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Physics > Part 5-10 (up until midterm) > Flashcards

Part 5-10 (up until midterm) Flashcards

1
Q

What creates loudness or strength of sound

A

Amplitude and intensity

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2
Q

Attenuation is

A

The reduction in intensity and amplitude with increasing imaging depth

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3
Q

What is absorption

A

Sound transferred to heat

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4
Q

Attenuation increases with

A

Frequency

Path length

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5
Q

Attenuation is caused by

A

Absorption
Reflection
Scattering

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6
Q

Attenuation coefficient (ac)

A

The attenuation that occurs with each centimeter* the sound wave travels

Unit: dB/cm

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7
Q

If sound intensity decreases by 2 what is the attenuation

A

3 dB

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8
Q

If sound intensity decreases by 10 what is the dB

A

10 dB

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9
Q

What does the attenuation depend on?

A
  1. Type of tissue
  2. Length/distance of travel
  3. Frequency
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10
Q

dB vs intensity - what do you do

A

dB you +

Intensity you x

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11
Q

Attenuation coefficient for bone

A

ac = 6-9 heavy attenuation

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12
Q

Attenuation coefficient for water

A

ac= 0.0022 - weak attenuation

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13
Q

Compare attenuation in lung vs. bone vs. soft tissue

A

Attenuation is higher in lung/air than soft tissue

Attenuation is higher in bone than soft tissue

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14
Q

Describe the relationship between imaging depth and frequency

A

Imaging depth decreases with increasing frequency

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15
Q

As frequency ____ attenuation _____

A

As frequency increases, attenuation increases

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16
Q

The average attenuation coefficient for soft tissue is _____ dB/cm for each MHz of frequency

A

0.5 dB/cm

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17
Q

The slope of the TGC in soft tissue is how much? Explain

A

1 dB/cm * MHz

Soft tissue attenuated at 0.5 dB * MHz, but also attenuated again coming back

To compensate for both directions of attenuation, TGC must give back 1 dB/cm * MHz

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18
Q

What can you adjust to compensate for attenuation

A

Time gain compensation

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19
Q

What does TGC slope do?

A

Amplifies far field echoes to promote uniformity in liver tissue. Displayed graphically with increasing deflection to the right. The slope compensated less at the shallowest portion and more at the deepest portion of the organ

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20
Q

What is impedance (formula)

A

The density of a medium * propagation speed

Unit: Rayls

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21
Q

What does impedance determine

A

How much of an incident sound wave is reflected and how much is transmitted

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22
Q

Average soft tissue impedance is how many rayls

A

1,630,000 rayls

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23
Q

There is no reflection when

A

No reflection when two media have the same impedance with perpendicular impedance

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24
Q

IRC

A

Intensity reflection coefficient - how much of the wave is reflected

IRC = Ir/Ii x 100

25
Q

ITC

A

Intensity transmission coefficient - how much is transmitted into second medium

ITC = It/Ii * 100

OR

ITC = 1-IRC

26
Q

The larger the impedance the _____ the IRC and echo intensity

A

The larger the impedance the greater the IRC and echo intensity

27
Q

If IRC increases, ITC _____

A

If IRC increases, ITC decreases

28
Q

If impedance is zero, what does that mean

A

There is total transmission

29
Q

If impedance nears 100%, what does that mean

A

Almost total reflection
Examples:
Bone and diaphragm

30
Q

If there is a perpendicular incidence what is the incident angle

31
Q

What happens with oblique incidence

A

With oblique incidence, the sound is refracted at a boundary between media for which propagation speeds are different

32
Q

When will no refraction occur?

A

If propagation speeds are equal

If the angle of incidence is 0 (perpendicular)

33
Q

If C1 < C2, what is the relationship between the corresponding angles?

A

Incident angle is less than transmission angle

34
Q

Two requirements for refraction

A

1) oblique incidence

2) diff propagation speed on either side of medium

35
Q

What does refraction cause

A
  1. Lateral position errors

2. Duplication of aorta

36
Q

What two angles are always the same at a boundary

A

Incidence and reflection angles

37
Q

When does scattering occur?

A

1) not smooth - rough/heterogeneous

2) small surface

38
Q

Scattering depends on

A
  1. Frequency

2. Scatterers size

39
Q

Explain the relationship between frequency, wavelength with scattering/reflection

A

Increase frequency decreases wavelength. Thus the object will be bigger than the wavelength, so there will be less scattering, more reflection, more echoes coming back to the transducer which increase the intensity of echo

40
Q

Explain the condition when increasing frequency increases scattering

A

Rayleigh scattering - when the object, like red blood cells, are much smaller and increasing frequency causes more scattering

41
Q

Explain how contrast agents work

A

They produce strong harmonics because air bubbles contact and expand creating non-linear propagation

42
Q

To position the echo on the screen, machine needs 2 pieces of info

A
  1. Direction echo came from
  2. Distance to reflector/scatterer
    - machine measures time travelled
43
Q

What is the pulse-echo round trip time used to determine the distance to a reflector

44
Q

Constructive interface

A

In phase, Reinforce - Intensity much stronger than individual sound waves

45
Q

Destructive interface

A

Out of phase, cancel out

46
Q

Acoustic speckle

A

A form of acoustic “noise” - the result of constructive and destructive interference of scattered sound waves

47
Q

What is HVLT

A

The thickness of a medium that decreases the intensity of a beam by 50% or 3dB of the initial intensity

48
Q

When perpendicular bean is transmitted, what happens

A

The bean moves through the second medium in the same direction as the incident sound beam

49
Q

What does it mean if there is no reflection at a boundary?

A

All is transmission, no echo, the medium impedances are equal

50
Q

What is acoustic impedance? It’s symbol? It’s unit? Equation with units?

A

Acoustic impedance is a measure of the resistance of a medium to the transmission of sound. Symbol is z, rayls

51
Q

What must the reflective/transmission coefficients add up to?

52
Q

What is refraction? What determines it? Why is it important?

A

Refraction is a change in the direction of sound as it crosses a boundary. Determined by the different acoustic velocity/impedance in the diff mediums. It contributes a great deal to the formation of our image.

53
Q

What is a specular reflector? What anatomy would produce one?

A

Flat smooth boundary that reflects well (mirror-like) it is produced at the artery walls or diaphragm

54
Q

Describe why TGC is needed

A

Reflectors with equal reflection coefficient will not result in echoes of equal amplitude arriving at the transducer if their travel distances are different) bc sound weakens as it travels

55
Q

What’s the difference between TGC and overall gain

A

TGC is depth specific

56
Q

Factors affecting TGC setting

A

Tissue composition
Attenuation coefficient
Transducer frequency

57
Q

If a medium has an attenuation coefficient of 2db/cm, what would be the best choice for the slope of the TGC

A

4db/cm

Slope for TGC should be double the attenuation coefficient

58
Q

How is speckle produced

A

Constructive/ destructive interference patterns within the returning echoes producing darker and brighter dots on the screen

59
Q

Units for amplitude

A

Pascal,
N/cm2
Kg/cm3
Volts

Physics (9 decks)

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