生物介质中弱衍射超声束的形成机理及应用研究

Limited-diffraction acoustic beams (LDABs) have shown their advantages with several distinctive features, such as non-diffracting and self-reconstruction, which make them very promising in the applications of biomedical ultrasound. However, up to date applications of LDABs in biological tissues are quite limited. One reason for that is, current theories of LDABs are based on ideal non-viscous fluid environment, while those for nonlinear, dispersive and scattering bio-medium are still missing. On the other hand, generation of LDABs have been highly dependent on transducers with complex structures, which are usually hard for fabrications..This project aims at promoting the applications of LDABs in biomedical ultrasound, to achieve which theoretical, numerical and application studies will be carried out. Firstly, new theories will be developed considering the above-mentioned characteristics of bio-tissues, during which analytical solutions of wave motion and acoustic radiation force will be presented with the help of fractional derivative theory, weak-scattering theory and angular spectrum decomposition technique. After that, simulations based on finite difference (FD) and finite element (FE) will be carried out, pursuing a spatial-phase-modulation (SPM) technique which could then be capable of generating LDABs. In the current design, SPM will be achieved with a kid of acoustic lens, which incorporates extraordinary acoustic transmission (EAT) and Fabre-Perot (FP) resonance. Experiments using the same SPM technique will help to generate LDABs in tissue phantoms, while acoustic field measurements will be carried out with both a hydrophone and a tissue-mimicking phantom. Besides, it is also planned that the side lobes of LDABs could be effectively reduced, and the scattering-induced beam distortion could be compensated, so that their spatial-precision as well as re-construction properties could be optimized..With the obtained beams, two biomedical applications utilizing LDABs will be developed, demonstrating the value and power of this new technique. One is to generate a focused ultrasound beam, the shape of whose focal area could be close to a sphere rather than an ellipsoid; to achieve that, Airy beams of three-dimensional will be pursued and exploited. The other application involves constructing a quasi-standing ultrasonic field of curved path, while two half-Bessel beams will be utilized to make contribution. .With all the plans carried out, it is expected that we could make a forward step on LDABs related nonlinear acoustics theory, boost the applications of LDABs in biomedical ultrasound, as well as making further impacts on ultrasonic imaging, therapy and particle manipulation.

弱衍射声束具有无衍射、自重构等特性，在生物医学超声中具有重要的应用前景，但其面向生物介质的理论和应用研究很不成熟。一方面，现有声学理论基于理想非粘滞流体，未考虑生物介质的复杂性；另一方面，弱衍射束的产生大多基于复杂的换能器结构，实用性差。本项目研究弱衍射超声束在生物介质中的传播理论、产生方法和具体应用。首先基于分数导数修正的非线性波动方程，研究介质的非线性、色散和散射对声束波动及力效应的影响；其次，研究基于声透镜的空间相位调制方法，实现弱衍射超声束的形成；进一步，通过对弱衍射声束进行性能优化如旁瓣抑制，研究提高其空间精度的方法并进行实验验证；最后，利用Airy声束构建焦域近似为球形的聚焦超声场；基于半Bessel声束形成具有弯曲路径的异形准驻波场，并应用于粒子操控。研究结果将丰富非线性声学理论，拓展弱衍射声束在生物介质的应用，并对超声成像、超声治疗和超声粒子操控产生积极影响。

针对弱衍射声束在生物医学中的重要应用前景，从理论、方法和应用方面进行了具体的研究。从理论出发，首先基于分数阶导数理论，对描述非线性声传播过程的KZK方程进行了修正，其中考虑了声速和声衰减随频率的变化；基于聚焦声场对所得理论进行检验，结果表明，改进的KZK方程可很好地预测生物介质中非线性焦点的偏移。对于1MHz和2.25MHz的聚焦声场，该偏移分别可达到1-3 mm和4-5 mm。随后，研究了近轴近似条件下的Airy声束和Gaussian-Airy声束，发现高斯分布的空间幅度调制可有效地抑制声束旁瓣的幅度；利用该声束形成双边聚焦声场，可有效地改善焦域的主瓣/旁瓣幅度比，辅以空间相位调制可进一步调节焦点距离。进一步，研究基于超声线阵发射脉冲形式Airy声束及其双边聚焦的方法，指出此类声束的“横向自加速”现象在瞬态声场中并不存在；同时，利用脉冲Airy声束进行了B超成像研究，所提出的方法可以克服近场存在的波长尺度障碍物，实现自愈。在此基础上，研究利用半Bessel声束进行物体操控，基于被动相位调制和主动相位编码相结合，构建了瓶状涡旋聚焦声场，研究了其在不同拓扑荷下的声场分布和辐射力场分布特征。最后，基于超声微流控器件，采用显微粒子成像测速技术，提出了对声辐射力/声流场进行分离成像的理论和方法。基于实验验证，证明了该方法可适用于驻波声场、聚焦声场等不同类型的声场分布；同时，对于小尺寸粒子，声辐射力的作用可忽略，而对于大粒子，声流的作用并不能简单地忽略。在上述研究中，针对实际遇到的问题提出了解决方案。一方面，针对软材料声学参数难以确定的问题，提出了两步测量方法；以PDMS为例，给出了在3-7MHz频率范围内纵波、横波的声速和衰减系数，得到了该材料的复弹性常数。另一方面，针对超声探头在宽带条件下电阻抗浮动的问题，提出了在线动态测量-匹配的理论和硬件解决方案，可在约100%带宽范围内使换能器电阻抗的相位为零，功率反射率达到较低水平。本项目所得理论、方法和器件将为弱衍射声束在生物医学中的进一步应用奠定基础，为新型医用超声设备的开发提供新方案。