The beam non-uniformity ratio (BNR) is defined as which of the following?

The beam non-uniformity ratio (BNR) is defined as which of the following?

• A. Ratio between spatial peak intensity and spatial average intensity
• B. Ratio between temporal peak intensity and average
• C. Ratio of energy to time
• D. Efficiency of energy transfer

Answer: (A)

Beam non-uniformity ratio shows how uneven the ultrasound field is across its cross-section. It is defined as the ratio of the spatial peak intensity (the highest intensity anywhere in the beam) to the spatial average intensity (the average intensity across the beam). This matters because tissue heating in therapeutic ultrasound follows the local intensity, so a beam with high non-uniformity creates hot spots that can cause unwanted tissue damage, while a lower BNR means a more uniform, safer, and more predictable heating pattern. For example, if the highest point in the beam is 6 W/cm^2 and the average across the beam is 2 W/cm^2, the BNR is 3:1. The other options describe temporal characteristics, energy-time relationships, or efficiency, which are not what BNR measures.

Beam non-uniformity ratio shows how uneven the ultrasound field is across its cross-section. It is defined as the ratio of the spatial peak intensity (the highest intensity anywhere in the beam) to the spatial average intensity (the average intensity across the beam). This matters because tissue heating in therapeutic ultrasound follows the local intensity, so a beam with high non-uniformity creates hot spots that can cause unwanted tissue damage, while a lower BNR means a more uniform, safer, and more predictable heating pattern. For example, if the highest point in the beam is 6 W/cm^2 and the average across the beam is 2 W/cm^2, the BNR is 3:1. The other options describe temporal characteristics, energy-time relationships, or efficiency, which are not what BNR measures.