A region on either side of the focal point where the beam is relatively narrow and where image detail is superior.

• A. Near field
• B. Far field
• C. Transition zone
• D. Focal zone

Answer: (D)

The concept here is the focal zone in ultrasound imaging—the region around the focus where the beam stays relatively narrow and image detail, especially lateral resolution, is at its best. The focal zone is defined as the area on either side of the focal point where the beam width remains small, giving superior detail. Before reaching the focus (the near field), the beam is still converging and not at its minimum width, so resolution isn’t as high. After the focus (the far field), the beam begins to diverge and image detail diminishes. The transition zone isn’t the standard term for this high-detail region and doesn’t capture the precise idea of a narrow beam around the focus.