Term
| What are the causes of attenuation? |
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Definition
- Absorption
- Diffraction
- Reflection |
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Term
| Is refraction a cause of attenuation? |
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Definition
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Term
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Definition
| When US energy is converted into a different type of energy, such as heat or vibrations. |
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Term
| How much attenuation is absorption responsible for in soft tissues? |
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Definition
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Term
| What factors affect absorption? |
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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) |
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Term
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Definition
| Half Value Layer is the thickness of a material (tissue) that will cause an intensity drop by 1/2. |
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Term
| What is the dB drop for HVL? |
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Definition
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Term
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Definition
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Term
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Definition
- Frequency of the US
- The medium through which the US travels |
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Term
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Definition
| The ability to bend around objects. Waves have this ability, particles do not. |
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Term
| What does diffraction cause? |
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Definition
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Term
| How do we predict diffraction in soft tissues? |
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Definition
| We can't, it's impossible. |
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Term
| How much does diffraction effect attenuation in soft tissues? |
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Definition
| Not very much, it's negligibly small. |
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Term
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Definition
| The redirection of a sound wave. |
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Term
| What are the types of reflection? |
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Definition
- Specular reflection
- Diffuse reflection (back-scatter)
- Rayleigh's reflection (non-specular reflection) |
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Term
| What is responsible for ultrasound of images? |
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Definition
| Reflection, but more specifically specular reflection. |
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Term
| What is specular reflection? |
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Definition
| The reflection of a wave off a very smooth surface. |
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Term
| What are two important aspects of specular reflection? |
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Definition
- Direction (where sound is redirected)
- Amount of energy redirected (P,I,A) that's redirected and transmitted |
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Term
| What is the law of reflection? |
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Definition
| The angle of incidence = the angle of reflection. |
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Term
| What is desirable in order to maintain maximum amount of echoes returned. |
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Definition
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Term
| What is acoustic impedance? |
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Definition
| The amount of resistance a medium has to a sound wave. |
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Term
| What does a difference in acoustic impedances between two mediums do? |
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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. |
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Term
| What is the reflection coefficient? |
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Definition
| The fraction of the incident intensity that the reflected wave will contain. |
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Term
| What is transmission coefficient? |
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Definition
| The amount of incident intensity that the transmission intensity will have. |
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Term
| What is Diffuse scattering? |
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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. |
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Term
| What is Rayleigh scattering? |
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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. |
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Term
| What is the overall attenuation of different objects in comparison to others? |
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Definition
| air > bones > lung > soft tissue > water |
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Term
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Definition
| A change in the direction of propagation when a wave enters a different medium. |
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Term
| According to Snell's law, what does the angle of transmission depend on? |
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Definition
| It depends on the angle of incidence, as well as the velocities in both media. |
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Term
| What does refraction only occur with? |
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Definition
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Term
| What does the angle of refraction NOT depend on? |
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Definition
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Term
| Where does the angle of transmittance bend when moving from a fast medium to a slow medium (v1>v2)? |
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Definition
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Term
| What happens to the angle of transmittance if sound waves travel from a slow medium to a fast (v2 >v1)? |
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Definition
| It'll bend away from normal. |
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Term
| What is a critical angle? |
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Definition
| When an ANGLE OF INCIDENCE corresponds to a 90 degree transmitted angle. The transmitted angle refracts a beam that travels along the interface. |
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Term
| When is a critical angle possible? |
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Definition
| When sound travels from a slow to a fast medium. |
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Term
| What happens if the angle of incidence is greater than the critical angle? |
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Definition
| Then the beam will reflect back into medium 1, known as total internal reflection. |
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Term
| What happens to the beam if v1 = v2? |
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Definition
@ = angle
Then sin@incidence / sin@transmission = 1. |
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Term
| What happens to the beam if the @inc = 0 degrees? |
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Definition
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Term
| When is logarithmic scale used? |
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Definition
| The logarithmic scale is used when comparing two values of significant difference. |
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Term
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Definition
| A decibel is a relative logarithmic scale. It is used to represent a RATIO OF FINAL TO INITIAL VALUES (intensities, power, amplitude, etc.) |
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Term
| What does 3db correspond to? |
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Definition
| A doubling of compared values. |
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Term
What is attenuation?
Units? |
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Definition
Attenuation is a decrease in intensity and amplitude as it moves through the medium.
Units: dB |
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Term
| Since attenuation is a decrease in intensity, it's always..? |
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Definition
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Term
-3 dB | ?
-6 dB | ?
-9 dB | ?
-10 dB | ? |
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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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