Which statement best describes the spatial distribution of beam intensity?

• A. Highest at the Center and Decreases Toward the Edges
• B. Uniform Throughout the Beam
• C. Highest at the Edges
• D. Randomly Varies

Answer: (A)

The main idea is how energy is distributed across the ultrasound beam at a given depth. The transducer forms a beam whose most intense portion sits along the central axis—the central lobe carries the highest energy density. As you move away from the center toward the edges, the waves interfere less constructively and the beam spreads due to diffraction, so the energy is spread over a wider area and the intensity decreases. In practice, the lateral intensity profile is a central peak that diminishes toward the edges, often with a Gaussian-like shape. Focusing can raise the peak in the focal zone, but the central region remains the area of greatest energy density.

Choosing a uniform distribution would require equal intensity everywhere, which isn’t how a focused ultrasound beam behaves. The edges being the brightest would imply a donut-like pattern, which isn’t the case for standard imaging beams. Random variation would ignore the predictable, physics-based beamforming and diffraction that establish a definite central peak.