Chapter 17 Harmonics and Contrast Agents

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Chapter 17Harmonics and Contrast Agents
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Harmonic Imaging What is it?

• The creation of an image from sound reflections
  at twice the frequency of the transmitted sound.
• Fundamental frequency transmitted sound
  frequency
• Harmonic frequency utilized to create the image
  is twice the fundamental frequency
• Aka, second harmonic frequency
• Harmonic frequency sound waves arise from
  non-linear behavior

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Fundamental vs. Harmonic Image

• Fundamental image
• Created by processing reflection having the same
  frequency of the transmitted sound
• Harmonic image
• Created by processing reflections having twice
  the fundamental frequency
• Two forms of harmonics in diagnostic sonography
• Tissue harmonics
• Contrast harmonics

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Linear vs. Nonlinear Behavior (Propagation)

• Linear propagation (behavior)
• Sinusoidal
• Symmetrical, proportional, even

• Nonlinear propagation (behavior)
• Asymmetrical, disproportional, uneven
• High pressure portion of the wave (compression)
  travels faster than the low pressure portions
  (rarefaction)

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Tissue Harmonics

• Created during transmission due to nonlinear
  behavior of sound velocity (propagation)
• Faster travel during compression
• Slower travel during rarefaction
• Variation in speed alters the shape of the sound
  beam

• Small amount of energy is transferred from the
  fundamental frequency to the harmonic frequency
• Strength of the harmonic wave grows as sound
  travels in tissue

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Tissue Harmonic Signal Strength

• Fundamental imaging
• Strong artifacts appear within the first few
  centimeters of tissue
• This occurs because the beam is very strong, and
• The many different superficial anatomic layers
  distort the sound beam
• Harmonic imaging
• Tissue harmonic signals do not exist at extremely
  superficial depths

• Harmonics increase with increasing depth but only
  to a point and then decrease
• Relationship between sound beam strength
  harmonic creation is nonlinear

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Distortion Degrades the Fundamental Image
Fundamental Image
Distortion masks gallbladder sludge
Harmonic Image
Sludge in the gallbladder is recognized
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Sound Beam Strength

• Weak sound beams
• Do not create tissue harmonics
• Intermediate strength sound beams
• Create only a tiny amount of tissue harmonics
• Strong sound beams
• Create significant tissue harmonic signals
• Importance of strong beams creating harmonic
  signals
• During sound wave propagation, most of the energy
  propagates along the beams main axis
• Harmonics are produced along the main beam
• Off-axis weak lobes create little to no harmonics
• Beams least likely to create artifacts are most
  likely to create harmonics
• Harmonics arise only in nondistorted beams,
  therefore harmonic image has less distortion

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Harmonics, identified in yellow, are produced in
the sound beams main axis
Harmonics
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Summary of Tissue Harmonics

• Created deeper in tissues
• Not present as sound leaves the transducer
• Created during transmission
• Created by nonlinear behavior
• Sound travels faster at wave compressions
• Sound travels slower at wave rarefactions
• More likely to be created along the beams main
  axis

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Contrast Agents

• Aka, microbubbles
• Gas bubbles entrapped in a shell
• Albumin, phospholipids or synthetic polymers
• Injected (intravenously) or ingested

• Must be
• Safe
• Metabolically inert
• Long lasting
• Strong reflector of US
• Small enough to pass through capillaries

Blood clot surrounded by contrast agent in a
cardiac chamber
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Contrast Harmonics

• Created during reflection
• Results from nonlinear behavior of microbubbles
  when sound strikes them

• Harmonics
  Fundamental frequency (green pulse)
  Harmonic frequency (red pulse)

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Nonlinear Behavior of Microbubbles

• Contrast harmonics result of nonlinear
  behavior, uneven changes in bubble size when
  exposed to a sound beam
• Known as resonance
• Sound wave compression high pressure
• Bubble shrinks, high internal bubble pressure
• Sound wave rarefaction low pressure
• Bubble expands, greater percentage than it shrank
  
• Small amount of energy is transferred from the
  fundamental frequency to the harmonic frequency

• Nonlinear Behavior
• Contrast bubbles expand (red bubble)
  to a greater extent than they
  compress (green bubble)

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Mechanical Index

• An indicator of the likelihood of cavitation in
  tissue
• The primary indicator of output power on
  ultrasound systems
• Displayed on all newer ultrasound equipment
• Directly related to the peak rarefactional, or
  negative pressure
• Indirectly related to frequency of the sound wave

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MI the Behavior of Microbubbles

• Microbubbles resonate when exposed to appropriate
  ultrasound frequencies
• Particular MIs enhance harmonic frequency
  creation

• Low MI - lt0.1
• Linear behavior
• No harmonics created
• Intermediate MI - 0.1 to 1.0
• Nonlinear behavior
• Moderate amount of harmonics are created
• High MI - gt1.0
• Extreme nonlinear behavior
• Very strong harmonics created
• Bubbles expand break apart

Nonlinear behavior
Bursting
Linear behavior
Low MI
Intermediate MI
High MI
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Mechanical Index Harmonics
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Contrast Agent Characteristics

• Important characteristics
• Nature of the outer shell
• Shells trap the gas
• Without a shell bubbles quickly vanish as the gas
  dissolves in blood
• Increase the effective life of a microbubble
• Designed to be flexible
• Rigid shells tend to fracture more readily
• Gas that fills the microbubble
• Determines the stability of the microbubbles
• Smaller gas molecules are more likely to pass
  through the shell and shrink the bubble
• Larger gas molecules find the shell less
  permeable and remain trapped within the bubble

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Summary of Contrast Harmonics

• Created during reflection
• Created by nonlinear behavior
• Resonance creates strong harmonics
• Bubble disruption creates very strong harmonics
• Related to the mechanical index
• Low MI no harmonics
• Intermediate MI ever increasing amounts of
  harmonics
• High MI associated with bubble disruption,
  causing a great deal of harmonics
• Determined by the shell and gas of the contrast
  agent

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Tissue Contrast Harmonics - Summary