What scenario is most likely to cause periosteal pain during ultrasound treatment?

Periosteal pain during ultrasound treatment is most likely to occur when there is a high beam nonuniformity ratio (BNR) in the ultrasound unit. The beam nonuniformity ratio refers to the variation in the intensity of the ultrasound beam, which indicates how concentrated the ultrasound energy is over a certain area.

When the BNR is high, this irregularity means that certain areas of the treatment surface may receive a significantly higher intensity of ultrasound energy than others. This uneven distribution can lead to hot spots during the treatment, causing localized heating and discomfort, leading to periosteal pain over bony areas where the periosteum is located.

In contrast, a low beam nonuniformity ratio suggests a more uniform distribution of ultrasound energy, minimizing the risks of overheating specific tissues and reducing the likelihood of causing discomfort. Additionally, the effective radiating area and its size primarily affect treatment depth and the overall effectiveness of the ultrasound, but they do not directly correlate to the risk of periosteal pain as influenced by the BNR. Thus, it is the high BNR that poses a risk for periosteal pain.