Which term describes echoes at twice the transmitted frequency that improve image quality?

• A. Harmonic frequency
• B. Primary frequency
• C. Transmitted frequency
• D. Echo frequency

Answer: (A)

Echoes at twice the transmitted frequency are called harmonics, specifically the second harmonic. As an ultrasound wave travels through tissue, it propagates nonlinearly and generates higher-frequency components. The second harmonic has a frequency about twice the fundamental and is often used to form images because harmonic signals tend to originate deeper in tissue and are less affected by near-field clutter.

Using these harmonics improves image quality by giving a narrower effective beam and reducing artifacts from the near field and side lobes, which sharpens boundaries and enhances contrast resolution. The other terms describe the transmitted frequency (primary or transmitted) or the actual received frequency after reflection (echo frequency), but they do not capture the doubling that defines harmonic imaging.