Influence of the mean scattering free-path on the temperature field

To avoid injuries caused by excessive heating during a therapeutic ultrasound treatment it is important to know the application time required to bring tissue to a reference temperature. Heating is assumed to be associated to ultrasonic absorption by tissues. However, it must be pointed that ultrasonic waves are scattered by heterogeneous media such as tissues. This leads to an increase in the average wave free-path and, consequently, to an extra ultrasonic absorption, which influences directly the thermal field and the correct determination of the thermal dose. To evaluate this phenomenon we analyzed the thermal field generated in silicone-based phantoms, with different percentages of graphite powder, subjected to continuous ultrasonic irradiation at therapeutic levels (0.5, 1.0 and 2.0 W/cm2 at 1.0 MHz). The phantoms had thermocouples inserted at four depths. Just to exemplify, we observed that the smaller temperature increase was 3∘C (compared to a phantom with no scatterers) with a phantom loaded with 0.25% graphite, after irradiated with 0.5 W/cm2 for 2 min. Thus temperature increase, due to the effect of ultrasonic scattering, must not be neglected but, on the contrary, should be taken into account, at least for certain treatments.