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Ultrasound Physics > 5 Sound + Media > Flashcards

5 Sound + Media Flashcards

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1
Q

What is attenuation?

A

Decrease in intensity, power and amplitude of a sound wave as it travels

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2
Q

What occurs the further US travels?

A

More attenuation

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3
Q

What is associated with less attenuation?

A

Lower frequency

Shorter path length

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4
Q

What is associated with more attenuation?

A

Higher frequency

Longer path length

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5
Q

What are the 3 components of attenuation?

A

Absorption
Scattering
Reflection

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6
Q 
List the following in greatest to least attenuation.
Water
Bone & Lung
Air
Soft tissue
A

Air&raquo_space; Bone & Lung > Soft tissue&raquo_space; Water

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7
Q

What does attenuation ultimately limit?

A

Max depth from which meaningful reflections are obtained

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8
Q

When does reflection occur?

A

Propagating sound energy strikes a boundary between 2 media and some returns to the transducer

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9
Q

What is specular reflection?

A

Reflections from a smooth reflector (mirror) and return in one direction

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10
Q

What is diffuse reflection or backscatter?

A

Reflection of sound in all directions

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11
Q

When does backscatter occur?

A

When the boundary has irregularities that are approx the same size as the sound’s wavelength

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12
Q

What is Rayleigh scattering?

A

When a reflector is much smaller than the wavelength of sound, sound is uniformly distributed in all directions (omnidirectional)

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13
Q

What is Rayleigh scattering related to?

A

Frequency^4

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14
Q

What is the attenuation coefficient?

A

Amount of attenuation per centimeter (dB/cm)

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15
Q

What is the relationship between frequency and attenuation coefficient?

A

Proportional

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16
Q

List the equation for total attenuation.

A

Total attenuation (dB) = path length (cm) x attenuation coefficient (dB/cm)

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17
Q

What is the attenuation coefficient in soft tissue?

A

Approx 1/2 of the the frequency (MHz)

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18
Q

What is impedance?

A

A number associated with a medium - it is calculated, not measured
Opposition (density) that the medium offers

19
Q

What is the unit for impedance?

A

Rayls (Z)

20
Q

What is the equation for impedance?

A

Impedance (rayls) = density (kg/m^3) x propagation speed (m/s)

21
Q

What does the reflection of an ultrasound wave depend upon?

A

Different acoustic impedances of the media on either side of the boundary

22
Q

What is normal incidence?

A 
"PORNN'
Perpendicular
Orthogonal
Right angle
Ninety degrees
23
Q

What is incident intensity?

A

Intensity of the sound wave at the instant prior to striking a boundary

24
Q

What is reflected intensity?

A

Portion of the incident intensity that, after striking a boundary, changes direction and returns back from where it came

25
Q

What is transmitted intensity?

A

Portion of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling

26
Q

List the equation for incident intensity.

A

Incident intensity = reflected intensity + transmitted intensity

27
Q

What is the intensity reflection coefficient?

A

The percentage of the US intensity that bounces back when the sound strikes a boundary

28
Q

What is the intensity transmission coefficient?

A

The percentage of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling

29
Q

What occurs at the boundary between 2 media?

A

“Conservation of energy”
When IRC and ITC are added, the result is 100%
When reflected and transmitted intensities are added, the result is the incident intensity

30
Q

What is the % reflection of the following biologic media?
Soft tissue - air
Soft tissue - bone
Soft tissue - soft tissue

A

99%
50%
<1%

31
Q

What is needed for reflection of normal incidence?

A

Reflection occurs only if the 2 media at the boundary have different acoustic impedances

32
Q

List the equation of the intensity reflection coefficient.

A

Intensity Reflection Coefficient (%) = [(Z2-Z1)/(Z2+Z1)]^2

33
Q

What is transmission with normal incidence?

A

Whatever is not reflected in normal incidence

34
Q

What occurs with reflection & transmission with oblique incidence?

A

Extremely complex physics regarding transmission & refection with obliquity

35
Q

What occurs with the Law of Conservation of Energy?

A

Reflection angle = Incident angle

36
Q

What is refraction?

A

Transmission with a bend

Change in direction as sound transmits from 1 medium to another

37
Q

What does refraction require?

A

Oblique incidence

Different speeds

38
Q

What does Snell’s Law describe?

A

Physics of refraction

39
Q

What is Snell’s law equation?

A

sin (transmission angle)/sin (incident angle) = propagation speed 2/propagation speed 1

40
Q

What is required for reflection with normal incidence?

A

Look for different impedances

41
Q

What is required for reflection with oblique incidence?

A

We do not know

42
Q

What is required for transmission?

A

Derive this from reflection information

43
Q

What is required for refraction?

A

Look for oblique incidence and different speeds

Ultrasound Physics (20 decks)

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