The region from the transducer to the minimum beam cross-sectional area is known as which zone?

• A. Fraunhofer zone
• B. Fresnel zone
• C. focus
• D. Snell's length

Answer: (B)

The Fresnel zone is the near-field region of an ultrasound beam—the space from the transducer surface up to the point where the beam reaches its smallest cross-sectional area. In this zone the wavefronts are still curving and diffraction effects cause the beam width to change rapidly with depth. That end point, where the beam is at its minimum width (often around the focus for a focused transducer), marks the boundary before the beam begins to diverge more in the far field.

In contrast, the Fraunhofer zone is the far-field region where the beam spreads out more predictably, and the focus is no longer the region of minimum cross-section. The focus itself is a location of minimum width for a focused transducer, not a whole region. Snell's length isn’t a standard term for these zones.