The end of the near zone marks the location of the

• A. Focus
• B. Transducer surface
• C. Far zone
• D. End of the transducer

Answer: (A)

The end of the near zone marks where the beam reaches its smallest width, i.e., the focus. In focused ultrasound, the near field (Fresnel zone) is the region in front of the transducer where wavefronts from the aperture are still converging toward a point. The point at which these converging waves form the tightest beam is the focal point, located at the end of the near zone. Beyond this point, the beam begins to diverge as it enters the far zone. So the end of the near zone corresponds to the focus. The transducer surface marks the start of the near field, not its end, and the far zone lies beyond that boundary. The end of the transducer is just the crystal face, not tied to the focus location.