Chapter 17 Flashcards by Kristin Phillips

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

Harmonic imaging

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Fundamental frequency

the frequency of sound created by the transducer and transmitted into the body

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twice the fundamental frequency

Harmonic frequency

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Harmonic frequency sound waves arise from _ behavior

Nonlinear

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Fundamental image

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

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

Harmonic image

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A portion of clinical studies are suboptimal because of _

sound beam distortion.

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Harmonic imaging improves poor quality images because _

harmonic frequency waves undergo less distortion than fundamental sound waves.

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Two forms of harmonics:

Tissue harmonics

Contrast harmonics

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proportional or symmetrical

Linear

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_ systems respond in an even manner

Linear

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irregular or disproportionate, asymmetrical

Nonlinear

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A system is nonlinear when it behaves _

Unevenly

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Harmonic frequency sound arises from _ behavior

Nonlinear

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As a sound wave travels in the body, a miniscule amount of energy is converted from _ to _. This conversion creates tissue harmonics during _

the fundamental frequency to the harmonic frequency.

Transmission

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Sound travels faster through _ and slower through _

Compressions

Rarefactions

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_ creates tissue harmonics in the biologic media and alters the shape of the sound beam.

Nonlinear variation in speed

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The strength of the harmonic wave _ as sound travels in tissue

Increases

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During fundamental imaging, strong artifacts appear in the first few centimeters of tissue due to:

The beam being very strong
Many different superficial anatomical layers distort the
sound beam

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Do tissue harmonic signals exist at superficial depths?

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The use of harmonics improves _

Signal to noise ratio

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The relationship between sound beam strength and harmonic creation is

Nonlinear

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Do weak sound beams create tissue harmonics?

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Intermediate strength beams create _ amount of tissue harmonics

Only a tiny

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Strong sound beams create _ tissue harmonic signals

Significant

_ are created when sound beams are distorted

Side/grating lobes

_ harmonics are created in the side/grating lobes

Very few

The main beam is strong and creates _ harmonics. This creates an image with _ distortion

Strong | Less

Beams that are most likely to create harmonics are _ likely to create artifacts

Least

Harmonic imaging improves the image quality in 3 primary ways:

1. The harmonic beam is much narrower 2. Grating lobe artifacts are eliminated 3. Improves signal-to-noise ratio

An imaging technique specifically designed to utilize harmonic reflections while eliminating distorted fundamental reflections

Pulse inversion harmonics

Disadvantage of pulse inversion

Degradation of temporal resolution due to requiring two pulses per scan line

Another technique designed to augment harmonic reflections while eliminating fundamental reflections. Two consecutive ultrasound pulses are sent down each scan line. The second pulse is twice the strength as the first pulse.

Power modulation harmonics

Power modulation reduces

Distortion and temporal resolution

Contrast agents are designed to _

create strong reflections that “light up” blood chambers, vessels, or other anatomic regions

Must meet the following 5 requirements:

1. Safe 2. Metabolically inert 3. Long lasting 4. Strong reflector of ultrasound 5. Small enough to pass through capillaries

As an ultrasound pulse interacts with contrast agents, _ are created

Much stronger harmonics

Contrast harmonics are created because

microbubbles act in a nonlinear manner when struck by ultrasound waves

Contrast harmonics are created during _ as_

Reflection | energy is converted from the fundamental frequency to the harmonic frequency

_ harmonics are stronger than _ harmonics

Contrast Tissue

When a microbubble is within a sound beam, the bubble grows and shrinks in relation to

the pressure variations (during compressions and rarefactions)

Contrast harmonics are created due to

the nonlinear changes in the bubble size.

The uneven behavior that causes contrast harmonics is called _

Resonance

Contrast harmonics: When exposed to high pressure (compressions), the bubble _ and the pressure

Shrinks | Increases

Contrast harmonics: When exposed to low pressure (rarefactions), the bubble

Expands

Bubbles _ to a greater extent than they _

Expand | Shrink

_ pressure is most important with regard to contrast harmonics

Peak rarefaction

A number related to the likelihood of harmful bioeffects from cavitation (bubble bursting)

Mechanical index

Mechanical index can estimate _

the amount of contrast harmonics produced

Mechanical index depends on

the frequency of the transmitted sound and the rarefaction pressure of the sound wave

MI =

peak rarefaction pressure/(square root of freq)

MI increases with

Lower frequency sound | Stronger sound waves (substantial pressure variation)

Lower MI

Small pressure variation | Higher frequency

Higher MI

Large pressure variation | Lower frequency

Low MI sound beams do not create harmonics because

the microbubbles expand and contract in a linear fashion.

If the MI is less than 0.1, bubbles only create

Backscatter

Intermediate MI’s (0.1 to 1.0) create a _ amount of harmonics

Relatively small

MI’s greater than 1 create _ harmonic signals. The bubbles may actually

Substantial | Expand and break apart

``` Low MI: <0.1: _ harmonics _ _ behavior _ frequency sound _ beam strength Bubbles expand _ ```

``` No Backscatter Linear Higher Low Very little ```

``` Higher MI: 0.1-1.0 _ harmonics _ _ behavior _ frequency sound _ beam strength Bubbles expands _ ```

``` Some Resonance Nonlinear Lower Higher Moderately ```

``` Highest MI > 1 _ harmonics _ _ behavior _ frequency sound _ beam strength Bubbles expands _ ```

``` Strongest Bubble disruption Extreme Lowest Highest Greatly ```

Two characteristics of contrast agents are important:

The nature of the outer shell | The gas that fills the microbubble

_ trap the gas and increase the effective life of a microbubble

Shells

Shells are designed to be

flexible so they can change shape

Smaller gas molecules are _ likely to leak through the shell and shrink the bubble.

Larger gas molecules remain _

Trapped in the bubble

``` Tissue harmonics: Created during_ Occurs as _ Results from_ _ harmonic signal ```

Transmission in tissue Sound propagates in tissue Nonlinear behavior of transmitted sound beam Weaker

``` Contrast harmonics: Created during _ Occurs only when_ Results from _ _ harmonic signal ```

Reflection off of microbubble Contrast agents are present and with MIs greater than 1 Nonlinear behavior of micro bubbles Stronger

Chapter 17 Flashcards

(68 cards)