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Ultrasound Physics > CH 17 - Harmonics and Contrast Agents > Flashcards

CH 17 - Harmonics and Contrast Agents Flashcards

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1
Q

the creation of an image from sound reflections at twice the frequency of the transmitted sound

A

harmonic imaging

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2
Q

is the fundamental frequency transmitted or received

A

transmitted

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3
Q

twice the transmitted frequency

A

harmonic frequency

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4
Q

harmonic frequency sound waves arise from _____ behavior

A

nonlinear

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5
Q

another name for harmonic frequency

A

second harmonic frequency

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6
Q

the image created by processing reflections that have the same frequency as the transmitted sound

A

fundamental image

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7
Q

the image created by processing reflections that are twice the fundamental frequency

A

harmonic image

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8
Q

two forms of harmonics that are important in diagnostic sonography

A

tissue harmonics

contrast harmonics

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9
Q

harmonic imaging is most useful in improving what

A

image quality

because harmonic frequency waves undergo less distortion than fundamental sound waves

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10
Q

linear systems respond in an even manner

A

linear behavior

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11
Q

other words for linear

A

proportional or symmetrical

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12
Q

a system is nonlinear when it behaves unevenly

A

nonlinear behavior

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13
Q

other names for nonlinear

A

irregular or disproportionate

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14
Q

does harmonic frequency sound arise from linear or nonlinear behavior

A

nonlinear

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15
Q

as a sound wave travels into the body, a small amount of energy is converted from the fundamental frequency to the harmonic frequency. this conversion creates _____ during transmission

A

tissue harmonics

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16
Q

why does sound travel slightly at uneven speeds through soft tissue

A

travels faster as it comprises and slower through rarefactions

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17
Q

this variation in speed creates ____ in the biologic media

A

tissue harmonics

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18
Q

the variation in speed alters the shape of the sound beam and a small amount of energy is transferred from ___ frequency to ___ frequency

A

fundamental to harmonic

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19
Q

the strength of the harmonic wave (increases/decreases) as it travels through tissue

A

increases

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20
Q

what is the significance of the gradual development of a harmonic signal?

A

harmonic signals cannot distort because they do not exist in the superficial depths

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21
Q

the use of harmonics (increases/decreases) signal to noise ratio?

A

increases

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22
Q

why do strong artifacts appear within the first few centimeters of tissue during fundamental imaging?

A

the beam is very strong and the many different superficial anatomic layers distort the sound beam

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23
Q

Is there another form of nonlinear behavior that further minimizes distortion with tissue harmonics?

A

the relationship between the sound beam strength and harmonic creation is nonlinear

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24
Q

what would happen if it were linear?

A

weak beams would create weak tissue harmonics. medium strength beams would create medium harmonics, and strong beams would create strong harmonics

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25
Q

Why do only strong sound beams create harmonic signals?

A

harmonics are only produced in the sound beams main axis, where the beam is strong

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26
Q

beams that are most likely to create harmonics are least likely to create _____

A

artifacts

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27
Q

tissue harmonics are not present as sound leaves the transducer. Where are they created?

A

in deep tissues during transmission

28
Q

what is the behavior of harmonics?

A

nonlinear

29
Q

harmonic frequencies are not created at the skins surface. this avoids what?

A

distortion and reduces noise

30
Q

an imaging technique that is specifically designed to utilize harmonic reflections, which are distortion free, while eliminating distorted fundamental reflections

A

pulse inversion harmonics

31
Q

this type of imaging creates images from the portion of the reflected sound that has the same frequency as the transmitted sound

A

fundamental imaging

32
Q

type of imaging that creates images only from the harmonic, which is twice the frequency of the transmitted sound

A

harmonic imaging

33
Q

instead of filters, we now use _________ to separate the harmonics from the fundamental frequencies

A

pulse inversion harmonics

34
Q

in pulse inversion harmonics, two consecutive ultrasound pulses are transmitted down each scan line. what are these pulses

A

first is a typical pulse

second is an inverted copy that is out of phase with the first

35
Q

wherever a compression exists in the first pulse, a _______ exists in the second pulse

A

rarefaction

36
Q

the reflections from these two pulses are combined where

A

in the receiver

37
Q

since the two waves are linear and one is out of phase, what happens to the waves

A

they destructively interfere and completely cancel each other out

38
Q

when these waves cancel out, the fundamental portions disappear but what portion stays?

A

the harmonic portion

39
Q

disadvantage of pulse inversion harmonics

A

2 pulses are transmitted down each scan line and twice the number of pulses are required to create each image which means the time required to create a Fram e is doubled, the frame rate is halved, and the temporal resolution is reduced

40
Q

does pulse inversion harmonics increase or decrease temporal resolution

A

decrease

41
Q

What is power modulation in harmonic imaging?

A

With power modulation, the harmonics are extracted by sending one strong pulse and one weak pulse down each scan line.
The fundamental portions of each wave completely cancel each other out and leave only the harmonic portion of the reflection. The harmonics create an image with less distortion, the frame rate is halved, and temporal resolution is decreased

42
Q

gas bubbles that are encapsulated in a shell that are injected or ingested

A

contrast agents or microbubbles

43
Q

what do contrast agents do

A

light up blood chambers, vessels, or other anatomic regions for diagnostic purposes

44
Q

5 requirements that contrast agents must meet

A 
safe
metabolically inert
long lasting
strong reflector of ultrasound
small enough to pass through capillaries
45
Q

are contrast harmonics created during transmission or reflection

A

reflection

46
Q

contrast harmonics are created during reflection as energy is converted from ____ frequency to ____ frequency

A

fundamental to harmonic

47
Q

what nonlinear behavior of a micro bubble creates contrast harmonics?

A

resonance

48
Q

what is resonance

A

when bubbles expand and compress when exposed to sound beams

49
Q

when exposed to high pressure, bubbles ____

A

compress

50
Q

when exposed to low pressures, the bubbles ___

A

expand

51
Q

bubbles expand (more or less) than they shrink

A

more

52
Q

because bubbles expand more than they shrink, this uneven, nonlinear change, a small amount of energy is transferred from the ___ frequency to the ___ frequency

A

fundamental to harmonic

53
Q

what does rarefaction of a sound wave do to a bubble

A

expands the bubble

54
Q

What is mechanical index?

A

a calculated number that is related to the likelihood of harmful bio effects from cavitation
depends on the frequency of the transmitted sound and the rarefaction pressure of the sound wave
effects the amount of contrast harmonics produced

55
Q

Higher MI

A 
large pressure variation
low frequency
strong harmonics
nonlinear behavior
high beam strength
bubble greatly expands
56
Q

Lower MI

A 
small pressure variation
higher frequency
low beam strength
bubble doesn't expand much
linear behavior
backscatter
no harmonics
57
Q

Why are micro bubbles strong scatterers of sound waves?

A

they resonate (oscillate) when exposed to sound in the 2-4 MHz range

58
Q

Is there another form of nonlinear behavior of a micro bubble that creates contrast harmonics?

A

yes
the relationship between the mechanical index and the harmonic creation is nonlinear
(if it was linear, a low MI would create weak harmonics, etc)

59
Q

Why don’t low MI sound beams create harmonics?

A

micro bubbles expand and contract evenly in a linear fashion

60
Q

why do beams with a high MI create harmonics?

A

the bubbles expand and may burst (extreme nonlinear behavior) which creates strong harmonic reflections

61
Q

What are the most important characteristics of contrast agents when used with harmonic imaging?

A

the nature of the outer shell

the gas that fills the microbubble

62
Q

How do the strengths of tissue and contrast harmonics compare?

A

contrast harmonics created by micro bubbles are much stronger than tissue harmonics

63
Q

Summary of contrast harmonics

A

created during reflection
nonlinear behavior
related to MI
affected by micro bubble’s shell and the gas within it

64
Q

tissue harmonics

A

transmission
occurs during sound propagation
results from nonlinear behavior of sound beam
weak harmonic signal

65
Q

contrast harmonics

A

reflection
occurs when contrast agents are present and when MI greater than 0.1
nonlinear behavior from microbubble
strong harmonics

Ultrasound Physics (24 decks)

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