a School of Mechanical and Aerospace Engineering, Nanyang Technological University , Singapore.
Int J Hyperthermia. 2017 Nov;33(7):758-769. doi: 10.1080/02656736.2017.1309579. Epub 2017 May 15.
HIFU has been emerging as an effective and safe modality for the treatment of solid tumours and cancers. The focus shifting range of phased array HIFU transducer is an important safety concern because of the presence of grating lobe in the pre-focal region. However, previous studies were only based on linear acoustic wave model.
The nonlinear wave propagation from a 256-element phased array through multiple layered media was simulated using the angular spectrum approach (ASA) in marching fractional steps with the consideration of diffraction, attenuation and non-linearity effects by a second-order operator splitting scheme. The distribution of acoustic intensities, temperature elevations, lesion sizes and grating lobe levels were calculated at various axially post-focal shifting distances and driving frequencies.
Axially shifting HIFU focus leads to significant increase of the acoustic intensity at the grating lobe, but decrease at the main lobe. The influences on the acoustic field, thermal field and lesion sizes are determined by the shifting distance and driving frequency, and variations can be fit monotonically and linearly. Prediction accuracies by simple regression models are satisfactory. Irreversible tissue coagulation could be generated by the grating lobe at certain conditions.
The established nonlinear wave propagation algorithm allows the accurate description of HIFU field and consequently the evaluation of grating lobe and steerability of focus. The influence of focus shifting may be predicted simply. The treatment planning of phased array HIFU ablation could be optimised by setting the appropriate exposure and focus scanning schemes.
高强度聚焦超声（HIFU）已成为治疗实体肿瘤和癌症的一种有效且安全的手段。相控阵 HIFU 换能器的聚焦移频范围是一个重要的安全关注点，因为在近场存在栅瓣。然而，以前的研究仅基于线性声波模型。
使用角谱法（ASA），在行进分数步长中考虑衍射、衰减和非线性效应的二阶算子分裂方案，模拟了 256 元相控阵通过多层介质的非线性波传播。在不同轴向远场移频距离和驱动频率下，计算了声强、温升、病灶大小和栅瓣级别的分布。
轴向移动 HIFU 焦点会导致栅瓣处的声强显著增加，但主瓣处的声强会降低。对声场、热场和病灶大小的影响取决于移频距离和驱动频率，且变化可单调线性拟合。简单回归模型的预测精度令人满意。在某些条件下，栅瓣可能会产生不可逆的组织凝固。
建立的非线性波传播算法可准确描述 HIFU 场，从而评估栅瓣和焦点的可控性。可简单预测焦点移频的影响。通过设置适当的照射和焦点扫描方案，可以优化相控阵 HIFU 消融的治疗计划。
