Which focusing method uses time-delayed excitation to achieve a fixed focal depth?

• A. Mechanical focusing uses a curved lens/backing to shape the beam.
• B. Electrical focusing uses time-delayed excitation for a fixed focal depth.
• C. Electronic focusing uses dynamic delays to adjust focus across depths.
• D. Adaptive focusing uses sensor feedback to adjust focus.

Answer: (B)

Understanding how ultrasound beams are focused starts with how the elements are driven. When the excitation signals are timed with precise delays across the array, the emitted waves arrive in phase at a specific point in space. This creates a converging wavefront that focuses the energy at a chosen depth. If those delays are set for one depth and kept fixed, the focus remains at that fixed depth—the beam is concentrated there rather than along a range.

This approach is what electrical (often called electronic) focusing does: using time-delayed excitation to shape the beam so it converges at a particular depth. If the delays were allowed to change with depth to keep the focus sharp across a range of depths, that would be dynamic or adaptive focusing, not a single fixed focus. Mechanical focusing, on the other hand, relies on a curved lens or backing to physically shape the beam, independent of timing delays. Adaptive focusing uses feedback to adjust focus, not simply a pre-set delay pattern.