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LMC Ultrasound > Physics Review Ch. 5, 6, 7 > Flashcards

Physics Review Ch. 5, 6, 7 Flashcards

1
Q

How do you calculate intensity?

A

a beam’s power divided by its area.

units: W/cm^2

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2
Q

What are other names for normal incidence?

A

perpendicular
orthogonal
right angle
90 degrees

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3
Q

Difference between normal and oblique incidence

A

normal incidence strikes the boundary at exactly 90 degrees

oblique incidence strikes the boundary at any angle other than 90 degrees (acute and obtuse)

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4
Q

Define incidence intensity

A

The sound waves intensity immediately before it strikes a boundary

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5
Q

Define reflected intensity

A

Is the intensity of the portion of the incident sound beam that, after striking a boundary, returns back from where it came

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6
Q

Define transmission intensity

A

Is the intensity of the portion of the incident beam that, after striking a boundary, continues forward in the same general direction that it was traveling

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7
Q

IRC

A

Intensity Reflection Coefficient; the percentage of intensity that bounces back, after it strikes the boundary between 2 media. (1%)

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8
Q

ITC

A

Intensity Transmission Coefficient; the percentage of intensity that passes in the forward direction between 2 media. (99%)

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9
Q

Define Impedance

A

The acoustic resistance to sound traveling in a medium. Calculated by multiplying the density of a medium by the speed at which sound travels in the medium.
Units: rayls; Z

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10
Q

Impedance-what is takes to achieve reflection

A

Reflection of an ultrasound wave depends upon the difference in acoustic impedances of the two media at a boundary.

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11
Q

Define specular reflectors

A

When the boundary is smooth, the sound is reflected in only on direction in an organized manner

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12
Q

Conservation of energy

A

Applies at the boundary between two media.

IRC + ITC = 100%

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13
Q

Reflection and transmission with oblique incidence

A

We cannot predict; it’s too complex

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14
Q

Define Refraction

A

Is a change in direction of wave propagation when traveling from one medium to another

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15
Q

Refraction occurs only when there is what?

A
  1. Oblique Incidence. (not normal)

2. Different Propagation speeds between the media.

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16
Q

What is refraction associated with?

A

Transmission

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17
Q

Attenuation

A

The decrease in intensity, power, and amplitude as sound travels.
Units: dB

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18
Q

Attenuation is determined by?

A
  1. Path length

2. Frequency of sound

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19
Q

What three processes contribute to attenuation?

A
  1. Reflection
  2. Scattering
  3. Absorption
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20
Q

What is impedance determined by?

A

Medium

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21
Q

Define scattering

A

Is the random redirection of sound in many directions. (Disorganized and chaotic)

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22
Q

Define Rayleigh scattering

A

Is a special form of scattering that occurs when the structure’s dimensions are much smaller than the beam’s wavelength. (Organized and omnidirectional)
*omnidirectional-in all directions

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23
Q

Relationship between scattering and frequency

A

directly related; Rayleigh scattering is proportional to frequency^4

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24
Q

Formula for Impedance

A

Impedance (rayls) = density (kg/m^3) * prop. speed (m/s)

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25
Q

Formula for IRC and ITC

A

100% = IRC + ITC

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26
Q

What are other names for decibel notation?

A

relative measurement
comparison
ratio
logarithmic

27
Q

Half-Value Layer

A

Is the depth of tissue that results in 3 dB of attenuation to the intensity.
May be called: penetration depth, depth of penetration, and half-boundary layer
Units: cm

28
Q

Half-value layer thickness depends on what two factors?

A
  1. The medium

2. The frequency of sound

29
Q

Half-Value Layer conditions

A 
Thin Half Value:
 -High frequency sound
 -Media with high attenuation rate
Thick Half Value:
 -Low frequency sound
 -Media with low attenuation rate
30
Q

Five key words that are related to intensity

A
  1. Spatial
  2. Peak
  3. Average
  4. Temporal
  5. Pulsed
31
Q

Six methods for measuring intensity

A
  1. SPTP
  2. SATP
  3. SPTA
  4. SATA
  5. SPPA
  6. SAPA
32
Q

What is the most relevant intensity with respect to tissue heating?

A

SPTA

33
Q

Importance of the study of bioeffects

A

The different methods evaluate tissue exposure to sound energy.

34
Q

T/F Spatial peak intensity is always higher than the spatial average intensity

A

True

35
Q

For continuous wave ultrasound, which intensities are the same?

A

Since the beam is always “on” the pulse average and temporal average intensities are the same. SPTA=SPPA and SATA=SAPA

36
Q

Rank of intensities from largest to smallest

A

SPTP -> Im -> SPPA -> SPTA -> SATA

37
Q

When a wave’s intensity doubles, the relative change is

A

+3 dB

38
Q

When intensity increases ten-fold, the relative change is

A

+10 dB

39
Q

When the intensity is reduced to one-half its original value, the relative change is

A

-3 dB

40
Q

When the intensity is reduced to one-tenth its original value, the relative change is

A

-10 dB

41
Q

Attenuation conditions

A 
More attenuation:
 -Longer distances
 -Higher frequencies
Less attenuation:
 -Shorter distances
 -lower frequencies
42
Q

A 15 MHz transducer is used for what?

A

Small parts- breast, thyroid, scrotum

43
Q

What frequency is used for vascular?

A

7 MHz-12 MHz

44
Q

When examining AB/OB the frequency should be between

A

3 MHz-6MHz

45
Q

Attenuation is not related to?

A

propagation speed

46
Q

Define Attenuation coefficient

A

Is the number of decibels of attenuation that occurs when sound travels one centimeter.
Units: dB/cm

47
Q

When sound travels to a depth of 5 cm, the attenuation coefficient remains 2 dB/cm. What is the total attenuation?

A 
10 dB
(5 cm * 2 dB/cm = 10 dB)

Total attenuation (dB) = atten. coef. (dB/cm) * distance (cm)

48
Q

Attenuation coefficient in soft tissue

A

Attenuation Coefficient = 0.5 dB/cm/MHz

49
Q

Attenuation in media

A 
Water- extremely low
Blood, urine- low
Fat- low
Soft tissue- intermediate
Muscle- higher
Bone and lung- even higher
Air- extremely high
50
Q

At the boundary, the reflected and transmitted intensities are added together. The result must equal?

A

The incident intensity

51
Q

If the intensity reflection coefficient is 45%, what percent of sound is transmitted into the body?

A

55%

45% + ? = 100%

52
Q

Refraction does not occur when

A

The speeds of the two media are identical..

The angles of incidence and transmission will be equal.

53
Q

When will the transmission angle be greater than the incident angle?

A

When the speed of medium 2 is greater than the speed of medium 1

54
Q

When will the transmission angle be less than the incident angle?

A

When the speed of medium 2 is less than the speed of medium 1

55
Q

Define reflection

A

As sound strike’s a boundary, a portion of the wave’s energy may be redirected back to the sound source

56
Q

Two types of reflection

A
  1. specular

2. diffuse or backscatter

57
Q

Diffuse reflection

A

When a wave reflects off an irregular surface, it radiates in more than one direction

58
Q

A sound wave with an intensity of 30 W/cm^2 strikes a boundary and is totally reflected. What is the intensity reflection coefficient?

A

100%, since the wave is totally reflected

59
Q

Conditions for impedance

A

Media have identical impedance-no reflection
Slightly different impedance-small reflection
Substantially different impedance-large reflection

60
Q

What is Time of Flight?

A

elapsed time from pulse creation to pulse reception. Also called go-return time

61
Q

What is the 13-microsecond rule?

A

for every 13 microseconds of go return time, the object creating the reflector is 1cm deeper in soft tissue.

62
Q

Relationship between time-of-flight and how deep a sound pulse travels

A

They are directly related. Greater distances prolong the time-of-flight. Lesser distances shorten the time-of-flight

63
Q

Snell’s Law

A

It quantifies the physics of refraction.

sin (transmission angle)/sin (incident angle) = speed of Medium 2/speed of Medium 1

LMC Ultrasound (32 decks)

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