Theoretical study of the temperature fluctuations induced during reflected-scanned planar ultrasound hyperthermia

Temperature fluctuations inside of a target volume during scanned planar ultrasound hyperthermia were investigated numerically by varying scan time, blood perfusion, and skin temperature. Three dimensional (3-D), transient, acoustic power deposition patterns induced by a scanning ultrasound reflector linear array system (SURLAS) were simulated and input into an homogeneous, 3-D, transient, bioheat transfer equation model. It was found that the largest temperature fluctuations were located at the ends of the linear scan path where the scanning reflector comes to a sudden stop and reverses direction. The smallest fluctuation was located at the center of the scan window. For a given scan distance, the magnitude of the temperature fluctuations increased linearly with increasing scan time, and increased as a weak exponential function of blood perfusion rate. A scan time of 20 s or less is necessary to keep the temperature fluctuations within ±0.5°C from the average temperature in a scan cycle for a scan window of 10 x 10 cm and a blood perfusion rate of 5 kg/m³s.