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Echo Sonography

Echo Tissue Harmonic Sonographic Imaging is a new technique that produces diagnostic images by recording harmonic echoes produced by nonlinear propagation of ultrasound in tissue. Tissues first compress and then relax as each pulse of sound passes. This physical alteration produces peaks and troughs in the pulse wave pattern because the speed of sound is slightly higher in compressed than noncompressed tissue. These repetitive alterations in the sound pressure wave cause low-amplitude harmonic echoes that increase in intensity as the beam propagates deeper. Multiple harmonics are produced, but the second - the initial echo of the fundamental transmitted pulse - is the only one strong enough to be captured and used by current clinical sonography equipment.
Images formed from the harmonic echoes have a somewhat different appearance from conventional scans produced by echoes of the fundamental transmitted beam. Because the harmonic beam is amplified rather than attenuated by passage through tissue, few harmonic echoes are produced by the skin and subcutaneous tissues. This results in a dramatic decrease in image aberration and deterioration caused by the body wall. The signal-to-noise ratio benefits not only from the dramatically decreased body wall factor, but also from a significant reduction in artifacts caused by reverberation and side lobes.

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These artifacts are weaker than the fundamental beam, so they produce much weaker harmonic echoes. Penetration can be improved using a low fundamental transmitted pulse and imaging at the higher harmonic frequency. More harmonics are produced at the center of a broadband pulse than in the periphery. This gives a narrower beam, which improves spatial resolution and further reduces side lobes, resulting in improved contrast resolution.

 

 

 

 
As much of the fundamental sonographic signal as possible must be removed to make these theoretical harmonic improvements a clinical reality. This can be done by either frequency-based or phase inversion methods. Previously published clinical studies have used the frequency-based method. This technique uses a narrow transmit frequency bandwidth. A high-pass or narrow band-pass filter is critical to suppress the fundamental signal and receive the second harmonic at twice the transmitted frequency. Any parts of the harmonic signal that overlap the fundamental are lost. This method has been successful in improving images of deep abdominal and pelvic structures and of obese patients.

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