环形阵列超声断层扫描高分辨率成像方法

介绍了针对环形阵列超声断层扫描的高分辨率全波形反演成像方法,不同于传统的延时叠加和渡越时间断层扫描,该方法充分考虑声传播的透射、反射、散射等特点,通过数据匹配的方法重构目标体声学参数图像。开发了环形超声换能器阵列构建超声断层扫描成像系统,并设计了适合检验成像算法的数值和物理模型,分别使用基于射线理论的回波延时叠加和初至波渡越时间断层扫描、以及基于波动理论的全波形反演方法对目标体进行图像重建,并对比其原理、分辨率、计算量等方面的特点,系统性分析了波动方法相对传统射线方法的优点和不足。结果表明,全波形反演这类充分考虑接收信号多种物理特征的方法可用于较高分辨率的声参数成像,适合与传统方法结合设计有效的超声断层扫描成像系统。     

高分辨率快速数字化光声CT乳腺肿瘤成像

提出了一种基于聚焦线性阵列探测器的快速光声计算机断层成像技术（光声CT）.在光声二维图像重建中,根据阵列探测器机械扫描和电子扫描相结合的组合扫描模式,提出了改进的有限场滤波反投影重建算法.一方面该算法适合多元探测器旋转扫描模式,另一方面探测器的指向性函数作为反投影的权重因子提高了系统的横向分辨率.同时,该成像系统还利用柱面声透镜实现Z轴方向上的聚焦扫描以实现三维层析成像.实验中,这套成像系统空间分辨率达到0.2mm,Z轴方向分辨率为1.5mm,扫描一幅二维图像仅需150s,得到 关键词： 光声CT 有限场滤波反投影算法 声透镜聚焦 乳腺肿瘤检测     

High-resolution ultrasound-modulated optical tomography in biological tissues

We present a novel implementation of high-resolution ultrasound-modulated optical tomography that, based on optical contrast, can image several millimeters deep into soft biological tissues. A long-cavity confocal Fabry-Perot interferometer, which provides a large etendue and a short response time, was used to detect the ultrasound-modulated coherent light that traversed the scattering biological tissue. Using 15-MHz ultrasound, we imaged with high-contrast light-absorbing structures placed >3 mm below the surface of chicken breast tissue. The resolution along the axial and the lateral directions with respect to the ultrasound propagation direction was better than 70 and 120 microm, respectively. The resolution can be scaled down further by use of higher ultrasound frequencies. This technology is complementary to other imaging technologies, such as confocal microscopy and optical-coherence tomography, and has the potential for broad biomedical applications.     

Boundary normal pressure-based electrical conductivity reconstruction for magneto–acoustic tomography with magnetic induction

As a kind of multi-physics imaging approach integrating the advantages of electrical impedance tomography and ultrasound imaging with the improved spatial resolution and image contrast, magneto–acoustic tomography with magnetic induction(MAT-MI) is demonstrated to have the capability of electrical impedance contrast imaging for biological tissues with conductivity differences. By being detected with a strong directional transducer, abrupt pressure change is proved to be generated by the gradient of the induced Lorentz force along the force direction at conductivity boundary. A simplified boundary normal pressure(BNP)-based conductivity reconstruction algorithm is proposed and the formula for conductivity distribution inside the object with the clear physical meaning of pressure derivative, is derived. Numerical simulations of acoustic pressure and conductivity reconstruction are conducted based on a 2-layer eccentric cylindrical phantom model using Hilbert transform. The reconstructed two-dimensional conductivity images accord well with the model, thus successfully making up the deficiency of only imaging conductivity boundary in traditional MAT-MI. The proposed method is also demonstrated to have a spatial resolution of one wavelength. This study provides a new method of reconstructing accurate electrical conductivity and suggests the potential applications of MAT-MI in imaging biological tissues with conductivity difference.     

基于角度-多普勒分辨的反射层析激光成像雷达研究

设计了一种基于角度-多普勒分辨的接发同轴反射层析激光成像雷达系统,给出了基本成像原理和数学表达,相应分析了目标横向距离分辨率和单个角度采样时间的关系。并在实验室平台上模拟远场衍射传输,获取探测目标的角度-多普勒分辨的反射投影图,采用滤波反投影算法实现目标横截面图像重建。此方法采用相干外差探测,可以大大提高雷达接收信号灵敏度,同时不涉及光频空间相位和时间相位,成像中没有相位匹配过程,降低了实施技术难度,具有一定的实际意义和使用价值。     

A measurement-domain adaptive beamforming approach for ultrasound instrument based on distributed compressed sensing: Initial development

High efficient acquisition of the sensor array signals and accurate reconstruction of the backscattering medium are important issues in ultrasound imaging instrument. This paper presents a novel measurement-domain adaptive beamforming approach (MABF) based on distributed compressed sensing (DCS) which seeks to simultaneously measure signals that are each individually sparse in some domain(s) and also mutually correlated with much few measurements under the Nyquist rate. Instead of sampling conventional backscattering signals at the Nyquist rate, few linear projections of the returned signal with random vectors are taken as measurements, which can reduce the amount of samples per channel greatly and makes the real-time transmission of sensor array data possible. Then high resolution ultrasound image is reconstructed from the few measurements of DCS directly by the proposed MABF algorithm without recovering the raw sensor signals with complex convex optimization algorithm. The simulated results show the effectiveness of the proposed method.     

The 10 MHz ultrasonic near-field: calculations, hydrophone and optical diffraction tomography measurements

The near-field from a 10 MHz ultrasound transducer has been studied using three different techniques; theoretical calculations, miniature hydrophone measurements and optical diffraction tomography (ODT). The motive was twofold; to evaluate the measurement possibilities of ODT for high-frequency ultrasound and to verify the calculated complexity of near-fields from real ultrasound transducers designed for use in blood perfusion measurements. Calculations of the field from an ideal piston transducer were done with surface integrals of the Kirchhoff function and the measured transducer was designed to approximate this ideal. Contour maps, from the three methods, of pressure amplitude from 1 to 30 mm along the beam are presented with beam profiles at 5 and 45 mm. The results imply that ODT has an increased spatial resolution compared with the 0.5 and 1.0 mm hydrophones. However, even better spatial resolution is needed to draw definite conclusions regarding the complexity of the near-field of the 10 MHz transducer.     

Schlieren visualization of ultrasonic wave fields with high spatial resolution

The visualization of ultrasonic wave fields in optically transparent liquids using the acousto-optic interaction is a well proven tool for the experimental investigation of wave propagation including wave field interaction effects with certain discontinuities and obstacles like reflection, refraction, and diffraction effects as well as for transducer testing and design. For high resolution visualization of wave fields including pulsed waveforms, pulsed light sources and sensitive optical imaging sensors with certain specifications are needed. In this paper the technical requirements of optical and electronic components for high resolution visualization of ultrasound wave fields will be presented. Also, specifications and operation results of a new designed, inexpensive Schlieren optical system will be presented, which is capable of pulsed wave field visualization in the MHz frequency range. The spatial resolution is high enough, not only for accurate beam shape and wave pattern visualization, but also for a gray-scaled display of wave amplitudes including amplitude zero crossings in ultrasound pulses. Consequently, ultrasonic wavelengths can be visualized quantitatively as well as wavelength changes of the ultrasound pulses while traveling through transparent media with different sound velocities. Results to be presented will include 2 MHz and 10 MHz experiments using single transducers as well as linear arrays of commercial medical scanners during their standard operation showing the system beamforming characteristics.     

Recent Advances of Diffraction Tomography in Geophysics,Ultrasonics, and Optics

A tutorial review of diffraction tomography is given along with an overview of recent advances of this technique in borehole geophysics, ultrasonic imaging, and optical microscopy. First, the basic principles of diffraction tomography are presented. Then, we discuss a generalized inversion algorithm, valid for irregularly spaced data and a non-uniform background, and present reconstructions based on ultrasonic water tank data and underground georadar data. Next, we discuss a hybrid filtered backpropagation algorithm for ultrasonic and optical imaging. Quantitative images, based on experimental data, are presented of objects embedded in water or in biological tissue and probed by ultrasound, and of fibers embedded in an index matching liquid and probed by laser light.     

Tomographic identification of gas bubbles in two-phase flows with the combined use of the algebraic reconstruction technique and the genetic algorithm

Combined use of the algebraic reconstruction technique (ART) and the genetic algorithm (GA) shows highly accurate and efficient tomographic reconstruction of line-of-sight projection images of two-phase flows compared with reconstructions obtained by separate use of these methods. A modified GA-based tomography uses the ART reconstruction result as preliminary information on the number, shapes, and sizes of bubbles to be reconstructed. This combined use of the two methods exploits faster convergence of the ART to the approximate solution space and more robust and accurate optimization of the GA to the ultimate solution space. In the investigation a computer-synthesized phantom field that consisted of five elliptical gas bubbles in liquid or solid surroundings was used.     

A general phase retrieval algorithm based on a ptychographical iterative engine for coherent diffractive imaging

Coherent diffractive imaging (CDI) is a lensless imaging technique and can achieve a resolution beyond the Rayleigh or Abbe limit. The ptychographical iterative engine (PIE) is a CDI phase retrieval algorithm that uses multiple diffraction patterns obtained through the scan of a localized illumination on the specimen, which has been demonstrated successfully at optical and X-ray wavelengths. In this paper, a general PIE algorithm (gPIE) is presented and demonstrated with an He-Ne laser light diffraction dataset. This algorithm not only permits the removal of the accurate model of the illumination function in PIE, but also provides improved convergence speed and retrieval quality.     

A general phase retrieval algorithm based on ptychographicaliterative engine for coherent diffractive imaging

Coherent diffractive imaging (CDI) is a lensless imaging technique and can achieve a resolution beyond the Rayleigh or Abbe limit. The ptychographical iterative engine (PIE) is a CDI phase retrieval algorithm that uses multiple diffraction patterns obtained through the scan of a localized illumination on the specimen, which has been demonstrated successfully at optical and X-ray wavelengths. In this paper, a general PIE algorithm (gPIE) is presented and demonstrated with an He-Ne laser light diffraction dataset. This algorithm not only permits the removal of the accurate model of the illumination function in PIE, but also provides improved convergence speed and retrieval quality.     

乳腺癌检测的三维微波热声成像技术

提出了三维微波热声成像技术检测早期乳腺肿瘤,成像同时具有高对比度和高分辨率的特点.该技术利用自适应的鲁棒Capon波束成形(RCB)算法进行信号处理和图像重建.基于电磁学和声学时域有限差分(FDTD)法,进行微波热声成像仿真,算例结果证实了该技术用于诊断早期乳腺肿瘤的可行性. 关键词： 乳腺癌检测 微波热声成像 鲁棒的Capon波束成形 时域有限差分     

菲涅尔区域的乳腺三维超声成像技术

该文提出了一种基于菲涅尔区域的乳腺三维超声成像技术,可实现病变组织早期的定性诊断与定量监测。主要研究内容包括：采用透射方式,以慢度作为反演参量,将正演与反演的透射时间差作为迭代判据,基于菲涅尔区域法实现乳腺组织的二维反演成像;根据柱面传感阵列的分布特点,将乳腺三维超声成像问题转为二维成像问题,进而实现乳腺三维超声成像;计算三维病变成像声速及尺寸误差,并分析菲涅尔区域法的成像分辨率以及入射频率对成像分辨率的影响。该文研究结果证明基于柱面传感阵列的菲涅尔区域法可实现乳腺三维超声成像并有效实现病变早期监测与诊断。     

Optimization-based image reconstruction in x-ray computed tomography by sparsity exploitation of local continuity and nonlocal spatial self-similarity

The additional sparse prior of images has been the subject of much research in problems of sparse-view computed tomography(CT) reconstruction. A method employing the image gradient sparsity is often used to reduce the sampling rate and is shown to remove the unwanted artifacts while preserve sharp edges, but may cause blocky or patchy artifacts.To eliminate this drawback, we propose a novel sparsity exploitation-based model for CT image reconstruction. In the presented model, the sparse representation and sparsity exploitation of both gradient and nonlocal gradient are investigated.The new model is shown to offer the potential for better results by introducing a similarity prior information of the image structure. Then, an effective alternating direction minimization algorithm is developed to optimize the objective function with a robust convergence result. Qualitative and quantitative evaluations have been carried out both on the simulation and real data in terms of accuracy and resolution properties. The results indicate that the proposed method can be applied for achieving better image-quality potential with the theoretically expected detailed feature preservation.     

A new algorithm for spatial impulse response of rectangular planar transducers

Previous solutions for spatial impulse responses of rectangular planar transducers require either approximations or complex geometrical considerations. This paper describes a new, simplified and exact solution using only trigonometric functions and simple set operations. This solution, which can be numerically implemented with a straightforward algorithm, is an exact implementation of the Rayleigh integral without any far field or paraxial approximation. Additionally, a nonlinear relationship was also established for spatial impulse responses from two field points which share the same projection point on the transducer surface plane. By incorporating this relationship in the algorithm, the computational efficiency of spatial impulse responses and continuous fields is improved about 20-folds and 14-folds, respectively. This algorithm has practical applications in designing l-D linear/phased arrays, 1.5-D arrays and 2-D arrays, as demonstrated through numerical simulations with array transducers. Experiments were also conducted to verify the new solution and results show that the algorithm is both accurate and efficient. The application of this method may include development of ultrasound imaging system for hard and soft tissue nondestructive assessment.     

Fast intersections on nested tetrahedrons (FINT): An algorithm for adaptive finite element based distributed parameter estimation

A variety of biomedical imaging techniques such as optical and fluorescence tomography, electrical impedance tomography, and ultrasound imaging can be cast as inverse problems, wherein image reconstruction involves the estimation of spatially distributed parameter(s) of the PDE system describing the physics of the imaging process. Finite element discretization of imaged domain with tetrahedral elements is a popular way of solving the forward and inverse imaging problems on complicated geometries. A dual-adaptive mesh-based approach wherein, one mesh is used for solving the forward imaging problem and the other mesh used for iteratively estimating the unknown distributed parameter, can result in high resolution image reconstruction at minimum computation effort, if both the meshes are allowed to adapt independently. Till date, no efficient method has been reported to identify and resolve intersection between tetrahedrons in independently refined or coarsened dual meshes. Herein, we report a fast and robust algorithm to identify and resolve intersection of tetrahedrons within nested dual meshes generated by 8-similar subtetrahedron subdivision scheme. The algorithm exploits finite element weight functions and gives rise to a set of weight functions on each vertex of disjoint tetrahedron pieces that completely cover up the intersection region of two tetrahedrons. The procedure enables fully adaptive tetrahedral finite elements by supporting independent refinement and coarsening of each individual mesh while preserving fast identification and resolution of intersection. The computational efficiency of the algorithm is demonstrated by diffuse photon density wave solutions obtained from a single- and a dual-mesh, and by reconstructing a fluorescent inclusion in simulated phantom from boundary frequency domain fluorescence measurements.     

Critical issues in breast imaging by vibro-acoustography

Clinically, there are two important issues in breast imaging: detection of microcalcifications and identification of mass lesions. X-ray mammography is the main imaging method used for detection of microcalcification, and ultrasound imaging is normally used for detection of mass lesions in breast. Both these methods have limitations that reduce their clinical usefulness. For this reasons, alternative breast imaging modalities are being sought. vibro-acoustography is an imaging modality that has emerged in recent years. This method is based on low-frequency harmonic vibrations induced in the object by the radiation force of ultrasound. This paper describes potential applications of vibro-acoustography for breast imaging and addresses the critical imaging issues such as detection of microcalcifications and mass lesions in breast. Recently, we have developed a vibro-acoustography system for in vivo breast imaging and have tested it on a number of volunteers. Resulting images show soft tissue structures and calcifications within breast with high contrast, high resolution, and no speckles. The results have been verified using X-ray mammography. The encouraging results from in vitro and in vivo experiments suggest that further development of vibro-acoustography technology may lead to a new clinical tool that can be used to detect microcalcifications as well as mass lesions in breast.     

融合结构先验信息的稳态扩散光学断层成像重建算法研究

扩散光学断层成像作为一种无辐射损伤、低成本的光学在体成像技术,有着良好的应用前景,但具有空间分辨率低、难以定量的缺陷.为了提高扩散光学断层成像的分辨率,实现光学参数分布的精确重建,基于有限元方法,提出了融合结构先验信息的稳态扩散光学断层成像重建算法.该算法以扩散近似作为成像模型,通过软先验的Laplace 正则化方法引入由MicroCT提供的空间结构信息.采用伴随法计算Jacobian矩阵,Levenberg-Marquardt方法用来进行迭代优化.仿真结果表明该算法不仅能获得精确的光学参数值分布,而且显著地提高了迭代收敛的速度.     

Simulation of laser generated ultrasound with application to defect detection

Laser generated ultrasound holds substantial promise for use as a tool for defect detection in remote inspection thanks to its ability to produce frequencies in the MHz range, enabling fine spatial resolution of defects. Despite the potential impact of laser generated ultrasound in many areas of science and industry, robust tools for studying the phenomenon are lacking and thus limit the design and optimization of non-destructive testing and evaluation techniques. The laser generated ultrasound propagation in complex structures is an intricate phenomenon and is extremely hard to analyze. Only simple geometries can be studied analytically. Numerical techniques found in the literature have proved to be limited in their applicability, by the frequencies in the MHz range and very short wavelengths. The objective of this research is to prove that by using an explicit integration rule together with diagonal element mass matrices, instead of the almost universally adopted implicit integration rule to integrate the equations of motion in a dynamic analysis, it is possible to efficiently and accurately solve ultrasound wave propagation problems with frequencies in the MHz range travelling in relatively large bodies. Presented results on NDE testing of rails demonstrate that the proposed FE technique can provide a valuable tool for studying the laser generated ultrasound propagation. PACS 02.70.Dh; 43.35.+d; 42.62.-b