散射介质中光传输行为时间特性的蒙特卡洛模拟研究

蒙特卡罗方法模拟了超短脉冲在皮肤的四层模型中的传输的时间特性,给出不同时间的表面漫射强度随径向距离和角度的变化,用傅里叶变换给出了表面漫射强度时间响应的频域结果;结果表明漫射强度随时间的变化会有数量级的不同,漫射强度的时间分辨为ps量级,这要求选择大动态范围ps时间分辨率的探测设备,频域分析的结果表明在达到很高的频率下,幅度和相位才有变化,实际上硬件难以实现。     

基于δ声波场的生物组织光学断层成像研究

利用δ声波场和近红外光漫射理论实现生物组织的断层成像是一种新颖医学 成像方法. 通过构建一个δ声波场并作用到生物组织中，仅改变组织内作用点上的光学特性参数，并影响近红外光在组织中的传播特性而改变组织表面的光强分布. 利用组织表面光强改变量与作用点上组织光学参数改变量之间的内在关系，可以通过对δ声波场作用点上断层的扫描实现组织内部的断层成像. 该方法具有广泛的临床医学应用前景，为乳腺癌的早期检测提供一种有效方法. 关键词： 光学断层成像 δ声波场 漫射方程 生物组织     

两层生物组织光学特性参数无损测量的模拟研究

生物组织的光学特性参数是与疾病的无损光诊断有关的重要的光学参数，一般的测量所应用的模型是假设生物组织为半无限厚的均匀结构。为了更能反映生物组织的真实特性，本文应用两层结构模型，即假设第一层为有限厚的均匀组织，第二层为半无限厚，由漫射方程经过傅立叶变换得到传输方程的漫射解。为检验漫射解的准确性，将漫射解的表面漫射光分布与Monte Carlo模拟结果进行了比较，结果表明两者符合得很好。为了研究由漫射解是否可以获得两层组织的光学特性参数，将漫射解与Monte Carlo模拟数据进行了非线性拟合，反演获得组织的光学特性参数，结果表明，由漫射解可以获得两层组织的有效散射系数和吸收系数。     

单像素探测频谱重构成像理论分析

迄今为止,基于频谱重建的单像素探测成像技术没有提供一个比较全面的理论基础,因此,本文对此成像技术进行了比较详细的理论分析.分析了若干因素对成像重建的影响,包括对来自成像物体散射光的直接探测,或间接探测即物体的散射光顺次经过多个漫射面漫射后由探测器探测;多通道探测即物体的散射光分别沿几个不同路径、各自经几个漫射面顺次漫反射后由探测器探测;探测器尺寸和位置.理论分析结果表明,无论是直接探测还是间接探测,无论是单通道探测还是多通道探测,只要能接收物体的散射光,就能够重构成像;探测器面积对成像重建的影响相当于位于不同位置的多个点探测器探测重建像的叠加.为了加快成像重建速度,导出了三步相移频谱重建公式.对表面光强反射率均匀的物体,采用三步相移技术,给出了成像重建结果.     

串级散斑场动态特性的研究

本文用统计光学方法推导了由两个漫射体形成的串级散斑场的统计性质。得出了漫射体的运动速度v与相关时间ι_c成线性的关系,讨论了经过三次随机调制的串级散斑场的斑纹平均半径较之一次调制斑纹半径为小。理论与实验结果符合得很好。     

漫射近似理论的有效使用范围分析

利用蒙特卡罗方法模拟了半无限大生物组织内部深度分辨的光能流率的分布情况。分析了漫射近似理论的有效使用范围。当组织厚度大于 3/ 2个光子的平均自由程时 ,应用漫射近似理论能得到很好的结果 ,反之 ,漫射理论不能使用。也就是说 ,当组织厚度z >0 1875cm时 ,应用漫射理论能得到很好的结果 ;当组织厚度z <0 1875cm时 ,漫射理论不能使用     

光在多层匹配生物组织中的时域传输模型

建立了匹配介质的n层半无限厚频域方程的解,并对频域进行傅里叶变换,计算出时域方程的时间分辨漫反射（Time-resolved Reflectance）.为检验漫射模型的准确性,将时间分辨漫反射与蒙特卡罗（Monte Carlo）模拟结果进行了比较,结果表明两者符合得很好.研究表明：该时域漫射模型可以判断生物组织和病变的光学特性.     

混合漫射近似模型的漫反射理论及其模拟实验研究

研究了一个改进的漫射近似模型.该模型将漫射近似中的漫射系数D用描述P₃近似的漫射系数D_asym替代.推导了这个混合的漫射近似模型在单点源近似和外推边界条件下的反射率R_Hybrid(ρ)的解析表达式,比较了有效反照率a′对标准漫射近似R_DA(ρ)和R_Hybrid(ρ关键词： 漫射近似 P₃近似')" href="#">P₃近似 混合漫反射模型 强吸收     

生物组织光传播的时域特征分析

采用蒙特卡罗方法对脉冲光在组织中传播的时域特征作了研究，并把计算结果与漫射理论的解析解作了比较，文中详细分析了组织光学特性参数与组织表面时间分辨的漫反射光分布之间的关系。结果表明：散射系影响着漫反射光强到达峰值的时间，吸收系数影响着漫反射光强在峰值之后随时间下降的速度，而ｇ因子对响应曲线的前沿影响较大，在后沿阶段基本上没有影响。     

漫射近似在测量生物组织光学性质中的适用范围

漫射近似理论经常被用来作为测量生物组织光学特性参数实验和方法的理论基础,但是漫射近似理论只是一种对辐射传输理论的近似结果,在一定条件下有其特定的适用范围.为了确定漫射近似理论在活体非侵入式无损测量要求下的适用范围,本文采用Monte Carlo模拟校验的方法,对漫射近似理论和Monte Carlo模拟计算的结果进行了比较,给出了基于单散射反照率a和各向异性因子g的漫射近似理论适用范围数值标准.这一标准为漫射理论的应用提供了参考依据.     

Transverse relaxation time constraints on resolution in one-dimensional,phase- and frequency-encoded nuclear magnetic resonance imaging

The theoretical dependence of the resolution on the relationship of sampling time to transverse relaxation time (T₂) for frequency-encoded, one-dimensional NMR imaging using constant field gradients has been investigated. A resolution function that is explicitly dependent on the sampling time is derived, and it is shown that the observed image of an object can be written as a convolution of the sample magnetization with this resolution function. This function is explicitly calculated for two cases of interest: (1) for sampling times much shorter than T₂, and (2) for sampling times much longer than T₂. These cases are illustrated for two examples: (1) a uniform magnetic bar, and (2) uniform periodic magnetic bars. When oscillating gradients are utilized, these results still hold in the limit of slow oscillation. The resolution in phase-encoded NMR imaging is not explicitly dependent on the sampling time.     

荧光谱线分布对共焦荧光显微镜分辨率的影响

研究了荧光谱线分布对共焦荧光显微镜中分辨率的影响,导出了荧光谱线分布的荧光功率传输函数、三维脉冲响应函数.数值计算了荧光谱线均匀分布的情况,结果表明：与采用单一中心荧光波长的分辨率比较,共焦荧光显微镜的横向分辨率、纵向分辨率随着荧光谱线范围和色散的增大而下降;谱线响应范围小的探测器有利于分辨率.     

Wavelet correlations of nonstationary signals

Two approaches to the analysis of nonstationary random signals are proposed and studied. The first approach is based on the adaptive Morlet wavelet that allows variations in time and frequency resolution of signals using an auxiliary control parameter. The second approach is related to the application of double correlation function that represents correlation of continuous wavelet transforms of two signals calculated in time and frequency domains. The advantages of the proposed correlation function in comparison with alternative correlation functions, in particular, analysis of both time and frequency correlations of nonstationary signals are outlined. Applications of the proposed approaches in the analysis of various transient processes in physics are discussed.     

Inference of DNA sequences from mechanical unzipping: an ideal-case study

The performances of Bayesian inference to predict the sequence of DNA molecules from fixed-force unzipping experiments are investigated. We show that the probability of misprediction decreases exponentially with the amount of collected data. The decay rate is calculated as a function of biochemical parameters (binding free energies), the sequence content, the applied force, the elastic properties of a DNA single strand, and time resolution.     

Submicrometer tomographic resolution examined using a micro-fabricated test object

To estimate the spatial resolution of microtomographs, a test object on the submicrometer scale was prepared by focused ion beam milling and subjected to microtomographic analysis. Since human tissues are composed of cells and extracellular matrices with micrometer and submicrometer structures, it is important to investigate the three-dimensional spatial resolution of microtomographs used to visualize microstructures of human tissues. The resolutions along the direction within the tomographic slice plane (in-plane resolution) and perpendicular to it (through-plane resolution) were determined from the modulation transfer function of square-wave patterns. The in-plane resolution was estimated to be 1.2 μm from the modulation transfer function of the non-zoom image. In contrast, the zoom image gave the in-plane resolution of 0.8 μm. This in-plane resolution is comparable to the through-plane resolution, which was estimated to be 0.8 μm. Although the two-dimensional radiographs were taken with the pixel width of half the X-ray optics resolution, these three-dimensional resolution analyses indicated that the zoom reconstruction should be performed to achieve the in-plane resolution comparable to the X-ray optics resolution. The submicrometer three-dimensional analysis was applied in the structural study of human cerebral tissue stained with high-Z elements and the obtained tomograms revealed that the microtomographic analysis allows visualization of the subcellular structures of the cerebral tissue.     

红外扫描和频振动光谱系统的激光线宽与光谱分辨率

本文报道了一个简化的利用可见光和红外光带宽来计算和频光谱分辨率的公式. 公式显示和频振动光谱的Voigt线宽可以通过振动模式的均匀线宽(洛伦兹线宽)、非均匀线宽(高斯线宽)、红外光与可见光的高斯线宽计算获得. 利用本实验室新搭建的频率分辨及偏振分辨的皮秒和频光谱系统验证了该公式的准确性. 实验结果显示,本激光系统获取的红外光的高斯线宽为1.5 cm^-1. 本激光系统的光谱分辨率约为4.6 cm^-1,结果与胆固醇单层膜光谱获取的光谱分辨率(3.5~5 cm^-1)基本一致.     

Rotating beamforming – motion-compensation in the frequency domain and application of high-resolution beamforming algorithms

A method is presented for the location of rotating sound sources by a microphone array. In contrast to other methods which are formulated in the time domain, this method works completely in the frequency domain and allows the application of advanced, high resolution beamforming techniques. Following the work by Lowis and Joseph for ducted sources, it is shown that a generalized cross-spectral matrix in the rotating frame of reference can be calculated which can serve as a starting point for advanced beamforming techniques. The Green's function of a moving point source under free space conditions is expressed in spherical coordinates, which are, for numerical reasons, more convenient than cylindrical coordinates. The microphones on the array have to be arranged in a ring. As a practical example high resolution beamforming deconvolution methods are applied to simulated data and measured data from rotating sources.     

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。     

指数掺杂GaAs光电阴极分辨力特性分析

通过建立和求解指数掺杂阴极中电子所遵循的二维连续性方程,得到了透射式指数掺杂阴极的调制传递函数表达式,并利用该表达式对阴极分辨力特性进行了理论计算和分析.计算结果显示,与均匀掺杂相比,指数掺杂能较明显地提高阴极的分辨力.当空间频率f在100—400 lp/mm范围时,分辨力的提高最为明显,如当f=200 lp/mm时,分辨力一般可提高20%—50%.与量子效率的提高相同,指数掺杂阴极分辨力的提高也是内建电场作用的结果. 关键词： 指数掺杂 内建电场 分辨力 调制传递函数