Physics Test 2
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What is the mathematical equation for acoustic impedance?
- z = ρ x c
- Acoustic impedance (z) is the product of density (ρ) and velocity (c)
What are the mathematical relationships of the variables in the acoustic impedance equation?
acoustic impedance, velocity and density are directly related
What are the units of measurement for acoustic impedance?
What is an interface?
Boundry of 2 media with different z values
What is the typical per cent reflection for a soft tissue/soft tissue interface, soft tissue/bone, and soft tissue/air?
- soft tissue/soft tissue: 1-15%
- soft tissue/bone: 60-65%
- soft tissue/air: 99.9%
Describe how the specific or characteristic acoustic impedance values change for gas, liquid, and solid media.
Solids have the highest acoustic impedance values, gasses have the lowest. (acoustic impedance= density x velocity)
What is the equation used to obtain the reflection coefficient?
What is normal incidence?
Direction of sound travel is perpendicular (90°) to interface
What is oblique incidence?
sound arrives at interface at any angle other than 90°
Describe the characteristics of a specular reflector.
- larger than incident wave length
- regular boarders
Describe the characteristics of a non-specular reflector.
- smaller than incident wave length
- irregular boarders
What is Rayleigh scatter?
- Very small reflector.
- Increase the frequency>> greater amount of Rayleigh scatter
If the reflected intensity is 5%, what is the transmitted intensity?
Provide clinical examples of a specular reflector.
- organ capsules
- vessel walls
Provide clinical example of a non-specular reflector.
What is the law that characterizes refraction?
What is the definition of attenuation?
Reduction in intensity as sound energy travels though a medium.
What are the two primary conditions that cause refraction?
List the primary factors that contribute to attenuation.
What is the primary cause for attenuation in soft tissue?
What is the primary cause for attenuation in bone?
absorption and reflection
What is the primary cause for attenuation in air?
reflection and scatter
Describe the role of transducer frequency in determining the attenuation rate.
increase the frequency (increase absorption rate) and increase the attenuation rate
What is the attenuation coefficient?
attenuation coefficient (dB/cm) = 0.5 x frequency (MHz)
What type of body tissue has the highest absorption coefficient?
What type of body tissue has the lowest absorption coefficient?
What type of body tissue has the highest attenuation rate? Why?
air/lung: because 99.9% is reflected and what remains is scatter.
Characterize acoustic enhancement in terms of acoustic enhancement.
lower attenuation rate
Characterize acoustic enhancement in terms of acoustic shadowing.
higher attenuation rate
What is the half intensity depth?
Distance required to decrease the sound intensity to 1/2 of the beginning value
What is the attenuation rate in soft tissue (0.5dB/cm) what the operating frequency is 5 MHz?
0.5 x 5 =2.5
If the transmitted intensity has been reduced to 10%, this is the equivalent to ___ dB.
What is the total attenuation in soft tissue (0.5 dB/cm) using 3.0 MHz at a depth of 10cm?
0.5 x 3 x 10 = 15
What is the relationship between frequency and the attenuation coefficient?
the greater the frequency the greater the attenuation rate
What is the mathematical relationship between path length and total attenuation?
the greater the path length the greater the attenuation rate
What is the relationship between attenuation coefficient and the total attenuation?
as the attenuation coefficient increases so does the total attenuation
What is the mathematical relationship between attenuation rate in tissue and the attenuation rate/cm?
the greater the tissues attenuation rate the greater the attenuation coefficient
Compare the HVL thickness at 3.0 MHz versus 6.0 MHz
double the frequency, half the HVL thickness
Compare the HVL thickness for soft tissue vs. bone.
- thicker for soft tissue (lower attenuation rate)
- thinner for bone (higher attenuation rate)
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