The impact of ultrasonic parameters on chest wall hyperthermia

A transient, three-dimensional acousto-thermal numerical model for chest wa ll 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 (xi (1)); (2) the depths of the soft-tissue-bone (d(b)) and soft-tissue-lung (d(u)) interfaces; (3) th e intensity reflectivities of these interfaces; and (4) the intensity atten uation coefficient of bone. Several important results were obtained. First, acoustic reflections from the underlying bone and lung surfaces may contri bute significantly to heating of the overlying soft-tissue. Secondly, a str ong dependence of optimal xi (1) values on d(b) and d(u) values was found. Chest wall volumes with 2-3 cm of soft-tissue overlying the ribs were optim al targets for unfocused ultrasound hyperthermia. Thirdly, the maximum stea dy state temperature in bone also strongly depended on xi (1). 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 degreesC and 0.8 degreesC. That is, the maximum temperature in the field , either during the transient period or at steady state, did not always occ ur in bone. It is concluded that control of power deposition penetrability offers great potential for improving hyperthermia to chest wall targets in real time.