血管网光声成像的机器学习抗散射仿真研究

乳腺癌已成为全球女性发病率最高的肿瘤疾病,微血管成像对乳腺癌的治疗方案和预后有重要意义。光声层析成像术(Photoacoustic Tomography, PAT)可有效对乳腺癌内微血管网进行成像,但肿瘤组织内部的异质微结构和钙化点的散射对成像质量影响较大。针对该问题,文章基于U-Net的卷积神经网络对不同颗粒散射条件下软组织中血管网图像散斑开展仿真研究。仿真结果表明,该神经网络可以学习光声散斑图像和成像目标之间的映射关系,提取出隐藏在噪声中的血管光声信号,并重建出轮廓清晰、背景清晰的高质量血管图像,表明U-Net网络可以从高度模糊的散射图像中提取出有效的光声信息,实现目标图像的高清重建,在乳腺癌的诊断成像中具有广阔的应用前景。     

南海海底粉质黏土沉积物声速和衰减的全浸式测量

提出了一种宽带声速和声衰减测量方法,该方法利用全浸式传感器测量技术(FITM)在实验室环境下对超细沉积物(平均粒径为5.27 μm,孔隙率为79%)的声速和声衰减进行了测量。为确定该实验室测量方法的可靠性,将该声速与声衰减关系与Kramer-Kronig关系进行了比较。同时,Hamilton经典曲线表明,该实验室测量结果符合声速与孔隙度关系,且声衰减在Hamilton曲线置信区间内。测量数据满足有效密度流体模型和颗粒剪切模型的声速和衰减预测曲线,进一步证明了实验室测量方法的准确性和有效性。     

新型木塑材料缺陷及损伤的声发射信号分析

综合新型木塑复合材料各类模式试样、源定位及信号的波形、常规参数、频谱、小波包最优树叶子节点能量谱等特征,对主损伤区附近的声发射事件,应用频谱分析和小波变换等信号处理手段提取特征参数,确定不同缺陷及损伤模式所对应的声发射特征信号,为日后进行神经网络模式识别奠定基础。由于新型木塑复合材料的声发射研究刚刚起步,对该材料的声发射特征还有待进一步的分析,常常需要借鉴其它复合材料的声发射检测结果,这势必会带来一定的局限性及适用性问题。对新型木塑复合材料的声发射参数的定量化还有待于大量实验数据的积累和归纳分析。     

应用SXR-CT技术研究闭孔泡沫铝微结构演化及变形分析

借助同步辐射硬X射线高强度、高能量、高准直、宽频谱以及可选能量等特点,对材料试件进行投影成像,并应用滤波反投影重建算法实现三维图像重建(Synchrotron X-Ray Computed Tomography).研究了闭孔泡沫铝在压缩过程中内部微结构的演化,得到了不同压缩状态下内部微结构图像,分析了闭孔泡沫铝在压缩过程中的变形及孔隙率变化.这些研究结果为泡沫铝制备工艺的改进和材料与结构的优化设计提供了有益的参考,并为泡沫铝压缩破坏机理的构建提供科学依据.     

基于差分模式激发光声光谱法的气体检测系统研究

本文介绍了一种新型光声光谱检测方法—差分模式激发光声光谱法,该方法通过计算同一光声池内两种不同声共振模式下光声信号强度的比值,间接测得被测气体浓度值。结合适当的滤光片,可实现对不同气体的测量。本文以高效数据处理芯片DSP作为核心处理单元,简化了系统结构,提高了响应速度,便于修改程序,改进检测算法。实验结果表明,在压力为98000Pa温度为295K的条件下,对丙酮蒸气的检测极限度可达23ppm.m-1。     

含孔隙碳纤维复合材料的超声衰减模型

从碳纤维复合材料孔隙率的超声脉冲检测原理出发,在假设其它缺陷的影响已在定性分析中排除、或以当量及修正的方式引入的前提下,对于根据孔隙直径适当给定的检测频率,建立材料的超声衰减模型,并导出超声衰减系数与材料和孔隙缺陷的主要参数之间的理论与统计关系,为建立新的检测方法提供理论支撑,改变现有检测方法的检测结果与理论模型的不一致现象.其中作为中间结果之一所建立的碳纤维声衰减模型及相应导出的声衰减系数与碳纤维含量和碳纤维半径之间的关系,不仅可服务于本文的直接应用目标,而且对于基于超声衰减原理的纤维增强复合材料无损检测的其它方面也具意义.作为中间结果之二所建立的分布孔隙率超声衰减模型,考虑到材料中的孔隙按其半径分布,而不同半径范围的孔隙表现出相异的声衰减特性这一实际情况,在由单一半径孔隙率声衰减模型导出的理论关系中引入了孔隙率与孔隙半径相关关系的统计特征,由此给出了更符合材料实际情况的修正模型和求解关系式.     

切向流作用下单层声衬噪声阻尼性能的数值研究

孔隙率是影响声衬噪声阻尼特性的主要结构参数,在声衬结构优化方面起到关键作用。首先,基于由一系列8 个完全相同的狭缝赫姆霍兹谐振器组成的单层声衬模型,利用COMSOL Multiphysics 软件研究切向流作用下单层声衬在不同孔隙率下的传输损失,然后优化设计一种双层声衬,并与单层声衬的吸声性能进行比较,通过数值结果与文献实验测量结果对比验证,讨论孔隙率、切向流及层数对声衬阻尼性能的影响。结果表明：相较于无流情况,切向流存在时,随着孔隙率的增加单层声衬的吸声能力逐渐增强,噪声衰减主要发生在共振频率附近,同时切向流作用也会降低声衬产生的传输损失、改变共振频率,双层声衬的优势在于明显拓宽消声频率范围。研究成果为今后实际应用中声衬的优化设计提供了一种可预测方法。     

泡沫铝微结构的分形特及与子L隙率的关系研究

采用分形理论对泡沫铝材料微结构进行了研究,利用面积周长分维算法证明了其微结构在一定尺度内具有自相似性的分形特征.利用Siterpinski地毯模型研究了包沫铝孔隙率与分维之间的关系.研究结果表明.引入分形维数的分析模型能够较好地反映泡沫铝微结构与孔隙率之间的关系.     

评价松质骨状况的一种背散射频谱方法

采用超声背散射信号的质心偏移量来评价松质骨,并对牛胫骨和人体跟骨中背散射信号的质心偏移量与松质骨表观密度的关系,以及人体跟骨松质骨中背散射信号频谱质心位置与年龄的关系进行了分析讨论。分析结果表明,随松质骨表观密度的增大,背散射信号频谱的质心向低频方向移动;随年龄的增大,质心位置越接近于发射超声的中心频率。根据超声背散射信号质心偏移量的大小,可用于评价松质骨健康状况。     

采用稀疏测量数据的有限角度光声层析成像的研究进展

生物光声层析(Photoacoustic Tomography,PAT)成像可以反映生物组织的光吸收分布,定量测量组织的光吸收系数和散射系数,进而分析组织成分,为疾病的早期诊断和治疗提供可靠的依据。由于成像目标特殊的几何结构以及成像装置的机械结构、空间位置和成像时间等的限制,超声探测器只能在有限的角度范围内扫描,采集到稀疏的光声测量数据,导致重建图像中出现伪影和失真。针对有限角度扫描和稀疏测量数据问题,对目前主流的光声图像重建算法进行综述和分析。     

Microstructural Modeling and Multiscale Mechanical Properties Analysis of Cancellous Bone

This paper is devoted to the microstructure geometric modeling and mechanical properties computation of cancellous bone. The microstructure of the cancellous bone determines its mechanical properties and a precise geometric modeling of this structure is important to predict the material properties. Based on the microscopic observation, a new microstructural unit cell model is established by introducing the Schwarz surface in this paper. And this model is very close to the real microstructure and satisfies the main biological characteristics of cancellous bone. By using the unit cell model, the multiscale analysis method is newly applied to predict the mechanical properties of cancellous bone. The effective stiffness parameters are calculated by the up-scaling multi-scale analysis. And the distribution of microscopic stress in cancellous bone is determined through the down-scaling procedure. In addition, the effect of porosity on the stiffness parameters is also investigated. The predictive mechanical properties are in good agreement with the available experimental results, which verifies the applicability of the proposed unit cell model and the validness of the multiscale analysis method to predict the mechanical properties of cancellous bone.     

Velocity dispersion of acoustic waves in cancellous bone

Measurement of ultrasonic attenuation and velocity in cancellous bone are being applied to aid diagnosis of women with high fracture risk due to osteoporosis. However, velocity dispersion in cancellous bone has received little attention up to now. The overall goal of this research was to characterize the velocity dispersion of human cancellous bone based on a spectral analysis of ultrasound transmitted through the bone specimens. We have followed a systematic approach, beginning with the investigation of a test material, moving on to the investigation of bone specimens. Particular attention is given to diffraction effect, a potential source of artifacts. Parametric images of phase velocity (measured at the center frequency of the pulse spectrum), slope of attenuation coefficient (dB/cm/MHz) and velocity dispersion were obtained by scanning 15 bone specimens. We have demonstrated that the diffraction effect is negligible in the useful frequency bandwidth, and that the ultrasonic parameters reflect intrinsic acoustic properties of bone tissue. The measured attenuation showed approximately linear behavior over the frequency range 200 to 600 kHz. Velocity dispersion of cancellous bone in the frequency range 200 to 600 kHz was unexpectedly found to be either negative or positive and not correlated with the slope of attenuation coefficient. There was a highly significant correlation between the slope of attenuation coefficient and phase velocity at the center frequency of the spectrum. This behavior contrasts with other biological or nonbiological materials where the local form of the Kramers-Kronig relationship provides accurate prediction of velocity dispersion from the experimental frequency dependent-attenuation for unbounded waves.     

Numerical analysis of variability in ultrasound propagation properties induced by trabecular microstructure in cancellous bone

The manner by which the trabecular microstructure affects the propagation of ultrasound waves through cancellous bone is numerically investigated by finite difference time-domain (FDTD) simulation. Sixteen 3-D numerical models of 6.45times6.45times6.45 mm with a voxel size of 64.5 mum are reconstructed using a 3-D microcomputed tomographic (muCT) image taken from a bovine cancellous bone specimen of approximately 20times20times9 mm. All cancellous bone models have an oriented trabecular structure, and their trabecular elements are gradually eroded to increase the porosity using an image processing technique. Three erosion procedures are presented to realize various changes in the trabecular microstructure with increasing porosity. FDTD simulations of the ultrasound pulse waves propagating through the cancellous bone models at each eroded step are performed in 2 cases of the propagations parallel and perpendicular to the major trabecular orientation. The propagation properties of the wave amplitudes and propagation speeds are derived as a function of the porosity, and their variability due to the trabecular microstructure is revealed. To elucidate an effect of the microstructure, the mean intercept length (MIL), which is a microstructural parameter, is introduced, and the correlations of the propagation properties with the MILs of the trabecular elements and pore spaces are investigated.     

超声功率谱特征参数在复合材料孑L隙率检测中的应用

基于材料超声检测系统模型及功率谱分析,考察了碳纤维增强环氧树脂基（CFRP）复合材料的孔隙率（0．03％～2．3％）与主频、中心频率、加权功率谱频率、有限频带能量（ELFB）及主频对应的能量（EPF）等参数之间的相关性。结果表明：孔隙率在0．03％～0．67％范围内时,ELFB与孔隙率呈线性递减关系;在0．67％～2．3％范围内,两者呈二次曲线递减关系;EPF与孔隙率的关系在0．64％左右两侧分别呈现出线性和二次曲线递减趋势。晶耶和踟能有效表征CFRP的孔隙率。     

Application of the Biot model to ultrasound in bone: inverse problem

This paper concerns the ultrasonic characterization of human cancellous bone samples by solving the inverse problem using experimentally measured signals. The inverse problem is solved numerically by the least squares method. Five parameters are inverted: porosity, tortuosity, viscous characteristic length, Young modulus, and Poisson ratio of the skeletal frame. The minimization of the discrepancy between experiment and theory is made in the time domain. The ultrasonic propagation in cancellous bone is modelled using the Biot theory modified by the Johnson-Koplik-Dashen model for viscous exchange between fluid and structure. The sensitivity of the Young modulus and the Poisson ratio of the skeletal frame is studied showing their effect on the fast and slow waveforms. The inverse problem is shown to be well posed, and its solution to be unique. Experimental results for slow and fast waves transmitted through human cancellous bone samples are given and compared with theoretical predictions.     

Development of a numerical cancellous bone model for finite-difference time-domain simulations of ultrasound propagation

The trabecular frame in cancellous bone has numerous porous spaces of various sizes and shapes. Their continual arrangement changes with position in the bone. Assuming that the complicated pore space is the aggregation of spherical pores, in this study, the trabecular structure was analyzed using a three-dimensional (3-D) X-ray microcomputed tomography (muCT) image. Analysis involved a 3-D cancellous bone model developed for numerical simulations of ultrasound propagation. In this model, the trabecular structure was simplified by regularly arranging spherical pores in a solid bone. Using a viscoelastic, finite-difference, time-domain (FDTD) method with the simplified cancellous bone model, ultrasound pulse waveforms propagating through cancellous bone were simulated in two cases of the propagations parallel and perpendicular to the main trabecular orientation. The porosity dependences of the propagation properties, attenuation, and propagation speed were derived from the simulated waveforms. Comparisons with simulated results using the realistic cancellous bone model reconstructed from a 3-D muCT image, assisted to further validate this simplified model.     

Signal-to-Noise Analysis of the Differential Open Photoacoustic Helmholtz Cell

When open photoacoustic cells are considered, attenuation of external noise is a serious issue. This paper describes how mechanical dimensions of a differential open photoacoustic Helmholtz cell influence its signal-to-noise (S/N) ratio. The analysis was performed by means of computer simulations based on the loss-improved transmission line model. This research showed that the mechanical parameters noticeably affect the signal-to-noise ratio. According to the presented results, optimal selection of the dimensions is a factor which can substantially improve the signal-to-noise ratio of the examined photoacoustic cell. Due to the very good acoustic properties of the cell, it should be possible to obtain an S/N ratio of over 100 dB.     

Propagation of ultrasonic waves through demineralized cancellous bone

Ultrasonic velocity is determined in a number of bovine cancellous (spongy) bone samples by using a double-probe-through-transmission ultrasonic pulse technique. The ultrasonic velocity, total mineral content, bone density, and solid volume fraction of the specimen were determined. The relation between fast velocity and each of the other parameters was examined to explore the best correlation using linear, logarithmic, power, and exponential relationships. There is a strong positive relationship between ultrasonic velocity and each of the other parameters. The exponential model fits the data better than the linear model, logarithmic model, and power model. Biot's theory also is used to predict the velocity of ultrasound in the demineralized bone. It is shown that the transmission of ultrasonic pulses in the cancellous bone samples can be adequately described using Biot's theory. The different parameters occurring in the Biot theory have been measured independently, and the calculation has been compared with measured velocity of water-saturated bone samples. The correlation coefficients for regression analysis between the experimental velocities and those predicted by Biot's theory are greater than 0.86.     

Multispectral quantitative photoacoustic imaging of osteoarthritis in finger joints

We present in vivo experimental evidence that multispectral quantitative photoacoustic tomography (qPAT) has the potential to detect osteoarthritis (OA) in the finger joints. In this pilot study, two OA patients and three healthy volunteers were enrolled, and their distal interphalangeal (DIP) joints were examined photoacoustically by a multispectral PAT scanner. Images of tissue physiological/functional parameters including oxyhemoglobin, deoxyhemoglobin, oxygen saturation, and water content, along with the tissue acoustic velocity of all the examined joints, were simultaneously recovered using a finite element reconstruction algorithm for multispectral photoacoustic measurements. The recovered multispectral photoacoustic images show that the OA joints have significantly elevated water content, decreased oxygen saturation, and increased acoustic velocity compared to the normal joints.     

A quantitative technique for comparing synthetic porous hydroxyapatite structures and cancellous bone

There are many different materials currently available for cancellous bone grafting. There is however, very little information relating the morphology of these materials to cancellous bone. Work was undertaken to develop a quantitative method for comparing synthetic hydroxyapatite bone structures with cancellous bone. The bases for comparison were mean plate thickness, mean plate distance, mineral area fraction, mineral volume fraction and plate orientation coupled with mechanical tests. The aim of this work was to develop a protocol for assessing whether these critical parameters which influence the success of bone implants were achieved in the synthetic materials. The methodology is successful in providing quantitative information about the mineral area fraction, the mean plate distance or pore size and the intercept frequency as a function of angle. Combining these three provides a quantitative measure of how much mineral there is and how it is distributed and oriented. The mechanical tests yield strengths and moduli values based on apparent density. The results of the mechanical tests can also be plotted as functions of the more discrete structural features such as those quantified in the image analysis to allow for even more equitable and systematic comparisons of different porous materials.