What is the location of the minimum cross-sectional area of the beam?

• A. Far zone
• B. Near zone
• C. Focus
• D. Fraunhofer zone

Answer: (C)

In ultrasound, the beam behaves like a diffracting wave from a circular aperture: it converges as it travels through the near field and then diverges in the far field. The narrowest part of the beam—the smallest cross-sectional area—occurs at the focal point, which is the end of the near field. Beyond that point, in the far field, the beam widens. So the location of the minimum cross-sectional area is the focus.