In ultrasound, which region is where the beam is narrowest?

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

Answer: (A)

The beam reaches its narrowest width at the focal point, where the transducer’s focusing effect brings waves from across the aperture together in phase. At this depth, constructive interference concentrates energy into the smallest lateral diameter, giving the best lateral resolution. Before this point, in the near field, the beam is still converging toward that narrowest spot; after passing the focus, in the far field, the beam begins to diverge and widens, reducing lateral resolution. So the region where the beam is narrowest is the focus.