chapter 17: harmonics and contrast agents

harmonic imaging

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

fundamental frequency

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

harmonic frequency

• twice the fundamental frequency

• AKA second harmonic frequency

harmonic frequency sound waves arise from what type of behavior from the system?

nonlinear behavior

fundamental image

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

harmonic image

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

harmonic imaging is most useful in improving poor quality images because:

harmonic frequency waves undergo less distortion than fundamental sound waves

what are the 2 forms of harmonics in diagnostic sonography?

• tissue harmonics

• contrast harmonics

linear behavior

• proportional or symmetrical

• these systems respond in an even manner

nonlinear behavior

• irregular or disproportionate

• these systems behave unevenly

• asymmetrical

tissue harmonics

• created during transmission in tissue

• occurs as sound propagates in tissue

• results from nonlinear behavior of the transmitted sound beam

• weaker harmonic signal

contrast harmonics

• created during reflection off of a microbubble

• occurs only when contrast agents are present and with MIs greater than 0.1

• results from nonlinear behavior of the microbubble

• stronger harmonic signal

what creates tissue harmonics?

as a sound wave travels through the body, a small amount of energy is converted from the fundamental frequency to the harmonic frequency

the use of harmonics increases the:

signal-to-noise ratio

at what depths do tissue harmonic signals exist?

at deeper depths in the tissue

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

create artifacts

pulse inversion harmonics

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

how do pulse inversion harmonics work?

• a positive and then a negative pulse is transmitted down each scan line

• the positive and negative reflections are combined, and the fundamental portions are eliminated leaving only the harmonic part of the reflection

disadvantage of pulse inversion harmonics:

• the frame time is doubled

• the frame rate is halved

• temporal resolution is reduced

power modulation harmonic imaging

an imaging technique specifically designed to augment harmonic reflections while eliminating fundamental reflections

how does power modulation harmonic imaging work?

harmonics are extracted by sending one strong pulse and one weak pulse down each scan line

contrast agents

• AKA microbubbles

• gas bubbles encapsulated in a shell

• they are injected in the circulation or ingested

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

what are the 5 requirements contrast agents must meet?

• safe

• metabolically inert

• long lasting

• strong reflector of ultrasound

• small enough to pass through capillaries

contrast harmonics

• as an ultrasound pulse interacts with contrast agents, much stronger harmonics are generated

• created during reflection as energy is converted from the fundamental frequency to the harmonic frequency

when a microbubble is exposed to a compression of a sound wave:

the bubble shrinks and the pressure inside the bubble increases

when a microbubble is exposed to a rarefaction of a sound wave:

the bubble expands to a greater extent than it shrinks

high mechanical index (MI)

• large pressure variation

• lower frequency

MI less than 0.1:

• no harmonics

• backscatter

• linear behavior

• higher frequency sound

• low beam strength

• bubble expands very little

MI 0.1-1.0:

• some harmonics

• resonance

• nonlinear behavior

• lower frequency sound

• higher beam strength

• bubble expands moderately

MI greater than 1:

• strongest harmonics

• bubble disruption

• extreme nonlinear behavior

• lowest frequency sound

• highest beam strength

• bubble expands greatly

what 2 characteristics of contrast agents are important?

• the nature of the outer shell

• the gas that fills the microbubble

what type of harmonics is stronger?

contrast harmonics created by microbubbles are much stronger than tissue harmonics