Heat distribution in multilayered tissues after therapeutic ultrasound (TUS) stimulation has been investigated in the last decades. However, there are still some concerns regarding wave parameters, dosage and physiological responses, leading to the need for further research. The aim of this study was to assess temperature changes in bone samples and phantoms (mimicking materials) caused by TUS.

It was used a sample of bovine ex vivo femur and two cortical bone phantoms (Sawbones®, USA): a 4-mm thick plate and a 5-mm thick cylinder put under a custom-made two-layered soft tissue phantom, and a TUS probe (1-MHz, intensity SATA of 1W/cm², continuous or 50%-pulsed regimen) was used in contact with it, in a fixed or moving configuration, for a 5-minutes stimulation. A thermometer captured temperature values with two K-type thermocouples: one in the bone-soft tissue interface, and the other between the two soft tissue layers. Everything was inside a water bath at 36.5°C and experiments were repeated five times. Wilcoxon tests (P<0.05) were used for statistical analysis.

The Fig. 1 depicts boxplots representing the difference between minimum and maximum temperatures for the five experiments concerning both pulsed regimens and interfaces, for fixed (fix) and moving (mov) transducers. It can be observed that fixed transducer leads to a higher increase in temperature compared to moving transducer, mainly for the continuous mode. The 50%-pulsed regimen raised maximum temperatures to more than 40°C. Some significant differences were observed (P<0.05).

Fixed transducer may be deleterious since it may lead to great temperature increases. Pulsed regimens seem to heat with less intensity compared with continuous applications. Heating by TUS on phantoms may differ from bone samples because of their different thermal constants and geometries.