Chapter 6-7 Flashcards
What is attenuation
weakening of the sound beam due to a decrease in intensity, power, and amplitude as sound travels through the body.
With attenuation, the weak reflections sent back to the transducer need to be amplified by the ultrasound system. How do we do this?
We use decibels to quantify the degree of attenuation or amplification
What are characteristics of decibles (dB) ?
They are logarithmic, a relative measurement, and a ratio
What kind of intensity do decibles require?
They require 2 intensities: a starting level and a level at time of
measurement
What does a positive dB mean?
the signal is increasing in strength or getting larger. +3 means it doubles and +10 means is multiplies by 10
What does a negative dB mean?
The signal is getting weaker or smaller. -3 means 1/2 and -10 means the signal has decreased 1/10 its original value .
What determines attenuation?
Path length and frequency. More distance and more frequent will lead to more attenuation.
What is the equation for total attenuation?
dB = attenuation coefficient x cm
What is the attenuation coefficient? Does it change or remain constant?
The number of decibels of attenuation that occurs in 1 cm. (measured in dB/cm) It remains constant regardless of how far the sound has travelled overall.
The attenuation coefficient does not change with depth. Is it related to frequency? What is the equation that proves this?
Yes it is directly related to frequency. Attenuation coefficient = frequency/2
What 3 processes contribute to attenuation?
Reflection, scattering, and absorption
What is half-value layer thickness?
the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value
When would the half-value layer be thin?
in tissues that attenuate sound greatly such as lung or bone or with high frequency.
Half-value layer depends on what two factors?
medium and frequency
What is reflection? How does it affect the beam?
Reflection is the portion of energy that is reflected back to the transducer. It weakens the beam that continues forward.
What are the necessary and limiting components of specular reflection?
Sound must have a smooth interface larger than the beam. It must have a normal incidence and impedence mismatch. It is limited if the beam is not 90 degrees, because the wave will not return to the transducer.
What are synonyms for when the sound strikes a boundary at 90 degrees?
Normal incidence, perpendicular, orthogonal, right angle
What is impedance
the acoustic resistance to sound traveling in a medium
What is the impedence equation?
Rayls= density x propagation speed
Diffuse reflection (backscatter) is when a wave bounces off an irregular surface. What are the necessary componants?
surface is larger than the beam, radiates in more than one direction, irregular-not smooth, impedence mismatch
If a beam is not 90 degrees, can a diffuse reflection still be produced?
Yes.
Which is weaker diffuse or specular reflection?
Diffuse reflected beams are weaker than specular reflections
What is scattering? When does it occur?
Random redirections of sound in many directions. It occurs when the boundary is small (equal or less than the beam’s wavelength)
Scattering is directly related to which property?
frequency.
What is rayleigh scattering? When would we find this clinically?
A special form of scattering where the bounday dimensions are much smaller than the wavelength of the beam. This is used when ultrasound imaging Red blood cells.
How does Rayleigh Scattering redirect or reflect sound?
equally and organized in all directions?
How is rayleigh scattering mathematically related to frequency?
Directly related to frequency ^4. When frequency x 2 = rayleigh scattering x 16.
What is absorption? How is it related to frequency?
Ultrasonic energy is converted into another form of energy. Bone undergoes excessive absorption- we can’t see through it. Absorption is directly related to frequency.
What is incident intensity? What is its equation?
sound wave’s intensity
immediately before it strikes a boundary. Incident intensity = reflected + transmitted because of conservation of energy.
What is reflected intensity?
the intensity of the portion of
the beam that is reflected after it strikes a boundary
What is transmitted intensity?
the intensity of the portion
of the beam that continues forward after striking a boundary
What is the intensity reflection coefficient (IRC) and intensity transmission coefficient (ITC)?
the percentage of the
intensity that bounces back when
sound strikes a boundary is IRC. The percentage that continues is ITC. They add up to 100%.
What is refraction?
Refraction is the change of a wave’s direction when moving from one media to another.
What are the two conditions for refraction? What happens when both are met?
Oblique incidence and different propagation speeds for the two mediums. If both conditions are met, the transmitted sound beam will bend.
What is the difference between mediums 1 and 2 in Snell’s law?
medium 1 is where the sound is currently traveling. medium 2 iis where the sound is entering.
What happens to refraction when there is an oblique incidence but the medium speeds are the same?
there will not be refraction if the speeds of the media are equal.
What happens with refraction when Speed 1 is less than Speed 2
The transmission angle will be greater than the incident angle. The transmitted beam is bending AWAY
from normal incident
What happens with refraction when Speed 2 is less than Speed 1
the transmission angle will be less than the incident angle. The transmitted beam bends towards
normal incident.
What is the echo ranging theory equation? What does this mean?
Distance = velocity x time. We can measure distance by knowing how fast something traveled and how long it took.
What is go-return time directly related to?
Depth. The deeper the structure, the longer the go-return time
What is the go-return time equation?
depth = (1.54 x go-return time) / 2
The 13 microsecond rule applies when sound travels through soft tissue. What does it mean?
For every 13 microseconds of go-return time, the object is 1 cm deeper in the body.
