Which statement best describes SPI?

• A. Spatial Peak Intensity; the greatest intensity anywhere within the beam
• B. Spatial Average Intensity; average intensity over the ERA
• C. Beam Nonuniformity Ratio; the ratio between SPI and SAI
• D. Time-Averaged Power; the average over time

Answer: (A)

In ultrasound beams, the energy isn’t spread evenly across the cross-section—the intensity varies from point to point. The quantity that captures the highest energy at any location within the beam is the Spatial Peak Intensity. It tells you the maximum potential for heating at a single spot inside the beam, which is why it’s the best description of SPI. This differs from Spatial Average Intensity, which averages the intensity over the effective radiating area, and from Time-Averaged Power, which averages power over time. The Beam Nonuniformity Ratio is related but describes how uneven the beam is by comparing SPI to SAI (a higher ratio means more variation).