How is spatial average intensity defined?

• A. The maximum intensity at the beam center
• B. The total power (watts) divided by the area (cm2) of the transducer head
• C. The energy delivered per second
• D. The time-averaged energy across the beam

Answer: (B)

Spatial average intensity gives the average power per unit area across the entire ultrasound beam cross-section. It answers how much acoustic power is spread over the whole beam, not at just one point.

This is calculated by taking the total acoustic power emitted by the transducer and dividing it by the beam’s cross-sectional area: I_s = P / A. The result has units of watts per square centimeter (W/cm^2).

Why this is the best choice: it directly reflects spreading of power over the beam area, rather than a single point (which would be spatial-peak intensity) or a time dimension (which would be temporal-average intensity). The other options describe either the maximum intensity at the center, the total power without regard to area, or the energy per unit time over time, none of which capture the idea of averaging across the beam’s cross-section.