基于超声散射元间距的生物软组织内部应变与弹性估计

超声弹性成像有希望成为生物软组织病变诊断和力学特性评价的有效手段．提出利用小坡变换检测组织背向散射信号的不连续性，估计规则散射元空间分布及其变化，由此得到组织内部应变分布与弹性模量的新方法．结果表明，利用小波变换的散射元定位方法具有较强的抗干扰能力，基于散射元间距的运动估计方法避免了超声散斑的解相关性，计算量较小，无误差积累．组织内部应变和弹性估计能够区分杨氏弹性不同的组织，具有较高的空间分辨力．方法同时提供了组织弹性与微结构的信息．     

强散射过程与双随机相位加密过程的等价性分析

薄层强散射介质的散射系统只会引起入射光波的振幅和相位分布变化,但不会导致总能量的衰减.这一过程可以看成光波被散射系统编码的过程,与双随机加密系统极为相似.本文首先证明了载有目标信息的光波在通过薄层强散射介质的散射系统时所产生散斑的分布特性,与双随机加密系统加密同一明文目标所得到的密文分布特性具有高度的相似性.然后,本文将该散射系统视为一个双随机加密系统,并利用相位恢复算法精确地计算出该散射系统所对应的两块随机相位密钥,同时证明了这两块密钥板还可以成功地从该散射系统所得到的其他任何散斑中恢复出对应的原始图像.最后,为了进一步证明二者的等价性,本文使用一种适用于双随机加密系统的唯密文攻击方法,成功地破解了薄层强散射介质的散射系统,得到了较好结果.     

混浊介质二维后向漫散射穆勒矩阵的测量

研制了一套实验系统用于测量混浊介质的后向漫散射穆勒矩阵.在系统中，激光被调制成所需偏振态，聚焦于混浊介质表面.利用CCD相机配合相应的偏振元件，测量入射点周围区域的后向散射光中特定偏振态的能量空间分布，并由测量值计算出介质的后向漫散射穆勒矩阵.描述了系统的组成、测量原理以及方法，分别测量了消偏振分光棱镜的反射穆勒矩阵和浓度1.5％的脂肪乳溶液的后向漫散射穆勒矩阵.实验证明:该系统能够完全消除样品表面镜面反射光的干扰，精确的测量混浊介质的后向漫散射穆勒矩阵；并可望用于生物组织的研究中.     

基于离散随机介质平均散射声功率的无损测温方法

生物组织中超声散射回波的平均功率与其温度有关．该理论包含：（１）生物组织近似为离散随机介质．（２）离散随机介质的平均散射功率与组织的衰减系数α和声速Ｃ有关。对于似水生物组织（如：猪肝）,当温度升高时,衰减系数的减小使平均散射声功率增加,但其影响大小与时间窗△ｔ有关,而声速的增大则使平均散射声功率减小。实验结果表明：生物组织的平均散射声功率随温度变化趋势明显,该结果提供一种肿瘤热疗无损伤检测温度的新方法。     

四元裂解位相调制实现相干光通过散射介质聚焦

光在不均匀介质中传播会受到散射的干扰,在这些散射材料中,例如粉末、生物组织、亚波长颗粒对入射光多次散射使得出射光无法聚焦,从而在接收平面形成散斑.本文提出四元裂解位相调制方法对入射相干光场进行调制,使其通过散射介质聚焦.此方法利用入射光场全场调制,充分考虑光场单元之间的干涉作用,从整个空间光调制器的调制面开始,逐层进行四元裂解及位相优化.运用此方法在实验中实现了相干光的前向散射和后向散射有效聚焦,这为生物医学领域中通过散射介质成像提供了新的思路和方法.     

随机粗糙表面光散射场分布特性的机理研究

为了研究随机粗糙表面光散射分布特性机理,采用线性滤波法生成高斯分布随机粗糙表面,以基尔霍夫近似作为电磁场边界条件,采用蒙特卡罗方法数值计算了一组金属和一组电介质粗糙表面在S偏振光和P偏振光照射下的散射光强度空间分布。计算结果显示:金属表面与电介质表面的散射光分布宽度、散射峰值、散射峰位置三个散射特征存在显著差异,经过分析发现这种特征差异的产生机理是由表面面元的斜率分布和面元反射率共同因素导致,与数值计算结果对比,二者具有良好的一致性。     

超声调制介质中漫散射光自相关性质研究

研究了超声调制随机介质中漫散射光自相关性质,推导出均匀无限介质中有超声场调制时漫散射光时间自相关函数的表达式,讨论了调制参数、调制幅度及其衰减与超声参数、光散射参数等的关系.用Monte Carlo方法研究了随机介质中隐含异物成像的可行性.结果表明,正常生物组织和病变生物组织的超声调制自相关函数有明显的差别,该方法为光学医学诊断提供了一种新参考 关键词： 自相关 漫散射光 超声调制 光学成像     

超宽频带THz脉冲在随机散射介质中传播的理论研究

研究了超宽频带THz脉冲入射到散射介质中时其透射脉冲的时空特性.根据Mie理论计算出随机散射介质的散射系数和各向异性因子,用时间分辨Monte Carlo方法模拟了超宽频带的THz脉冲在随机散射介质中的传播,研究了在不同散射颗粒半径,不同频宽的THz入射脉冲对透射脉冲的影响和散射对成像分辨率的影响.结果表明:散射会降低THz脉冲在随机散射介质中的成像分辨率,散射颗粒越小,散射介质越厚,其成像分辨率越差.     

基于稀疏重构的尾波干涉成像方法

尾波干涉成像是利用尾波时延和扩散近似敏感核来反演散射介质中微小速度扰动空间分布的技术.该问题本质上是一个欠定问题,一般无确定解,导致其难以精确定位介质中微小波速变化的区域.为解决上述缺陷,本文利用速度扰动分布在空间上具有稀疏性的特点,提出了一种基于压缩感知理论中稀疏重构算法的尾波干涉成像方法.该方法可在散射介质中较准确地获取速度扰动的空间位置和范围,同时具有较高的计算效率.数值仿真和实验结果表明:相比于现有的线性最小二乘差分成像方法,无论是单个还是多个扰动区域,该方法均能更精确地进行定位成像,同时明显减少了计算时间.     

短脉冲激光与介质相互作用的扩散模型

推导了扩散近似方程,通过半无限大均匀介质计算,用扩散理论分析解验证了数值方法的有效性.模拟了光在非均匀介质内的传输过程,给出了介质内光通量随时间变化的空间分布.结果表明,该基于扩散模型的数值方法能够模拟短脉冲光在强散射介质中的传播过程以及漫散射光的时间变化特性,并且借助于光通量空间分布能够准确模拟非均匀介质内内含物的位置.     

Spatial Localization of Biosensor Fluorescence Signals in Human Skin under the Effect of Eequalization of the Refractive Index of the Surrounding Medium

A priority line of biomedical applications of optics is the development of noninvasive diagnostic methods based on the scanning of fluorescence radiation of biosensors embedded in biological tissue. Their main advantage is a high sensitivity and selectivity with respect to given parameters of tissues and their variations. In this study, we present a method for and results of modeling of excitation and propagation of fluorescence radiation in a multilayer randomly inhomogeneous highly scattering and absorbing medium imitating human skin. The model takes into account the spatially inhomogeneous distribution of skin fluorophores and their photophysical characteristics. Both the spatial distribution of fluorescence of skin tissues and the possibility of localization of a detected fluorescence signal are studied. The spatial distribution of fluorescence centers (fluorophores) in the medium is assumed to closely follow the spatial distribution of collagen fibers of the skin. The equalization of the refractive indices at the air-skin interface is shown to lead to a higher degree of localization of the fluorescence signal detected from a biosensor located in a near-surface skin layer.     

Fluorescence tomography with simulated data based on the equation of radiative transfer

The quantification of a nonuniform quantum yield or fluorophore absorption distribution is of major interest in molecular imaging of biological tissue. We introduce what is believed to be the first fluorescence image reconstruction algorithm based on the equation of radiative transfer that recovers the spatial distribution of light-emitting fluorophores inside a highly scattering medium from measurements made on the surface of the medium. We obtain images of either the quantum yield or the fluorophore absorption.     

随机介质中电磁波空间分布特性研究

采用随机介质统计模型和FDTD方法模拟了随机介质电场空间分布根据光子局域化理论,讨论了电场的空间分布和介电常数涨落的关系,结果表明,在一定的介质密度下,电磁场是局域化的,其空间分布和介质介电常数的涨落密切相关.     

Coherent backscattering spectroscopy

Coherent backscattering (CBS) of light in random media has been previously investigated by use of coherent light sources. Here we report a novel method of CBS measurement that combines low spatial coherence, broadband illumination, and spectrally resolved detection. We show that low spatial coherence illumination leads to an anomalously broad CBS peak and a dramatic speckle reduction; the latter is further facilitated by low temporal coherence detection. Thus CBS can be observed in biological tissue and other media that previously were beyond the reach of conventional CBS measurements. We also demonstrate, for the first time to our knowledge, spectroscopic analysis of CBS. CBS spectroscopy may find important applications in probing random media such as biological tissue in which depth-selective measurements are crucial.     

二维随机介质中模式的空间局域化分布

基于随机激光的时域理论,利用时域有限差分法(FDTD)数值求解麦克斯韦方程组和速率方程组,利用分区域计算法,分别计算了整体和局部泵浦二维随机介质时,不同泵浦强度下,不同区域的辐射谱,得出了随机介质中模式的空间局域化分布。计算结果显示,同样条件下不同区域的辐射谱是不同的,且随着激励源泵浦强度的变化,随机激光辐射始终集中在介质中某几个固定区域,但各个区域的随机激射效率不同,在区域之间存在模式的空间范围重叠。利用该方法,可以分析介质的随机分布对随机激射的影响,理论上可以为高激射效率的伪随机介质制备提供一定指导。     

Localized modes in orientation-disordered one-dimensional media with uniaxial scatterers

Localized modes in one-dimensional (1D) media with uniaxial scatterers that are assumed to be order in spatial location but disorder in spatial orientation of their optical axis are investigated. Based on the holistic effect model in random laser, I.e., the random laser is due to the interaction of the complex localized modes in active random media with local aperiodic quasi-structure with appropriate pump light, a physical model on this type of random media is found. Its disorder degree is defined by D = no/ne. Then, the typical transmission spectrum through the random media and the light field intensity distribution corresponding to the defect modes in photonic band-gap are calculated numerically by means of the transfer matrix method, and the condition that the localized mode appears is discussed. Results show that the medium disorder plays an important role in determining the lightwave state. The localized state appears when the medium disorder is strong enough, and a new mechanism creating random laser phenomenon is brought forward.     

Localization of radiation in two-dimensional random media of finite extent

The localization of electromagnetic waves in a two-dimensional random medium composed of strong scatterers is studied theoretically. It is shown that an allowance for the spatial radiation intensity distribution inside the medium, along with an analysis of the distance dependence of the transmission coefficient, is needed to reveal the localization states in media of finite extent.     

Near field of a low-frequency source of forced vibration on a layered biological tissue

Results of a numerical calculation of the near acoustic field excited by a low-frequency source of forced vibration positioned on the surface of a layered biological tissue are presented. The latter is simulated by a soft tissue layer varying in thickness and overlying a solid halfspace. The spatial distribution of the acoustic field in the biological tissue is obtained.     

Ultrasonic strain imaging and reconstructive elastography for biological tissue

Mechanical properties of biological tissue represent important diagnostic information and are of histological and pathological relevance. In order to obtain non-invasively mechanical properties of tissue, we developed a real-time strain imaging system for clinical applications. The output data of this system also allow an inverse elastography approach leading to the spatial distribution of the relative elastic modulus of tissue. The internal displacement field of biological tissue is determined using the above mentioned strain imaging system by applying quasi-static compression to the considered tissue. Axial displacements are calculated by comparing echo signal sets obtained prior to and immediately following less than 0.1% compression, using the fast root seeking technique. Strain images representing mechanical tissue properties in a non-quantitative manner are displayed in real-time mode. For additional quantitative imaging, the stiffness distribution is calculated from the displacement field assuming the investigated material to be elastic, isotropic, and nearly incompressible. Different inverse problem approaches for calculating the shear modulus distribution using the internal displacement field have been implemented and compared. The results of an ongoing clinical study with more than 200 patients show, that our real-time strain imaging system is able to differentiate malignant and benign tissue areas in the prostate with a high degree of accuracy (sensitivity=76% and specificity=89%). The reconstruction approaches applied to the strain image data deliver quantitative tissue information and seem promising for an additional differential diagnosis of lesions in biological tissue. Our real-time system has the potential of improving diagnosis of prostate and breast cancer.     

生物组织光学特性参数的无创检测理论与技术

无创定量确定生物组织的光学特性在医学诊断和治疗领域中有着很重要的意义和广泛的应用前景。作为生物组织光学特性参数无创检测技术的理论基础 ,首先简要介绍了描述散射介质中光传播及分布的漫散射理论及蒙特卡罗模拟算法 ;然后分别介绍了用漫散射理论和蒙特卡罗模拟算法无创定量确定生物组织光学特性参数的具体方法 ;最后总结了到目前为止无创定量确定生物组织光学特性参数的研究成果 ,并对今后的工作作了展望