Liu Xiaozhou, Li Junlun, Gong Xiufen, Zhang Dong
State Key Lab of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing 210093, China.
Ultrasonics. 2006 Dec 22;44 Suppl 1:e27-30. doi: 10.1016/j.ultras.2006.06.035. Epub 2006 Jun 30.
In recent years the propagation of the high intensity focused ultrasound (HIFU) in biological tissue is an interesting area due to its potential applications in non-invasive treatment of disease. The base principle of these applications is the heat effect generated by ultrasound absorption. In order to control therapeutic efficiency, it is important to evaluate the heat generation in biological tissue irradiated by ultrasound. In his paper, based on the Khokhlov-Zabolotkaya-Kuznetsov (KZK) equation in frequency-domain, the numerical simulations of nonlinear absorption in biological tissues for high intensity focused ultrasound are performed. We find that ultrasound thermal transfer effect will be enhanced with the increasing of initial acoustic intensity due to the high harmonic generation. The concept of extra absorption factor is introduced to describe nonlinear absorption in biological tissue for HIFU. The theoretical results show that the heat deposition induced by the nonlinear theory can be nearly two times as large as that predicated by linear theory. Then, the influence of the diffraction effect on the position of the focus in HIFU is investigated. It is shown that the sound focus moves toward the transducer compared with the geometry focus because of the diffraction of the sound wave. The position of the maximum heat deposition is shifted to the geometry focus with the increase of initial acoustic intensity because the high harmonics are less diffraction. Finally, the temperature in the porcine fat tissue changing with the time is predicated by Pennes' equation and the experimental results verify the nonlinear theoretical prediction.
近年来，高强度聚焦超声（HIFU）在生物组织中的传播成为一个有趣的研究领域，因为其在疾病的非侵入性治疗方面具有潜在应用价值。这些应用的基本原理是超声吸收产生的热效应。为了控制治疗效果，评估超声辐照生物组织中的产热情况很重要。在本文中，基于频域中的Khokhlov-Zabolotkaya-Kuznetsov（KZK）方程，对高强度聚焦超声在生物组织中的非线性吸收进行了数值模拟。我们发现，由于高次谐波的产生，随着初始声强的增加，超声热传递效应会增强。引入了额外吸收因子的概念来描述生物组织中HIFU的非线性吸收。理论结果表明，非线性理论诱导的热沉积几乎是线性理论预测值的两倍。然后，研究了衍射效应对HIFU中焦点位置的影响。结果表明，由于声波的衍射，声焦点相对于几何焦点向换能器移动。随着初始声强的增加，最大热沉积位置向几何焦点移动，因为高次谐波的衍射较小。最后，用Pennes方程预测了猪脂肪组织中温度随时间的变化，实验结果验证了非线性理论预测。
