difference.
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acoustic impedance is a property of the
medium
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units for impedance is
rayles
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reflection depends on the difference in acoustic impedances of
2 mediums at a boundary
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angles of incidence
right, normal, orthogonal, perpendicular, oblique
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oblique is any incidence that is not ___ degrees
90
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incident intensity is the
sound wave's intensity before it strikes a boundary
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reflected intensity is
the intensity of the portion of the incident sound beam after striking the boundary returns back in the direction from which it came
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transmitted intensity is the
portion that after striking a boundary continues on in the same general direction. it keeps going
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incident intensity =
reflected + transmitted intensity. this will always equal 100%
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The IRC intensity reflection coefficient is the
percentage of the intensity that returns back when the sound beam strikes the boundary between 2 media
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The ITC intensity transmission coefficient
the percentage of the ultrasound intensity that continues in the same direction when the beam strikes the interface between 2 media
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Reflection with normal incidence
-No reflection will occur if the two media have identical impedances
-Conversely, there will only be a small reflection if there is only a small difference in impedances and a large reflection with a large difference.
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Reflection with oblique incidence
-With oblique incidence, we cannot predict whether the sound will reflect or transmit after striking a boundary
-When sound strikes a boundary obliquely reflection and or transmission may or may not occur.
-This is a very vague area with no real physics to predict the outcome.
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Reflection with oblique incidence 2 rules that do apply are:
- -The conservation of energy
- -Transmitted and reflected energy must equal 100%
-Reflection angle = incident angle
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When a sound wave strikes a boundary of media with different propagation speeds obliquely the sound beam will be
refracted
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Refraction of the sound beam is a bending off of the
normal path of transmitted portion
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Refraction can only occur when there is
- oblique incidence
- different propagation speed
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snell's law
The transmission angle will be greater than the incident angle when the speed of medium 2 is greater than the speed of medium 1. (away from normal)
-The transmission angle will be less than the incident angle when the speed of medium2 is less than the speed of medium 1. ( towards normal )
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bigger number the bigger the
angle
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larger the speed the
further away from normal
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the smaller the speed the
closer to normal
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Effects of Refraction
-Shifting of the sound wave will cause an artifact.
- -An artifact is the breaking of rules. An artifact is the breaking of the sound rule of the assumption that sound travels straight out and straight back to the transducer. When a rule is broken there are consequences.
- -edge shadowing and displacement of a structure
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Critical angle
-This will occur when there is so much refraction that none of the incident beams will transmit across the interface.
-Total reflection will occur
-Different propagation speeds and a curved surface or oblique incidence that often will result in shadowing- refractive shadowing.
-This is thought to be around 22 degrees.
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Half Value Layer is
the depth at which half of the sound beam’s original power has been attenuated.
The distance that sound travels in tissue that reduces the sound to one-half its original intensity. Remember higher frequency has less penetration than lower frequency. This is due to attenuation. Therefore a low-frequency sound beam will travel further before half of its energy is used up.
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A high frequency will have a _____HVL
thin
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A low frequency will have a ____HVL
thick
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distance and attenuation are ___ related
directly
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frequency and attenuation are ___ related
directly
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attenuation is ______ the further the sound beam travels
greater
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attenuation is _____ the higher the frequency
greater
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Attenuation is the
decrease in the amplitude or intensity of a wave as it propagates through a medium.
So when saying the beam was attenuated is saying that the beam was decreased by X amount of decibels.
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Attenuation limits imaging depths and must be
compensated by the diagnostic equipment.
As a sound beam travels through a medium there is a continual loss if intensity and amplitude as a function of time, attenuation.
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Reflection – occurs when
a portion of the sound beam is returned from the media boundary.
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Absorption- the only process whereby sound energy is
dissipated by the medium. All other interactions decrease the ultrasonic beam intensity by redirecting the energy of the beam. Absorption is the process of ultrasonic energy conversion to other energy forms, primarily heat. Absorption is normally the dominant contributor in the attenuation of the sound beam.
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normal incidence the wave direction is
perpendicular when the waveform is traveling so that the line of incidence is perpendicular to the medium. It will form a 90 degree angle to the medium. The incident wave is parallel to the normal line which is always perpendicular to the medium.
The incident sound beam strikes the boundary at exactly 90 degrees.
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when dealing with a specular reflector the angle of incidence is equal to
the angle of reflection
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specular reflection occurs when
the surface of a medium is large and smooth with respects to the wavelength of the incident sound beam. The reflection is directed in only one direction.
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Angle of transmission when a waveform encounters
encounters a boundary or an interface between 2 media a percentage of the wave will be reflected back and a percentage of the wave will continue to propagate through the second medium.
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Scattering is directly related to
to frequency
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most half value layer ranges from
0.25 to 1 cm
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Echogenicity
Describes the strength of the signal reflection
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Anechoic
No to little echoes. Ex: cyst
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Hypoechoic
low echogenicity, echoes but subtle, this will appear darker than the surrounding tissues. Ex: liver mets
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Hyperechoic
Very echogenic, bright, highly reflective. Ex: hemangioma, bone, diaphragm
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Calcified
strongly echogenic with acoustic shadowing. Ex: bone, plaque
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Complex
mixed echogenicity with or without shadowing, may have both. Many levels of echoes, may range from anechoic to calcified. Ex: complex or ulcerated plaque, old hemangiomas, some masses such as cancerous breast masses.
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Uniformity
homogenous, smooth
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Speckle is
When a pulse encounters scatter and then arrives back at the transducer in the form of constructive/ destructive, or partially constructive interference the result will give a speckle appearance.
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