Harmonics

Tissue harmonics and contrast agents both rely on what idea

non-linear behavior

when the non-linear behavior is the result of a sound beam interacting with the soft tissue

tissue harmonics are created

when the non-linear behavior is the result of sound beams interacting with the bubble

contrast harmonics are created

compressions vs rarefraction speeds

compressions travel faster

rarefractions travel slower

fundamental frequency

the frequency of the transmitted sound wave. linear, predictable

harmonic frequency

twice the transmitted frequency, aka second harmonic. NON-LINEAR, unpredictable, created in the MEDIA.

Harmonics

multiples of the transducer frequency, and are created in the tissues (not in the transducer or receiver)

Non-linear behavior

creates harmonics, as a sound wave travels in the body, an amount of energy is converted from fundamental frequency to harmonic frequency due to nonlinear behavior

The small difference in speeds between rarefractions and compressions is

non-linear behavior (distorts the sound wave and creates harmonics)

A transducer transmits a sound pulse with a fundamental frequency of 2MHz. In the harmonic mode waht is the new MHz

4MHz

The less sinusoidal the wave becomes

the stronger harmonics created

near field harmonics

okay harmonics, less distortion

mid field

good harmonics, best picture

far field

strongest harmoincs, too much attenuation to use

Harmonics improves the SNR where

in the near and mid field of the image

What kind of beams are required for harmonics

strong beams (best lateral resolution close to the focal point)

Harmonic signal improves the image in what 3 ways

1-harmonic beam is narrow, improving lateral resolution

2-grating lobes (and their artifacts) are eliminated because only strong beams can create

3-near field artifacts and sound distortion cannot occur

what happens to axial resolution with a narrow bandwidth

degrades axial resolution.

Pulse inversion harmonics

positive and negative pulses transmit down each scan line (fundamental frequency) The negative pulse is the “inverse” of the positive, cancelling the fundamental reflections because of destructive interference. Only harmonic reflections remain and create the image.

Major disadvantage of pulse inversion

frame rate is half that of fundamental imaging.

DEGRADES temporal resolution IMPROVES spatial resolution

Pulse inversion harmonics use what type of transducers

wide-bandwidth/broadband

Power modulation harmonics

a weak beam is emitted, another beam 2x stronger than the weak beam is emitted, fundamental echoes return and the machine doubles the weak echoes. Just the harmonics are left.

contrast agents and examples

microbubbles, used to highlight anatomy

definity, optison, imogent, lumason

3 ways contrast agents help

1- identify lesions when the echogenicity is similar to surrounding

2- characterize lesions through documentation of arterial and venous phases

3- added doppler detection in weak flow areas

5 contrast characteristics

1- safe

2- metabolically inert

3- long persistence (lasting)

4- strong reflector of ultrasound

5-small enough to pass through capillaries

compression causes the bubble to

shrink

rarefraction causes the bubble to

expand

what type of impedance difference do contrast agents have

large impedance difference, creating strong reflections

formula for peak rarefraction pressure

mechanical index= peak rarefraction pressure/ square root of frequency

peak rarefraction pressure

how little pressure at the rare fraction portion of the wave. the amount of expansion the bubble will go under is

Mechanical index

force of a sound beam has on a media.

Lower mechanical index

small pressure variations, higher frequency

higher mechanical index

large pressure variations, lower frequency

the microbubbles are the size of what

red blood cells, resonate at frequencies between 2 to 4 MHz.

Low, mid, high MI in relation to harmonics

Low <1= no harmonics

Mid 0.1-1= some harmonics (non linear behavior of bubbles)

High >1= greatest harmonics, bubbles burst too much expansion, strong beamq