测量与理论模型结合方法定征高强度聚焦超声非线性声场

发布时间：2018-05-20 06:24

本文选题：高强度聚焦超声 + 非线性声场　；参考：《南京大学》2014年博士论文

【摘要】：高强度聚焦超声(High Intensity Focused Ultrasound, HIFU)是一种治疗实体良／恶性肿瘤的非侵入性新兴治疗技术,焦点的声强可高达103W/cm2,能够在短时间内(秒级)使焦域组织凝固性坏死。为了更好的控制HIFU治疗过程中的临床效果,需要对HIFU的非线性声场进行准确描述。本文首先介绍了HIFU非线性声场的测量方法和相关理论模型,在此基础上讨论了冲击波产生后,HIFU声场的定征方法。针对大张角强聚焦换能器,本论文提出了频域-时域结合算法计算其非线性声场分布的方法。利用椭球坐标系非线性方法(SBE)描述声场分布,在频域计算声场衍射和衰减效应,在时域计算非线性效应。该方法计算结果与Rayleigh积分及传统频域算法计算结果吻合的很好,证明了该计算方法的可靠性。冲击波产生后,传统频域算法计算时会产生明显的波形振荡,利用该方法不仅可以有效的避免冲击波波形振荡而且还缩短了计算时间。冲击波产生后,由于其极限条件(强声压,宽频谱,高空间分辨率),直接测量其声场分布不仅耗时且对测量设备的要求较高。本论文提出实验测量与理论计算相结合的方式定征大张角强聚焦换能器的非线性声场。首先,忽略换能器封装等因素,将其视作表面均一振动的振子,在线性声场条件下确定其有效参数；其次,利用焦点波形二次谐波与基波之比,结合实验测量结果和模型计算结果得到驱动电压与声源表面声压幅值的关系；最后,利用SBE模型计算在任意驱动下的非线性声场分布。将本方法得到的焦点波形与光纤水听器测量结果做对比,结果表明：在较低驱动下,由于非线性效应不足以产生足够高的二次谐波,本方法不能用于描述低驱动下的声场分布；当冲击波产生后,测量波形与仿真结果在峰峰值以及相位上存在差距,两者的频谱在50MHz范围内吻合的很好,这说明冲击波产生后,光纤水听器的测量波形可能受限于带宽等原因,与实际结果存在一定的差距。本章中提出的方法有助于准确定征冲击波产生后的HIFU声场。本论文利用实验测量与理论计算相结合的方法定征HIFU非线性声场,该工作将进一步推进对HIFU声场非线性分布的研究,更加准确的预测超声在活体组织中的传播,从而促进高强度聚焦超声在临床的应用。
[Abstract]:High intensity focused ultrasound (HIFU) high Intensity Focused Ultrasound, HIFU) is a new non-invasive treatment technique for solid benign and malignant tumors. The focal sound intensity can be up to 103 W / cm ~ 2 and can cause coagulation necrosis of focal area tissue in a short time (second order). In order to better control the clinical effect of HIFU, it is necessary to accurately describe the nonlinear sound field of HIFU. In this paper, the measurement method and related theoretical model of HIFU nonlinear sound field are introduced, and the method of determining the acoustic field after shock wave is discussed. In this paper, a method for calculating the nonlinear acoustic field distribution of a large angle strong focusing transducer is proposed by combining frequency-domain and time-domain algorithm. The nonlinear method of ellipsoidal coordinate system (SBE) is used to describe the distribution of sound field. The diffraction and attenuation effects of sound field in frequency domain and nonlinear effect in time domain are calculated. The calculated results of this method are in good agreement with the Rayleigh integral and the traditional frequency domain algorithm, which proves the reliability of the method. After the shock wave is produced, the traditional frequency-domain algorithm will produce obvious waveform oscillation, which can not only effectively avoid the shock wave oscillation but also shorten the calculation time. After the shock wave is produced, due to its limit conditions (strong sound pressure, wide frequency spectrum, high spatial resolution), it is not only time consuming to measure the sound field distribution directly, but also the requirement of measuring equipment is high. In this paper, the nonlinear sound field of the strong focus transducer with large angle of tension is determined by combining the experimental measurement with the theoretical calculation. First of all, the transducer packaging is ignored, and it is regarded as a vibrator with homogeneous surface vibration, and its effective parameters are determined under the condition of linear sound field. Secondly, the ratio of the second harmonic to the fundamental wave of the focus waveform is used. The relationship between the driving voltage and the amplitude of sound pressure on the sound source surface is obtained by combining the experimental results with the results of the model calculation. Finally, the nonlinear sound field distribution under arbitrary driving is calculated by using the SBE model. The focus waveform obtained by this method is compared with the measurement results of fiber-optic hydrophone. The results show that the method can not be used to describe the sound field distribution under low driving because the nonlinear effect is not enough to produce high second harmonic. When the shock wave is produced, there is a gap between the measured waveform and the simulation result in peak, peak and phase. The spectrum of the two waves is in good agreement in the range of 50MHz, which indicates that after the shock wave is produced, The measurement waveform of fiber-optic hydrophone may be limited by bandwidth, which is far from the actual results. The method proposed in this chapter is helpful to determine the HIFU sound field after the shock wave is produced. In this paper, the nonlinear acoustic field of HIFU is characterized by the combination of experimental measurement and theoretical calculation. This work will further promote the study of the nonlinear distribution of HIFU sound field, and more accurately predict the propagation of ultrasound in living tissue. So as to promote the clinical application of high intensity focused ultrasound.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：R445.1

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