Which factor most directly influences the risk of hotspots during ultrasound therapy?

• A. BNR
• B. Treatment duration
• C. Frequency
• D. Patient age

Answer: (A)

Hotspots during ultrasound therapy are driven by how uneven the ultrasound beam is. This is captured by the beam nonuniformity ratio (BNR), which compares the highest peak intensity in the beam to the average intensity. A high BNR means there are pronounced peaks, causing localized overheating or hotspots even when the overall average intensity is within limits. So, BNR directly reflects the potential for hotspot formation.

Treatment duration affects the total amount of energy delivered and the overall thermal dose, but it doesn’t describe how concentrated the energy is within the beam. Frequency changes how deep and how much energy is absorbed, not how unevenly that energy is distributed. Patient age has no direct effect on the beam’s uniformity or hotspot risk. In practice, selecting a transducer with a low BNR, moving the head during treatment, and ensuring good coupling help minimize hotspots.