Numerical and in vitro evaluation of temperature fluctuations during reflected-scanned planar ultrasound hyperthermia

Temperature fluctuations inside a target volume during reflected-scanned planar ultrasound hyperthermia were investigated numerically and in vitro. The numerical approach consisted of integrating an ultrasonic power deposition model for a scanning ultrasound reflector linear array system (SURLAS) designed for simultaneous thermoradiotherapy, and a three-dimensional transient version of Pennes' bioheat transfer equation. The in vitro approach consisted of delivering hyperthermia to a fixed-perfused canine kidney phantom using a SURLAS prototype. Both approaches allowed the study of temperature fluctuations for several important clinically relevant parameters: scan time, scan distance, perfusion rate and skin cooling. The simulation results showed that the largest temperature fluctuations were located at the opposite ends of the scan window where the scanning reflector comes to a sudden and complete stop and reverses direction. The smallest fluctuations were located at the centre of the scan window. For a given scan distance, the magnitude of the temperature fluctuations increased linearly with increasing scan time, and increased almost linearly as a function of blood perfusion rate. For a scan window of 10 cm x 10 cm and a blood perfusion rate of 5 kg/m³ s, the simulated temperature fluctuations were within ± 0.5°C from the average temperature for scan times less than or equal to 20 s. The in vitro results agreed well with the numerical findings. The measured temperature fluctuations were less than 1.0°C for flow rates into the renal artery of less than 200 ml/min and scan times less than 20 s.