In ultrasound, the Fraunhofer zone corresponds to which region?

• A. focus
• B. near zone
• C. penetration depth
• D. far zone

Answer: (D)

Understanding where the Fraunhofer zone lies is about the far-field part of the ultrasound beam. The Fraunhofer zone is the far-field region, the area beyond the transducer’s near field where the wavefronts become nearly planar and the beam’s angular spread is determined mainly by the aperture and the wavelength. This region starts after the near-field (the Fresnel zone) and, for a circular aperture, roughly begins at the Rayleigh distance (about 2D^2/λ). In this zone the beam has already radiated away from the transducer and continues to diverge in a predictable way. The focus and penetration depth describe different aspects of imaging, so they do not define this region.