Which term describes the spreading of the ultrasonic beam as it propagates and can degrade lateral resolution?

• A. Beam spread
• B. Attenuation
• C. Refraction
• D. Doppler shift

Answer: (A)

Beam spread describes how the ultrasonic beam widens as it travels. This widening comes from diffraction and the finite size of the transducer aperture, so the beam width grows with depth. As the beam becomes wider, the ability to distinguish structures that are next to each other in the lateral direction diminishes, which degrades lateral resolution. Attenuation refers to loss of signal strength with distance, refraction is the bending of the beam at interfaces, and Doppler shift is a frequency change due to motion; none of these capture the widening of the beam that leads to poorer lateral resolution.