Chapter 17 - Harmonics and Contrast Agents. Understanding Ultrasound Physics

harmonic imaging

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

harmonic frequency sound waves arise from nonlinear behavior

fundamental frequency

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

The frequency that we start with.

harmonic frequency

twice the fundamental frequency (also called second harmonic frequency).

Fundamental image

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

Creates images from sound reflections with the same frequency as the transducer frequency.

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.

Two forms of harmonics

tissue harmonics and contrast harmonics

linear behavior

Linear means proportional or symmetrical. Linear systems respond in an even manner.

nonlinear behavior

nonlinear means irregular or disproportionate. A system is nonlinear when it behaves unevenly.

Note: Harmonic frequency sound arises from nonlinear behavior.

tissue harmonics

-created during transmission in tissue

-occurs as sound propagates in tissue

-results from nonlinear behavior of transmitted sound beam

-weaker harmonic signal

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

Harmonic signals cannot distort. In essence, harmonics get a "free ride" through the superficial tissues, thereby remaining distortion free with less noise. The use of harmonics increases the signal-to-noise ratio.

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

with tissue harmonics, the relationship between sound beam strength and harmonic creation is also nonlinear.

What is it so significant that only strong sound beams create harmonic signals?

Beams that are most likely to create harmonics are least likely to create artifacts.

Harmonics are only produced in the sound beam's main axis.

Pulse inversion harmonics

designed to utilize the harmonic reflections (which are distortion free, eliminating distorted fundamental reflections)

Two consecutive ultrasound pulses are transmitted down each scan line. The first is a typical pulse. The second pulse is an inverted copy that is out-of-phase with the first. Wherever a compression exists in the first pulse, a rarefaction exists in the second.

-lower frame rate = lower temporal resolution

what is power modulation harmonic imaging?

two consecutive ultrasound pulses are sent down each scan line; the second pulse is twice the strength of the first one.

Since harmonics exhibit nonlinear behavior, the weaker first reflection does not contain harmonics. the stronger second reflection contains harmonics.

With power modulation, the harmonics are extracted by sending one strong pulse and one weak pulse down each scan line.

contrast agents

Also called micro-bubbles, are gas bubbles encapsulated in a shell.

Designed to create strong reflections that actually "light up" blood chambers, vessels, or other anatomic regions.

different acoustic fingerprint than blood or tissue.

Five requirements:

-safe

-metabolically inert

-long lasting

-strong reflector of ultrasound

-small enough to pass through capillaries

contrast harmonics

As an ultrasound pulse interacts with micro bubbles a small amount of energy is converted from the fundamental frequency to the harmonic frequency

Contrast harmonics are created because micro-bubbles act in a nonlinear manner when struck by sound waves.

What nonlinear behavior of a microbubble creates contrast harmonics?

When a micro-bubble is within a sound beam, the bubble grows and shrinks in relation to these pressure variations. When exposed to sound beams of adequate strength, contrast harmonics are created because of the nonlinear changes in bubble size. This uneven behavior is called resonance.

Mechanical Index (MI)

an estimate of the amount of contrast harmonics (depends o the frequency of transmitted sound & the pressure of the sound wave)

Lower MI

-small pressure variation

-higher frequency

Higher MI

-more pressure

-lower frequency

is there another form of nonlinear behavior of a microbubble that creates contrast harmonics?

The relationship between the mechanical index and harmonic creation is nonlinear.

Low MI: less than 0.1

No harmonics,

backscatter,

linear behavior,

higher frequency sound,

low beam strength,

bubble expands very little

Higher MI: 0.1 to 1.0

Strong harmonics

Resonance

Non-linear behavior

Lower frequency sound

Higher beam strength

highest MI: greater than 1

-strongest harmonics

-bubble disruption

-extreme nonlinear behavior

-lowest frequency sound

-highest beam strength

-bubble expands greatly

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

-the nature of the outer shell

-the gas that fills the micro-bubble

Tissue harmonics (summary)

-created during transmission in tissue

-occurs as sound propagates in tissue

-results from nonlinear behavior of transmitted sound beam

-weaker harmonic signal

contrast harmonics (summary)

- created during reflection off of microbubble

- occurs only when contrast agents are present and with MI's greater than 0.1

-results from nonlinear behavior of microbubble

-stronger harmonic signal