What is the Fresnel (near) zone in ultrasound?

• A. Boundary layer
• B. Far field region
• C. Region from the transducer to the focal point
• D. Near field region

Answer: (C)

The Fresnel (near) zone is the portion of the ultrasound beam where the wavefronts are still being shaped by the transducer and are converging toward the focal point. In this region, the pressure field is created by interference from all elements of the aperture, so the beam width and intensity change rapidly with depth as it approaches focus. Beyond the focus, the beam enters the far field, where the wavefronts become more planar and the beam begins to diverge. So describing the Fresnel zone as the region from the transducer up to the focal point captures its defining behavior: the beam is still forming and tightening toward the focus.