Lateral resolution is improved by focusing and higher frequency; it is determined by which beam property?

• A. Beam width along the scan plane
• B. Wavelength
• C. Depth of field
• D. Beam width across the scan plane

Answer: (D)

Lateral resolution is governed by how narrow the ultrasound beam remains across the image plane. The closer two reflectors can be side by side and still be distinguished depends on the beam’s width in that lateral direction. Focusing concentrates energy into a narrower cross‑section at the focal zone, and using a higher frequency shortens the wavelength, both of which reduce the beam width across the scan plane and sharpen lateral detail. Depth of field describes how long the beam stays narrow with depth, not the instantaneous lateral width itself. While wavelength plays a role, the direct determinant of lateral resolution is the beam width across the scan plane.