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(B) Unit 2
MIT231
44
Medical
Undergraduate 2
06/04/2014

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Term
What are the causes of attenuation?
Definition
- Absorption
- Diffraction
- Reflection
Term
Is refraction a cause of attenuation?
Definition
NOPE
Term
What is absorption?
Definition
When US energy is converted into a different type of energy, such as heat or vibrations.
Term
How much attenuation is absorption responsible for in soft tissues?
Definition
Up to 80%.
Term
What factors affect absorption?
Definition
- Frequency (high f, means high absorption)
- Depth
- Viscosity (more viscous means more more absorption)
- Relaxation time (time it takes molecules to return to their original positions)
Term
What is HVL?
Definition
Half Value Layer is the thickness of a material (tissue) that will cause an intensity drop by 1/2.
Term
What is the dB drop for HVL?
Definition
3 dB
Term
Units for HVL?
Definition
cm
Term
What does HVL depend on?
Definition
- Frequency of the US
- The medium through which the US travels
Term
What is diffraction?
Definition
The ability to bend around objects. Waves have this ability, particles do not.
Term
What does diffraction cause?
Definition
An intensity decrease.
Term
How do we predict diffraction in soft tissues?
Definition
We can't, it's impossible.
Term
How much does diffraction effect attenuation in soft tissues?
Definition
Not very much, it's negligibly small.
Term
What is reflection?
Definition
The redirection of a sound wave.
Term
What are the types of reflection?
Definition
- Specular reflection
- Diffuse reflection (back-scatter)
- Rayleigh's reflection (non-specular reflection)
Term
What is responsible for ultrasound of images?
Definition
Reflection, but more specifically specular reflection.
Term
What is specular reflection?
Definition
The reflection of a wave off a very smooth surface.
Term
What are two important aspects of specular reflection?
Definition
- Direction (where sound is redirected)
- Amount of energy redirected (P,I,A) that's redirected and transmitted
Term
What is the law of reflection?
Definition
The angle of incidence = the angle of reflection.
Term
What is desirable in order to maintain maximum amount of echoes returned.
Definition
A normal incidence.
Term
What is acoustic impedance?
Definition
The amount of resistance a medium has to a sound wave.
Term
What does a difference in acoustic impedances between two mediums do?
Definition
- A difference in acoustic impedances will cause a fraction of the sound wave to be reflected.
- The bigger differences in acoustic impedance means the MORE sound wave will be reflected.
- Fraction of wave doesn't depend on order of impedance, it could pass through medium with a big impedance and then through a small one, or pass through a medium with a small impedance and then a large one, and the fraction of wave reflected will be the same no matter what.
Term
What is the reflection coefficient?
Definition
The fraction of the incident intensity that the reflected wave will contain.
Term
What is transmission coefficient?
Definition
The amount of incident intensity that the transmission intensity will have.
Term
What is Diffuse scattering?
Definition
Also known as backscattering it is the reflection of sound waves off a rough surface. Sound waves are still returned in the general direction of sound wave, but the echoes received by the probe are much weaker.
Term
What is Rayleigh scattering?
Definition
It is also known as non-specular reflection, it is when a sound wave to strikes a very small object in comparable size to the wavelength, the objects are given their own frequency and emit sound waves in all directions. This happens with RBC's.
- Proportional to the frequency squared to the frequency to the sixth.
Term
What is the overall attenuation of different objects in comparison to others?
Definition
air > bones > lung > soft tissue > water
Term
What is refraction?
Definition
A change in the direction of propagation when a wave enters a different medium.
Term
According to Snell's law, what does the angle of transmission depend on?
Definition
It depends on the angle of incidence, as well as the velocities in both media.
Term
What does refraction only occur with?
Definition
Oblique incidence
Term
What does the angle of refraction NOT depend on?
Definition
Impedence.
Term
Where does the angle of transmittance bend when moving from a fast medium to a slow medium (v1>v2)?
Definition
Bends toward the normal.
Term
What happens to the angle of transmittance if sound waves travel from a slow medium to a fast (v2 >v1)?
Definition
It'll bend away from normal.
Term
What is a critical angle?
Definition
When an ANGLE OF INCIDENCE corresponds to a 90 degree transmitted angle. The transmitted angle refracts a beam that travels along the interface.
Term
When is a critical angle possible?
Definition
When sound travels from a slow to a fast medium.
Term
What happens if the angle of incidence is greater than the critical angle?
Definition
Then the beam will reflect back into medium 1, known as total internal reflection.
Term
What happens to the beam if v1 = v2?
Definition
@ = angle

Then sin@incidence / sin@transmission = 1.
Term
What happens to the beam if the @inc = 0 degrees?
Definition
sin0/sin@tr = v1/v2 = 0.
Term
When is logarithmic scale used?
Definition
The logarithmic scale is used when comparing two values of significant difference.
Term
What is a decibel?
Definition
A decibel is a relative logarithmic scale. It is used to represent a RATIO OF FINAL TO INITIAL VALUES (intensities, power, amplitude, etc.)
Term
What does 3db correspond to?
Definition
A doubling of compared values.
Term
What is attenuation?
Units?
Definition
Attenuation is a decrease in intensity and amplitude as it moves through the medium.
Units: dB
Term
Since attenuation is a decrease in intensity, it's always..?
Definition
A negative number.
Term
-3 dB | ?
-6 dB | ?
-9 dB | ?
-10 dB | ?
Definition
-3dB = 1/2 I initial
-6dB = 1/4 I Initial
-9 dB = 1/8 I initial
-10 dB = 1/10 I initial
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