In ultrasonic imaging, the number of transmit focal zones affects lateral resolution at depth. Which statement is true?

• A. Degrades frame rate significantly
• B. Improves lateral resolution over a depth range
• C. Increases slice thickness
• D. Reduces contrast

Answer: (B)

Understanding how focusing affects the beam helps here. When the ultrasound beam is focused at a particular depth, its width becomes narrow there, which improves lateral resolution at that depth. If you use several transmit focal zones, the beam is narrowed at multiple depths, so the lateral resolution is enhanced over a wider depth range rather than just at one location. That’s why this statement is true: more focal zones extend the range over which you get better side-to-side detail.

Keep in mind the trade-off: adding focal zones requires more transmissions per line, which lowers the frame rate. This approach does not inherently change slice thickness (elevational resolution) or contrast in a direct, consistent way—the former is governed more by the elevational beam and the latter by tissue properties and imaging settings.