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Harmonic Imaging

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Harmonic Imaging

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    1. Harmonic Imaging Lee W. Goldman Hartford Hospital

    2. Harmonics

    3. US Harmonic Formation Harmonics can be produced by tissue or by contrast agents (microbubbles). Harmonics are generated by tissue during propagation of an acoustic pulse. Contrast agent harmonics are produced by resonance of microbubbles when struck by the US pulse

    4. Waveform Distortion Waveform distortion from nonlinear propagation. Zero crossings of time waveform propagate at speed c0 but peaks & troughs propagate faster slower, respectively. Solid line is initial waveform. Dashed lines are waveform at two later points along propagation path.

    5. Tissue Harmonic Production (a) Acoustic pulse leaving transducer (dotted) and same pulse slightly distorted, at 10-cm depth (solid line). (b) Frequency spectrum of pulse. At 0 cm, only fundamental band is present. At 10 cm, multiple harmonic bands have been generated.

    6. Signal Strength vs Depth Fundamental and 2nd harmonic intensity vs depth. Fundamental intensity decreases with depth by attenuation and energy loss to harmonics. Harmonic intensity builds with depth but eventually declines due to attenuation and energy loss to higher harmonics

    7. Harmonic Signal Strength Important Points: Harmonics are produced beyond the body wall (and thus past the source of most noise & clutter associated with technically difficult patients) Harmonic strength is proportional to fundamental strength (IH ~ IF2 ) Harmonic signals much weaker than fundamental (~1% or 20dB beyond build-up region)

    8. PULSE SHAPING (a) Fundamental and harmonic bands overlap. Noise and clutter from fundamental signal corrupt harmonic signal. (b) Fundamental band is shaped so that there is no overlap. Image formation can be done from pure harmonic signal.

    9. Reverberation in Cardiac US Reverberation artifact removal in cardiac mages: transmitted fundamental (blue) and 2nd harmonic (red). Reflections of fundamental between transducer and ribs are eliminated in harmonic image

    10. Harmonic Image Characteristics Improved Signal-to-Noise Ratio: although signals are much weaker, SNR is higher due to: Minimal effects of noise and clutter from body wall elimination of side lobes, reverberations and scatter (since IH ~ If2, these noise sources are undetectable) Spatial Resolution: Improved because axial resolution of harmonic image is function of fH Dynamic Range: lower (already ~20 dB weaker) Overall: affected by units sensitivity and ability to shape pulse and separate frequency bands

    11. Harmonic Images Fundamental image (left) shows poor delineation of kidney and a vague mass that is difficult to interpret. NTHI image (right) clearly shows a complex renal mass

    12. Harmonic Images - Cont Fundamental image of spleen (left) suggests normal anatomy. NTHI image (right) clearly shows ascites in anterior and posterior aspects. .

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