UltraSound Physics Chapter 17

Harmonic Imaging

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

Fundamental Frequency

Transmitted Frequency

nonlinear behavior

Harmonic frequency sound arises from ________________ _______________

tissue harmonics and contrast harmonics

Two important forms of harmonics

by processing reflections that are twice the fundamental frequency

How is a harmonic image created?

in an even or symmetrical manner

Linear systems respond how?

unevenly or asymmetrically

A system is considered non-linear when it behaves ______________

compressions and rarefactions of the sound beam

What creates the nonlinear speed variation that causes harmonic imaging?

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

List the 2 reasons that artifacts arise within the first few cm of tissue

because they do not exist at superficial depths

Why can harmonic signals not distort?

Non-linear

Describe the relationship between tissue harmonics and sound beam strength

-None

-only a tiny amount

-significant amount

What amount of tissue harmonics are created by

-Weak sound beams

-intermediate strength sound beams

-strong sound beams

Least Likely

The Beams that are most likely to create harmonics are also ________ __________ to create artifacts

Not present

Tissue harmonics are ___________________ as sound leaves the transducer

in the tissue during transmission

Where are tissue harmonics created?

Pulse inversion harmonics

This technique is specifically designed to utilize harmonic reflections which are distortion free while eliminating distorted fundamental reflections

Harmonic imaging

This type of imaging creates images only from the harmonic.

Pulse inversion harmonics

Two consecutive pulses transmitted down each scan line with the second pulse being an inverted copy that is out of phase with the first pulse describes

Double the pulses are needed to create an image

time frame is doubled

frame rate is halved

temporal resolution is reduced

What are the disadvantages of pulse inversion harmonics?

Power modulation harmonic imaging

Two consecutive pulses are transmitted down each scan line with the second pulse being double the strength of the first pulse.

Double the pulses

frame rate is halved

temporal resolution is reduced

What are the disadvantages of power modulation harmonic imaging?

microbubbles

Another name for contrast agents

-safe

-metabolically inert

-long lasting

-strong reflector of ultrasound

-small enough to pass through capillaries

What five requirements must contrast agents meet?

contrast agents

Designed to create strong reflections that "light up" blood chambers and vessels

generates much stronger harmonics

What effect do contrast agents have on tissue harmonics?

because microbubbles act in a non-linear manner when struck by sound

Why do contrast agents increase the harmonics?

compressions and rarefactions

A sound wave is an oscillation of pressure or a series of _________________ and _________________

grows and shrinks

When a microbubble is within a sound beam, the bubble __________ and _________ in relation to pressure changes

resonance

The uneven behavior of bubble size when exposed to sound

expand to a greater extent than they shrink

Bubbles ___________ to a greater extent than they ___________

Peak rarefaction pressure

___________ __________ pressure is the most important with regard to contrast harmonics

rarefaction expands, compression shrinks

Rarefaction pressure ______________ the bubble, while compression pressure ____________ the bubble

the measurement of the amount of contrast harmonics produced

What is the mechanical Index?

-lower frequency sound

-stronger sound waves

The numerical value of the MI increases with

>1.0

Beams must have a MI of _______ to create substantial harmonic signals

-no harmonics

-backscatter

-linear behavior

-higher frequency sound

-low beam strength

-bubble expands very little

Low MI < 0.1

-some harmonics

-resonance

-nonlinear behavior

-lower frequency sound

-higher beam strength

-bubble expands more moderately

Higher MI 0.1 - 1.0

-strongest harmonics

-bubble disruption

-extreme non linear behavior

-lowest frequency sound

-highest beam strength

-bubble expands greatly

Highest MI >1.0

-the nature of the outer shell

-the gas that fills the microbubble

What are the 2 important characteristics of contrast agents

the relationship between the shell and the internally trapped gas

What determines the contrast agents stability and longevity while circulating in the blood?

wide-bandwidth or broadband

Pulse inversion harmonics uses ___________________ transducers

transmission, reflection

Tissue harmonics are created in the tissue during _________________ while contrast agent harmonics are created during _________________

sound propagates thru tissue

Tissue harmonics occur as

contrast agents are present and MI is > 0.1

contrast harmonics occur when