Transfer functions of US transducers for harmonic imaging and bubble responses

Current medical diagnostic echo systems are mostly using harmonic imaging. This means that a fundamental frequency (e.g., 2 MHz) is transmitted and the reflected and scattered higher harmonics (e.g., 4 and 6 MHz), produced by nonlinear propagation, are recorded. The signal level of these harmonics is usually low and a well-defined transfer function of the receiving transducer is required. Studying the acoustic response of a single contrast bubble, which has an amplitude in the order of a few Pascal, is another area where an optimal receive transfer function is important.

We have developed three methods to determine the absolute transfer function of a transducer. The first is based on a well-defined wave generated by a calibrated source in the far field. The receiving transducer receives the calibrated wave and from this the transfer functions can be calculated. The second and third methods are based on the reciprocity of the transducer. The second utilizes a calibrated hydrophone to measure the transmitted field. In the third method, a pulse is transmitted by the transducer, which impinges on a reflector and is received again by the same transducer. In both methods, the response combined with the transducer impedance and beam profiles enables the calculation of the transfer function.

The proposed methods are useful to select the optimal piezoelectric material (PZT, single crystal) for transducers used in reception only, such as in certain 3D scanning designs and superharmonic imaging, and for selected experiments like single bubble behavior.

We tested and compared these methods on two unfocused single element transducers, one commercially available (radius 6.35 mm, centre frequency 2.25 MHz) the other custom built (radius 0.75 mm, centre frequency 4.3 MHz). The methods were accurate to within 15%.     

传输电缆对换能器阻抗特性的影响

本文通过传输线方法讨论了电缆对换能器的影响,随频率增加,电缆对换能器的影响交蛎,主要表现为,最大电导增大,串联谐振率下降,这些都是由于电缆传输系数随频率变化引起的,并且与换能器的阻抗特性有关。     

高频聚焦超声声场和温度场的仿真研究

为探究临床常用的7 MHz高频聚焦超声在多层生物组织中的声传播以及毫秒级时间内的生物传热规律问题,基于Westervelt方程和Pennes传热方程,使用有限元方法建立高频聚焦超声辐照多层组织的非线性热黏性声传播及传热模型。首先分析了线性模型和非线性模型之间的差异,然后在非线性模型下探究换能器的参数对声场和温度场的影响。仿真结果显示：在7 MHz频率下,当换能器输出声功率超过5 W时,声波传播的非线性效应不可忽视(p <0.05);当声功率从5 W增大到15 W时,非线性模型与线性模型预测的温度偏差从20%增加到34.703%;高频聚焦超声波的非线性行为比低频更加显著,基频能量向高次谐波转移的程度增大,声功率为10 W和15 W时4次谐波与基波之比分别达到7.33%和12.12%;高频换能器参数的改变对组织中声场和温度场分布的影响较大,换能器焦距从12 mm减小到11.2 mm,焦点处最高温度增加了77%。结果表明,7 MHz聚焦超声的非线性声传播需要考虑到4次谐波的影响。该文提出的多层组织非线性仿真模型可为高频聚焦超声换能器参数优化及制定安全、有效的术前治疗方案提供理论参考。     

Controlled Space Radiation concept for mesh-free semi-analytical technique to model wave fields in complex geometries

Numerical modelling of the ultrasonic wave propagation is important for Structural Heath Monitoring and System Prognosis problems. In order to develop intelligent and adaptive structures with embedded damage detector and classifier mechanisms, detailed understanding of scattered wave fields due to anomaly in the structure is inevitably required. A detailed understanding of the problem demands a good modelling of the wave propagation in the problem geometry in virtual form. Therefore, efficient analytical, semi-analytical or numerical modelling techniques are required. In recent years a semi-analytical mesh-free technique called Distributed Point Source Method (DPSM) is being used for modelling various ultrasonic, electrostatic and electromagnetic wave field problems. In the conventional DPSM approach point sources are placed along the transducer faces, problem boundaries and interfaces to model incident and scattered fields. Every point source emits energy in all directions uniformly. Source strengths of these 360° radiation sources are obtained by satisfying interface and boundary conditions of the problem. In conventional DPSM modelling approach it is assumed that the shadow zone does not require any special consideration. 360° Radiation point sources should be capable of properly modelling shadow zones because all boundary and interface conditions are satisfied. In this paper it is investigated how good this assumption is by introducing the ‘shadow zone’ concept at the point source level and comparing the results generated by the conventional DPSM and by this modified approach where the conventional 360° radiation point sources are replaced by the Controlled Space Radiation (CSR) sources.     

Single-element ultrasonic transducer modeling using a hybrid FD–PSTD method

In a recent publication [E. Filoux, S. Callé, D. Certon, M. Lethiecq, F. Levassort, Modeling of piezoelectric transducers with combined pseudospectral and finite-difference methods, J. Acoust. Soc. Am. 123 (6) (2008) 4165–4173], a new finite-difference/pseudospectral time-domain (FD–PSTD) algorithm was presented and used to model the generation of acoustic waves by a piezoelectric resonator and their propagation in the structure and the surrounding water. In this paper, the model has been extended to simulate the two-dimensional behaviour of a complete single-element transducer, composed of the resonator, a backing and a front matching layer. This further version of the model takes into account the mechanical loss in materials, and enables the calculation of electrical impedance, which is a characteristic of high interest to optimize the performance of ultrasonic transducers. The impedance curves of a PZT [URL: http://www.ferroperm-piezo.com (last viewed 04/2008); B. Jaffe, R.S. Roth, S. Marzullo, Piezoelectric properties of lead zirconate-lead titanate solid-solution ceramics, J. Appl. Phys. 25 (1954) 809–810] plate-based high-frequency transducer, with a 50 MHz thickness resonant frequency, were compared to those of a KLM model [R. Krimholtz, D.A. Leedom, G.L. Matthei, New equivalent circuit for elementary piezoelectric transducers, Electron. Lett. 6 (1970) 398–399] in the one-dimensional case. The acoustical properties were also found to be in good agreement with those obtained using the finite element (FE) method of ATILA^® software in two-dimensional configuration.     

Molekül‐Detektive. Biosensoren

Biosensors are analytical devices incorporating biological material (receptor) intimately associated with or integrated within a physicochemical transducer. Advantages are the high selectivity for analyte detection. Examples given comprise the very successful commercial blood glucose biosensors made for the self‐control by the diabetic patients. Other biosensors are part of an analytic system, including the sensor chips of surface plasmon resonance or interferometry based devices, piezoelectric or reflectometric sensors capable of direct measurement of mass changes, and thermometric and other reagentless sensors. The development of nanotubes‐based devices allows for significant enhancment of the signal‐tonoise ratio of the biosensors. A milestone on the way towards miniaturization and parallelization of biosensors is the recently developed and prize‐winning electronic DNA chip.     

压电式摆动圆柱形偶极发射换能器优化研究

为了实现偶极信号有效辐射,获得足够高的信噪比,全面分析弯曲波频散,本文提出了一种新型的频带范围为0.5–5 k Hz的压电式摆动圆柱形偶极声波测井换能器,并采用有限元方法对其进行了模拟和优化设计。此外,论文还对换能器进行了实验样机制作和实验测量,将实验测量与数值模拟结果进行了对比。研究表明,提出的新型偶极声波换能器比传统换能器具有更高的发射灵敏度,尤其是在低频响应范围。本文为新型偶极声波测井换能器的研发工作奠定了基础。     

LON智能变送器设计与实现

智能化仪器仪表设计研究成为新一代工业制造技术突破的重要瓶颈之一。LON智能变送器是基于LonWorks总线,具有多种信息智能处理的智能变送器,能够将标准的模拟信号、传感器输出的非标准的弱模拟信号及其脉冲信号转换为数字信号,与其它控制仪表实现双向数字通讯。LON智能变送器除了具有数字调零、数字滤波等功能外,其重要特征是具有传感器故障诊断与处理功能,在传感器失效后,能够在一定时间内继续输出有效数据,保证控制系统的稳定和安全。     

毛细管流动池在光声检测中的应用

设计了适于激光诱导光声流动检测的毛细管流动池,讨论了池的特点及其在分析化学中的应用,用该池检测Co~(2+),检测限相当于2×10~(-6)cm~(-1)光吸收。     

基于阵列波导光纤温度特性的分布式光纤布喇格光栅解调法

设计了一种使用阵列波导光栅对分布式光纤布喇格光栅中心波长较大范围变化的解调方法.根据阵列波导光栅各通道的中心波长可随温度变化而改变的特性,通过控制软件循环地在几分钟内使加在阵列波导光栅的芯片温度从30℃线性增加到90℃,同时用微机高速采集各通道的数据并分别找出各通道数据的最大值时刻所对应的阵列波导光栅芯片温度,从而根据其波长-温度关系在微机上报告此时各光纤布喇格光栅的中心波长.实验结果表明,系统有效地解决了分布式解调的问题, 微机以小于10 min的周期报告出每通道0.6 nm范围变化的光纤布喇格光栅中心波长(共40个通道),测量相对误差小于2%.