What is the principle of tissue harmonic imaging and why does it improve resolution and reduce near-field artifacts?

• A. Tissues generate harmonics at f0 due to linear propagation; receiving at f0 improves resolution.
• B. Tissues generate harmonics at 2f0 due to nonlinear propagation; receiving at 2f0 reduces side lobes and clutter, improving lateral resolution and reducing near-field artifacts.
• C. Harmonics are generated at f0/2; receiving at f0 reduces artifacts.
• D. Harmonics are generated at 3f0; receiving at 3f0 improves depth penetration.

Answer: (B)

Tissue harmonic imaging relies on nonlinear propagation in soft tissue, which distorts the wave and creates energy at multiples of the transmitted frequency. The most useful of these is the second harmonic, generated at about twice the fundamental frequency. In practice, the transducer emits at f0 but the received image is formed from echoes at 2f0.

Because the second harmonic has a higher frequency, it comes with a shorter wavelength, which improves spatial detail and sharpens the image (better axial resolution). The harmonic signal tends to originate mainly from the focal region and is less contaminated by widespread fundamental-pulse artifacts, so the effective beam is more confined. This narrowing of the image-forming beam reduces lateral blur and clutter, producing cleaner edges.

Near-field artifacts, such as strong reverberations and clutter close to the transducer, are also reduced because the processing emphasizes the 2f0 content and attenuates energy at the fundamental frequency. In short, nonlinear tissue propagation creates a strong 2f0 signal, and receiving at this second harmonic yields higher-resolution detail with fewer near-field artifacts.