The impact of ultrasonic parameters on chest wall hyperthermia

A transient, three-dimensional acousto-thermal numerical model for chest wall anatomies was developed to evaluate the impact of ultrasonic parameters on thermal coverage. The following independent variables were considered: (1) the relative output intensities of the low and high frequency components of an unfocused dual-frequency ultrasonic beam (xi1); (2) the depths of the soft-tissue bone (d(b)) and soft-tissue-lung (d(u)) interfaces; (3) the intensity reflectivities of these interfaces; and (4) the intensity attenuation coefficient of bone. Several important results were obtained. First, acoustic reflections from the underlying bone and lung surfaces may contribute significantly to heating of the overlying soft-tissue. Secondly, a strong dependence of optimal xi1 values on d(b) and d(u) values was found. Chest wall volumes with 2-3 cm of soft-tissue overlying the ribs were optimal targets for unfocused ultrasound hyperthermia. Thirdly, the maximum steady state temperature in bone also strongly depended on xi1. Finally, the largest difference between the maximum temperature in bone and the maximum temperature in soft-tissue during initial transient heating was between -1.4 degrees C and 0.8 degrees C. That is, the maximum temperature in the field, either during the transient period or at steady state, did not always occur in bone. It is concluded that control of power deposition penetrability offers great potential for improving hyperthermia to chest wall targets in real time.