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Ultrasound Physics: Chapter 6
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Flashcards
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Terms in this set (75)
What is Attenuation?
decrease in intensity, power, and amplitude as sound travels
Decibel notation
a relative measurement, a comparison, a ratio, logarithmic
logarithms
Method of rating numbers
Decibel
a logarithmic unit of sound intensity
Positive decibels
Report signals that are INCREASING in strength, or GETTING LARGER
a +3 dB change indicates intensity does what
doubles
a -3 dB change indicates intensity does what
halves
negative decibels
Describe signals that are DECREASING in strength or GETTING SMALLER
Attenuation is determined by TWO factors:
path length and frequency of sound
distance an attenuation have a ______ relationship
direct
Frequency and attenuation are ______ related.
directly
what unit is used to measure attenuation?
Decibels
What three processes contribute to attenuation?
1. Reflection
2. Scattering
3. Absorption
Reflection
As sound strikes a boundary a portion of the wave's energy may be redirected back to the sound source
specular reflection
When a wave is reflected in a single direction by a smooth surface
diffuse reflection
When a wave is reflected by a rough surface and the reflected rays are scattered in lots of different directions
backscatter
scattered sound waves that make their way back to the transducer and produce an image on the display
Scattering
random redirection of sound in many directions; occurs when tissue interface is small (equal to or less than the wavelength of the incident sound beam)
Rayleigh Scattering
Special form of scattering that occurs when the structure's dimensions are much smaller than the beam's wavelength
Absorption
most sizeable component of attenuation; occurs when ultrasonic energy is converted into another energy form
Absorption and frequency are ____ related.
directly
Scattering and frequency are ____ related.
directly
Total attenuation depends on what three things?
Frequency of the sound, distance the beam travels, tissue through which the sound wave travels
Attenuation Coefficient
the number of decibels of attenuation that occurs when sound travels one cm
What units are used to describe the attenuation coefficient?
dB/cm
how is the attenuation coefficient calculated in soft tissue?
frequency/2
Half-value layer thickness
the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value
What units are used to describe half-value layer thickness?
cm or any other unit of length
What is the typical value of half-value layer thickness?
0.25 to 1 cm
What other terms are used to describe half-value layer thickness?
penetration depth, depth of penetration, half-boundary layer
what affects the half-value layer thickness?
the medium and frequency of sound
what is the relationship between half-value layer thickness and frequency?
inverse
transmission
critical to ultrasound's ability to image structures located deep in the body
Impedance
acoustic resistance to sound traveling in a medium
how is impedance calculated?
Z = density (kg/cm3) X propagation speed (c)
What units are used to describe impedance?
Rayls
What are typical values for impedance?
1,250,000 to 1,750,000 rayls (1.25-1.75 Mrayls)
Impedance is determined by
medium only
Acoustic Impedance can also be called ________
characteristic impedance
What are the 3 types of angles?
acute, right, obtuse
Angles with a measure other than 90 degrees are also called ______ angles.
oblique
Normal Incidence occurs when
the incident sound beam strikes the boundary at exactly 90 degrees
Normal incidence is also known as?
perpendicular, orthogonal, right angle, or 90 degree angle
Oblique Incidence occurs when
the incident sound beam strikes the boundary at any angle other than 90 degrees.
Incident intensity
the sound wave's intensity immediately before it strikes a boundary
Reflected Intensity
the intensity of the portion of the incident sound beam that, after striking a boundary, returns back from where it came
Transmitted Intensity
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
All intensities have units of?
W/cm2
Intensity Reflection Coefficient (IRC)
the percentage of the intensity that bounces back when a sound beam strikes the boundary between two media
In clinical imaging what is the typical value for IRC?
<1%
Intensity Transmission Coefficient (ITC)
the percentage of intensity that passes in the forward direction when the beam strikes an interface between two media
In clinical imaging what is the typical value for ITC?
>99%
IRC+ITC=_____
100%
reflection only occurs at a normal incidence if...
the media on either side of the boundary have different impedances
at a normal incidence, no reflection will occur if ____
the two media have identical impedances
at a normal incidence, if the impedances are only slightly different a ______ reflection will occur
small
at a normal incidence, if the impedances are substantially different a ______ reflection will occur
large
With normal incidence, ______ of the sound is transmitted when the two media have the same impedances
ALL
With oblique incidence reflection ________ occurs
sometimes
What 2 physical principles always apply to reflection with oblique incidence?
conservation of energy; reflection angle = incident angle
What is conservation of energy?
Energy cannot be created or destroyed, only transferred
how does conservation of energy relate to reflection with oblique incidence?
the sum of the percentage of sound reflected and the percentage of sound transmitted must equal 100%
in cases of oblique incidences, the reflection angle ______ incident angle
always equals
in cases of oblique incidences, transmission of any or all of the beam is _______
uncertain
in cases of oblique incidences, if part of the wave is transmitted it may travel....
straight ahead or bend/ change direction (refraction)
refraction
The bending of a wave as it passes at an angle from one medium to another
Refraction occurs with ________ waves and _______ waves.
Light and sound
Refraction only occurs if two conditions are satisfied:
oblique incidence AND different propagation speeds of the two media
Snell's Law
quantifies the physics of refraction
Snell's Law of Refraction
sin( transmission angle) Speed medium 2
—————————————— = ——————————-
sin( incident angle) Speed medium 1
under what conditions will the transmission angle equal the incident angle?
oblique incidence AND equal propagation speeds
under what conditions will the transmission angle be greater than the incident angle?
when the speed of medium 2 is greater than the speed of medium one
under what conditions will the transmission angle be less than the incident angle
speed of medium 2 is less than the speed of medium 1
dB notation
dB notation
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