Ultrasound Hyperthermia Treatments Research Articles

Simulations of scanned focused ultrasound hyperthermia. the effects of scanning speed and pattern on the temperature fluctuations at the focal depth

A transient three-dimensional simulation program has been developed to investigate the effects of scanning speed, scanning pattern, blood perfusion, and transducer choice on the temperature fluctuations that occur during scanned focused ultrasound hyperthermia treatments. The model uses the bioheat transfer equation with uniform tissue properties to solve for the temperature field. The results show that the largest temperature fluctuations are always located on the scanning path in the acoustical focal plane and that the temperature fluctuation pattern and magnitudes are essentially the same, regardless of the focal depth. The results also show that the magnitude of these temperature fluctuations increases linearly with increasing scan times (decreasing scanning speeds) and increases as a weak exponential with the magnitude of the blood perfusion rate. Moreover, the smaller the diameter of the focus of the power field, the larger the temperature fluctuations. To avoid temperature fluctuations inside the scanned volume, scan time of 10 s of less were needed when single 2-cm-diameter circular scans were simulated at practical blood flow values. The general trends predicted by the simulations agree with the trends present in previously reported experiments, indicating that the simulations could be an important tool in patient treatment planning and temperature field approximations.

Temperature elevation at muscle-bone interface during scanned, focused ultrasound hyperthermia.

The effect of a muscle-bone interface on the temperature distribution during scanned, focused ultrasound hyperthermia was studied in vivo in dogs' thighs. The detailed temperature maps showed a sharp temperature maximum at the interface. The magnitude of the temperature elevation was between two and four times as large as obtained in resting muscle with similar intensities. The muscle tissue close to the bone could be heated to a therapeutic temperature level without overheating the bone when higher frequency or multiple-focused beams were used. The results showed that bone pain associated with ultrasound hyperthermia treatments is probably caused by extreme temperatures in bone, which can be avoided by sonicating the tumour in such a manner that the intensities at the bone surface are low enough so as not to cause large temperature elevations. Therefore, treatment planning which takes into account the location of bones is necessary for successful, clinical utilization of scanned, focused ultrasound hyperthermia.