The primary advantage of annular phased arrays is that ________ create each scan line of the image.

• A. A single focal zone
• B. A uniform aperture
• C. Multiple transmit focal zones
• D. A fixed focus

Answer: (C)

Dynamic focusing along depth is what makes annular phased arrays powerful. By using concentric rings that can be excited with different time delays, the beam can be focused at chosen depths. This lets the system create multiple transmit focal zones, so each scan line can be formed with the beam focused at various depths as needed. That flexibility improves lateral resolution and image quality along the line, which is the key advantage of this type of array. Limiting itself to a single focal depth or a fixed focus would reduce that depth-dependent control, and a uniform aperture isn’t the feature that enables the line-by-line focusing across depths.