Lin W L, Liauh C T, Yen J Y, Chen Y Y, Shieh M J
Institute of Biomedical Engineering, National Taiwan University, Taipei.
Int J Radiat Oncol Biol Phys. 2000 Jan 1;46(1):239-47. doi: 10.1016/s0360-3016(99)00421-6.
To examine the optimal ultrasound frequency and the treatable domain determined by the tumor size and tumor depth when an external ultrasound heating system is employed for the brain tumor hyperthermia.
This work employs a simplified model of a scanned ultrasound transducer power deposition (a cone with convergent/divergent shape) and a search algorithm to investigate the optimal frequency and the treatable domain. The distributions of temperature and SAR (specific absorption rate) ratio are used to determine the appropriateness of the acoustic window size and the input power level for a yielded set of tumor conditions. The factors considered are the acoustic window size, tumor size and depth, ultrasound frequency, and the acoustic absorption of the post-target bone behind the tumor.
Simulation results demonstrate that the optimal frequency depends on the tumor depth and the acoustic absorption of the post-target bone. However, it is almost independent of the acoustic window size. The optimal frequency shifts to a higher level for a deeper tumor heating to reduce the effect of the high acoustic absorption of post-target bone. Moreover, the treatable domain is proportional to the acoustic window size and related to the ultrasound frequency.
It may not be possible to deliver appropriate ultrasonic energy to heat a brain tumor without overheating the normal brain tissue and/or the post-target bone under the constraints of the available acoustic window size for the ultrasonic beam, ultrasonic attenuation of brain tissue, high absorption of post-target bone, and high blood perfusion rate. The results of this study can be a guideline for designing an optimal ultrasound heating system, arranging the transducers, and implementing further treatment planning for the brain tumor hyperthermia.
研究在使用外部超声加热系统进行脑肿瘤热疗时，最佳超声频率以及由肿瘤大小和肿瘤深度所确定的可治疗区域。
本研究采用简化的扫描超声换能器功率沉积模型（一个具有会聚/发散形状的圆锥体）和一种搜索算法来研究最佳频率和可治疗区域。利用温度分布和比吸收率（SAR）来确定对于一组给定肿瘤条件下的声学窗口大小和输入功率水平的适宜性。所考虑的因素包括声学窗口大小、肿瘤大小和深度、超声频率以及肿瘤后方靶后骨的声吸收。
模拟结果表明，最佳频率取决于肿瘤深度和靶后骨的声吸收。然而，它几乎与声学窗口大小无关。对于更深的肿瘤加热，最佳频率会向更高水平偏移，以降低靶后骨高声吸收的影响。此外，可治疗区域与声学窗口大小成正比，并且与超声频率有关。
在超声束可用声学窗口大小、脑组织的超声衰减、靶后骨的高吸收以及高血液灌注率的限制下，可能无法在不使正常脑组织和/或靶后骨过热的情况下输送适当的超声能量来加热脑肿瘤。本研究结果可为设计最佳超声加热系统、布置换能器以及实施脑肿瘤热疗的进一步治疗计划提供指导。
