Harmonics

History of Harmonics

1990's - harmonic frequencies can be generated in native tissues

Harmonics

Frequencies that are integer multiple of the fundamental frequency

Original/Fundamental Frequency

One frequency

Sine wave

Attenuation begins as soon as pulse enters body

Harmonic Frequency

Multiples of the original frequency - often integer multiples of 2

Non-linear wave/non-linear propagation

Generated at center of beam where intensity is the highest

Begins after skin line, strengthens with depth, then looses intensity

Returning harmonic signals is less intense than the original frequency

Linear Sound Propagation

Sine wave

Small acoustic pressure

Tissue shows elasticity

No new harmonic frequencies created

Non-Linear Sound Propagation

Sawtooth wave

High acoustic pressure

New harmonic frequencies created

Tissue is compressible, but response to greater pressure is different

Non-linear distortion of the fundamental/Sine Wave depends on...

Amplitude of the emitted pulse

Distance traveled into the tissue

Fundamental Imaging

Pulses emitted from transducer contain central frequency

Transducer receives returning echoes

Frequency of emitted pulse and returning echoes are the same

Tissue Harmonic Imaging

Done by eliminating returning fundamental/original echoes and keeping harmonic echoes

Relationship between harmonic frequencies and depth

More harmonic frequencies are generated as the sound travels deeper into the body — to a point

Skin line - only fundamental frequencies

Depth past 2-3 cm - sine wave becomes distorted, so harmonic frequencies are generated

Bandwidth Receive Filtering

Lower fundamental frequencies are filtered out by receiver

Uses higher harmonic frequencies to create image

Requires narrow bandwidth = decreased axial resolution

Pulse Inversion/Phase Cancellation/Temporal Cancellation

Two pulses are emitted (one after the other) along the same line with a 180° phase difference

Fundamental echoes are canceled out

Harmonic echoes remain and are amplified

Does not affect axial resolution - contrast preserved

Affected by tissue motion

Can lower frame rate

Side-by-Side Phase Cancellation

Two pulses are emitted (one after the other) along adjacent lines with a 180° phase difference

Fundamental echoes are canceled out

Harmonic echoes remain and are amplified

Pulse-Coded Harmonics

Pulses are encoded with a unique code and then transmitted

Some echoes return with fundamental frequency codes and are cancelled

Some echoes are not coded and return with harmonic frequencies and remain

Advantages of Tissue Harmonic Imaging

Represents cystic structures well

Can image abnormalities that contain, calcifications, air

Better signal to noise ratio

Improved contrast resolution

Improved lateral resolution

Reduces some artifacts - side/grating lobes, reverb

Enhances some artifacts - enhancement, shading, comet tail

Does not affect MI and TI values

Helps with technically difficult patients (hard scans)

Disadvantages of Tissue Harmonic Imaging

Narrowed bandwidth

Decreased axial resolution

May be worse than fundamental imaging in some situations - like high attenuation

Pulse-inversion is sensitive to tissue motion

Microbubbles

Generate harmonic frequencies based on how they resonate with sound beam

Contrast Harmonics

Generated from reflection of microbubbles

Need pulse cancellation techniques to cancel out fundamental frequency and only display bubble frequency