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Optimization of matching layer design for medical ultrasonic transducer.

机译：医用超声换能器匹配层设计的优化。

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摘要

This thesis work contains two major parts. In the first part, ultrasonic wave propagation in multilayer structure is investigated. Delaminations between ceramic and electrode layers in multilayer capacitors and multilayer actuators are common defects, which are difficult to detect using traditional ultrasonic imaging method if the size is smaller than 50 microns in diameter. The T-Matrix method is used to treat wave attenuation in periodic structures with alternating ceramic and electrode layers. Multiple penny-shaped delaminations are assumed perpendicular to the incidence wave, and the forward scattering amplitude of the wave from delaminations is calculated by substituting the average effective crack opening displacement into the scattered wave displacement. The effective phase velocity, wave amplitude and the attenuation coefficient have been calculated for different crack densities. The results provide a theoretical base for potential attenuation based ultrasonic non-destructive evaluation (NDE) method.;The second part is a study on matching layers. Matching layers are crucial components in ultrasonic transducers for medical imaging. Without proper matching layers, large acoustic impedance mismatch between piezoelectric resonator and the human body tissue will cause most of the ultrasound energy to be reflected at the interface. For a given frequency, the matching layer thickness should be one quarter of the wavelength and its acoustic impedance should be the geometric mean of the acoustic impedances of piezoelectric material and the imaging body. There are no natural materials that can precisely meet such requirements. Therefore, solid particle/polymer composites are commonly used as matching layer materials. The acoustic impedance of such composites is generally in the range of 2-15 MRayls. It is a routine task to make such composite for low frequency transducers, but for transducers with operating frequency higher than 40 MHz, the powder size must be sub-microns in order to reduce wave scattering because the wavelength is much smaller. High volume loading fine powder composite is very difficult to make using conventional composite fabrication technique because air bubbles will be trapped in the mixture. Therefore, all ultrahigh frequency transducers currently used or under development are not properly matched because the lacking of desired matching layer materials. This problem hinders the development of finer resolution ultrahigh frequency ultrasonic imaging.;There are some progress made in the past 3 years and there are sol-gel SiO2/polymer nano-composites being developed that can have acoustic impedance up to 5.7 MRays. In this thesis work, TiO2 nano-structured material has been developed. The material has porous nano-structure with the volume fraction of voids being controlled by the amount of bonding amorphous phase in the material and its acoustic impedance can be further tuned by heat treatment at slightly elevated temperatures. Using the quarter wavelength thickness characterization method, the acoustic properties of this nanostructured material were accurately characterized. It was found that the acoustic impedance can reach as high as 7.19 MRayls, which is a concrete improvement compared to that of the best nano-composites available.;Because of recent rapid development of the single crystal PMN-PT and PZT-PT materials for medical ultrasonic transducer applications, there is a new excitement to develop transducers with very broad bandwidth because the electromechanical coupling coefficient of these single crystals are better than 90%. For such very broad bandwidth transducers, the front matching layer will be the limiting factor because the quarter wavelength matching layer acts like a filter whose bandwidth is generally less than 100%. One of the main tasks of this thesis is to investigate matching layer design with gradient acoustic impedance to achieve much broader bandwidth (>100%). Wave propagation within an inhomogeneous multilayer structure has been analyzed and simulated using the finite difference time domain numerical technique. By adjusting the acoustic impedance distribution function, we have found the best gradient design which has super broad bandwidth. In fact, the passband only has a low frequency cut-off and it works for almost all frequencies beyond the cut-off frequency. To certain extend, this optimized design is universal so that such a matching layer can be used in all medical transducers of different frequencies.
机译：论文工作分为两大部分。在第一部分中，研究了超声波在多层结构中的传播。多层电容器和多层执行器中陶瓷层和电极层之间的分层是常见的缺陷，如果尺寸小于直径50微米，则很难使用传统的超声成像方法进行检测。 T-Matrix方法用于处理具有交替的陶瓷层和电极层的周期性结构中的波衰减。假定垂直于入射波有多个便士形分层，并且通过将平均有效裂纹开口位移代入散射波位移来计算波从分层的前向散射幅度。计算出不同裂纹密度下的有效相速度，波幅和衰减系数。研究结果为基于势能衰减的超声无损评价（NDE）方法提供了理论基础。第二部分是对匹配层的研究。匹配层是用于医学成像的超声换能器中的关键组件。如果没有合适的匹配层，压电谐振器与人体组织之间的大声阻抗失配将导致大部分超声能量在界面处反射。对于给定的频率，匹配层的厚度应为波长的四分之一，并且其声阻抗应为压电材料和成像主体的声阻抗的几何平均值。没有天然材料可以完全满足这些要求。因此，固体颗粒/聚合物复合材料通常用作匹配层材料。这种复合材料的声阻抗通常在2-15 MRayls范围内。为低频换能器制造这种复合材料是一项常规任务，但对于工作频率高于40 MHz的换能器，粉末的尺寸必须为亚微米，以减少波散射，因为波长要小得多。使用常规的复合材料制造技术很难制造高容量的细粉复合材料，因为气泡将被捕获在混合物中。因此，由于缺少所需的匹配层材料，所以当前使用中或正在开发中的所有超高频换能器都无法正确匹配。这个问题阻碍了更高分辨率的超高频超声成像的发展。在过去的三年中取得了一些进展，并且正在开发具有高达5.7 MRays的声阻抗的溶胶-凝胶SiO2 /聚合物纳米复合材料。在本文工作中，开发了TiO2纳米结构材料。该材料具有多孔纳米结构，空隙的体积分数由材料中键合非晶相的数量控制，其声阻抗可通过在略微升高的温度下进行热处理来进一步调节。使用四分之一波长厚度表征方法，可以精确表征这种纳米结构材料的声学特性。结果发现，声阻抗可以达到7.19 MRayls，与目前可用的最佳纳米复合材料相比，这是一个具体的改进。由于最近单晶PMN-PT和PZT-PT材料的快速发展，在医疗超声换能器应用中，开发具有非常宽带宽的换能器具有新的激情，因为这些单晶的机电耦合系数优于90％。对于这种带宽非常宽的换能器，前匹配层将成为限制因素，因为四分之一波长匹配层的作用就像一个滤波器，其带宽通常小于100％。本文的主要任务之一是研究具有梯度声阻抗的匹配层设计，以实现更大的带宽（> 100％）。已经使用有限差分时域数值技术分析和模拟了非均匀多层结构内的波传播。通过调整声阻抗分布函数，我们发现了具有超宽带宽的最佳梯度设计。实际上，通带只有一个低频截止点，它几乎适用于截止频率以外的所有频率。在某种程度上，这种优化设计是通用的，因此这样的匹配层可以用于所有不同频率的医疗换能器中。

著录项

  • 作者

    Zhu, Jie.;

  • 作者单位

    The Pennsylvania State University.;

  • 授予单位 The Pennsylvania State University.;
  • 学科 Engineering Biomedical.;Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2008
  • 页码 172 p.
  • 总页数 172
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 生物医学工程;工程材料学;
  • 关键词

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