Where is lateral resolution best?

• A. Best at the focal point
• B. Best in the near field
• C. Best in the far field
• D. Best at maximum depth

Answer: (A)

Lateral resolution is how well you can distinguish two structures that are side by side in the image, and it depends on how narrow the ultrasound beam is in the image plane. The beam is designed to be narrow at the focal point, where focusing concentrates energy and tightens the beam cross-section. That narrow beam means points separated laterally within that region can be resolved more distinctly than anywhere else.

As you move away from the focal point, the beam widens due to diffraction, so the ability to separate side-by-side structures worsens. The far field and maximum imaging depth don’t inherently improve lateral resolution; they generally come with a broader beam and lower image quality, especially for resolving lateral details. So the best lateral resolution is at the focal point, where the beam width is smallest.