Ultrasound Department, Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw, Poland.
Ultrasonics. 2011 Dec;51(8):997-1005. doi: 10.1016/j.ultras.2011.05.016. Epub 2011 Jun 15.
This work addresses the difficulties in the measurements of the nonlinear medium parameter B/A and presents a modification of the finite amplitude method (FAM), one of the accepted procedures to determine this parameter. The modification is based on iterative, hybrid approach and entails the use of the versatile and comprehensive model to predict distortion of the pressure-time waveform and its subsequent comparison with the one experimentally determined. The measured p-t waveform contained at least 18 harmonics generated by 2.25 MHz, 29 mm effective diameter, single element, focused PZT source (f-number 3.5) and was recorded by Sonora membrane hydrophone calibrated in the frequency range 1-40 MHz. The hydrophone was positioned coaxially at the distal end of the specially designed, two-section assembly comprising of one, fixed length (60mm), water-filled cylindrical container and the second, variable length (60-120 mm) container that was filled with unknown medium. The details of the measurement chamber are described and the reasons for this specific design are analyzed. The data were collected with the variable length chamber filled with 1.3-butanediol, which was used as a close approximation of tissue mimicking phantom. The results obtained provide evidence that a novel combination of the FAM with the semi-empirical nonlinear propagation model based on the hyperbolic operator is capable of reducing the overall uncertainty of the B/A measurements as compared to those reported in the literature. The overall uncertainty of the method reported here was determined to be ±2%, which enhances the confidence in the numerical values of B/A measured for different, clinically relevant media. Optimization of the approach is also discussed and it is shown that it involves an iterative procedure that entails a careful selection of the acoustic source and its geometry and the axial distance over which the measurements need to be performed. The optimization also depends critically on the experimental determination of the source surface pressure amplitude.
这项工作解决了非线性介质参数 B/A 的测量难题，并提出了有限振幅法（FAM）的一种改进方法，这是确定该参数的一种公认方法。该改进方法基于迭代、混合方法，需要使用多功能和全面的模型来预测压力-时间波形的失真，并将其与实验确定的波形进行比较。所测量的 p-t 波形至少包含 18 个由 2.25MHz、29mm 有效直径、单个元件、聚焦 PZT 源（f-number 3.5）产生的谐波，并由在 1-40MHz 频率范围内校准的 Sonora 膜水听器记录。水听器同轴放置在专门设计的两段式组件的远端，该组件包括一个固定长度（60mm）的充水圆柱形容器和第二个可变长度（60-120mm）的容器，该容器填充有未知介质。详细描述了测量腔室的细节，并分析了这种特定设计的原因。数据是在可变长度腔室中充满 1.3-丁二醇的情况下收集的，1.3-丁二醇用作组织模拟体模的近似物。所获得的结果表明，FAM 与基于双曲算子的半经验非线性传播模型的新颖组合能够降低 B/A 测量的整体不确定性，与文献中报道的结果相比。报告的方法的整体不确定性确定为±2%，这增强了对不同临床相关介质测量的 B/A 值的数值的信心。还讨论了该方法的优化，并表明它涉及迭代过程，需要仔细选择声源及其几何形状以及需要进行测量的轴向距离。优化还严重依赖于声源表面压力幅度的实验确定。
