How does adjusting the focal depth influence the lateral resolution across the image?

• A. Lateral resolution is best at the far field; away from focus it degrades
• B. Lateral resolution is best at all depths
• C. Lateral resolution is best at the focal depth; away from focus it degrades
• D. Lateral resolution is unaffected by focal depth

Answer: (C)

Focusing the beam narrows the ultrasound beam where it converges, and that narrower beam directly improves lateral resolution at that depth. Lateral resolution depends on the beamwidth: a thinner beam provides finer detail across the image lines. Away from the focal point, diffraction makes the beam spread out, increasing beamwidth and worsening lateral resolution. By adjusting the focal depth, you place the region of best lateral resolution at the depth of interest; outside that focal zone, resolution degrades.