Which zone is used to describe the far-field region of an ultrasound beam?

• A. Fresnel zone
• B. Near zone length
• C. Focal distance
• D. Fraunhofer zone

Answer: (D)

In wave propagation, the far-field region is described by the Fraunhofer zone. After the near field, where wavefronts are still curved and patterns vary quickly with distance, the ultrasound beam settles into a simpler, diffraction-dominated pattern. In this Fraunhofer zone, the angular distribution becomes more stable and the beam behaves like a diffracted wave from a finite aperture.

This zone begins after the near field length, which marks the end of the Fresnel region and depends on the transducer’s diameter and the wavelength (roughly D^2/4λ). The term Fresnel zone or near zone length refers to the region before that transition, while focal distance is simply the distance from the transducer to its focal point and is not a far-field zone.