In a water tank scan, which region of the sound beam has the highest intensity?

• A. Focal point
• B. Focal zone
• C. Fresnel zone
• D. Fraunhofer zone

Answer: (A)

Highest intensity occurs at the focal point. When the ultrasound beam is focused, the waves converge to a very small spot, so the same amount of acoustic power is concentrated into the smallest cross-sectional area. That concentration raises the power per unit area, giving the peak intensity right at the focus. The surrounding focal zone remains fairly narrow and high in intensity, but not as high as the exact focal point. In the near-field (Fresnel) region the beam is still evolving and not yet at its smallest cross-section, while in the far-field (Fraunhofer) region the beam has diverged and the intensity diminishes with distance.