基于倒置结构的直接体积重建

通过多个点状探测器在三维空间中的适当分布的排列,得到了基于倒置结构Ｘ射线锥形束投影的直接体积重建三维成像七锥形束边界重建相结合,针对倒置结构构造的重建算法及其数值仿真表明,新的成像模式相对于传统的锥形束边界重建,在实际存在Ｘ射线投影噪声情形下有明显的优点。     

局部扫描情况下多项式拟合的X射线硬化校正

在进行X-CT成像时,容易诱发射束硬化效应。通过多项式拟合建立起多色投影同透射距离的数学模型,然后将多色投影校正为单色投影,是一种简单高效的射束硬化校正方法。对于局部扫描的情况,在重建图像域通过正投影方法计算得到的透射距离不再代表真实的射线穿过长度,拟合数据不再有效。因此借助一圆柱模体,根据圆柱自身的几何特性,通过解析的方法计算得到射线真实的透射距离,并用于射束硬化校正。实验结果表明,该方法能够有效抑制射束硬化引起的图像伪影,提高CT重建图像质量。     

代数重建技术在板壳结构断层重建中的应用

针对板壳结构用锥束射线倾斜扫描时数据缺失等问题,利用代数重建技术(ART)进行了断层重建。计算机模拟结果表明,由ART重建出的切片图像质量远好于先前用滤波反投影(FBP)重建出的切片图像质量。对影响ART算法的各种因素进行了调整,即选取适当的初始图像和收敛因子就可获得最佳的图像质量。     

螺旋锥束计算机断层成像倾斜扇束反投影滤波局部重建算法

螺旋锥束计算机断层成像(CT)作为常用的临床诊断工具,如何尽可能地减少其辐射剂量是热点研究领域之一.局部成像利用准直器减小射线直照区域,能够有效降低CT辐射剂量.然而,局部成像会造成投影数据横向截断,产生局部重建问题.现有螺旋反投影滤波(BPF)算法只能实现局部曲面重建,难以实现局部体区域重建.在圆轨迹扇束BPF算法的基础上,通过加权修正和坐标扩展,提出了螺旋锥束CT倾斜扇束反投影滤波(TFB-BPF)重建算法.该算法把重建区域按层划分,对各层构建倾斜的扇形束几何,并使用经过加权修正的TFB-BPF算法逐层进行重建.该算法最大的优势是滤波线(与原始螺旋锥束BPF算法中PI线等价)在二维平面内选择,算法更加简洁高效,并且能够应用于局部体区域的重建.实验结果表明,算法能够有效重建物体局部体区域,并且重建图像质量较好,没有明显的截断伪影.     

倒置结构锥形束计算机层析的精确成像与Grangeat算法实现

研究了一种新的圆加垂直单弧段的锥顶轨迹的锥形束计算机层析（CT）的完全性条件和精确重建的Grangeat方法，提出了倒置结构锥形束计算机层析的便于实现的圆加垂直多弧段的成像结构及进行精确重建的完全性条件。利用倒置成像结构与现有成像结构之间的等价关系，给出了倒置结构锥形束计算机层析的重建算法。最后，对上述几方面进行了计算机模拟。     

针孔单光子发射计算机断层成像的空间分辨率研究

针孔单光子发射计算机断层(SPECT)成像的空间分辨率通常是根据Anger经验公式来进行估算,与实际测量存在较大偏差.本文通过对针孔成像的物理过程进行分析,提出了一个近似度更高的计算公式.利用精确的蒙特卡罗方法模拟针孔SPECT成像,采用OSEM(ordered subsets expectation maximization)算法对投影数据进行图像重建,并与模具实验进行比较,验证了理论公式的适用性.同时还讨论了体素尺寸、几何映射获取投影矩阵以及探测器尺寸与成像物体尺寸比值对断层图像空间分辨率的影响.实验结果显示,该理论公式所估算的空间分辨率比实验值平均偏小约10%,而Anger经验公式所估算的空间分辨率比实验值平均偏大约60%.因此,该理论公式能更好地估算针孔SPECT成像的空间分辨率,可为针孔SPECT系统的设计和使用提供有价值的参考.     

CT不完全投影数据重建算法综述

本文主要针对计算机断层成像(computed tomography,CT)不完全投影数据重建中探测器全覆盖稀疏角度重建和探测器部分覆盖截断数据重建问题,综述了其在研究方法上国内外的进展.在探测器对被检测物体可以完全覆盖的情况下,针对稀疏均匀采样和视角受限采样,探讨了离散模型迭代重建算法和压缩感知采样重建算法.在探测器对被检测物体不能完全覆盖的情况下,探讨了锥束螺旋BPF重建算法、加型迭代重建算法和锥束FDK改进算法.论文可以为CT重建领域的研究工作者提供全面的方法梳理和总结,并指出了当前研究的重点和未来研究的方向.     

Computer simulation of acoustic nonlinear parameter tomography

I.IntroductionUltrasonicimaginghasbcenwide1yusedinthcfie1dofclinicaldiagnosis,becauseitcanvisualizethetissuetharacterizationandinternalstructureofbiologicalobjectsbyacousticwave.Usingconventionalultrasonicimagingtechnique,theimagesofacousticlinearparameterssuchassoundve1ocity,acousticimpedenccandattenuationcoefficientmaybeobtained.Thesehavebecometheeffectivemethodsofu1trasonicdiagnosis.How-ever,inordertoobscrvctheearlystageofcanccr,weintendtoobtainmoreaccurateandmorecompleteinformationaboutth…     

Synthetic aperture microscopy for high resolution imaging through a turbid medium

We report on synthetic aperture microscopy through a highly turbid medium. We first recorded a transmission matrix for the turbid medium with an angular basis of 20,000 complex images covering 0.6 NA. This effectively converts the medium into a lens of the same NA. Distorted images of a target object are then taken at 500 different angles of illumination covering 0.6 NA. For each of the distorted images, the original object image is reconstructed from the transmission matrix by the recently developed turbid lens imaging (TLI) technique. All 500 reconstructed images are synthesized to enhance the NA to 1.2 and thereby generate an object image with twice the enhanced spatial resolution of the individual images. Our method of applying aperture synthesis for TLI makes it possible to enhance the resolving power without increasing the number of transmission matrix elements. This relieves the demand for data acquisition and processing that has impeded the practicality of TLI.     

Interferometric key readable security holograms with secrete-codes

A new method is described to create secrete-codes in the security holograms for enhancing their anti-counterfeiting characteristics. To imitate these codes is difficult as pure phase objects having complex phase distribution function are used to modulate the object beam that is recorded in conjunction with an encoded interferometric reference beam derived from a key hologram. Lloyd’s folding mirror interferometer is used to convert phase variations of the reconstructed wave-front into an intensity pattern for hologram authenticity verification. Creating the secrete-codes through an interferometric reference beam from the key hologram facilitates a multi-stage authenticity verification as well as easy repositioning of the security hologram through a specific Moiré pattern generated during the verification process.      

Mean pulse analysis and spectral character study of pulsar PSR B2111+46

PSR B2111+46 is studied and analyzed by the method of Gaussian fit separation of the average profile (GFSAP). It has been observed at six frequencies,408, 610, 925, 1408, 1640, and 4850 MHz, serving as an excellent sample for a multifrequency analysis. Applying the GFSAP method we find that PSR B2111+46 hasfive components in its profile at all six frequencies. The result supports Rankin's "core-double cone" model in emission region. On the basis of component decomposition, we discuss the geometrical structure of core, inner cone and outer cone, the emission beam radius, the emission region altitude and the geometry symmetry. We also examine the spectrum characteristics of core, inner cone and outercone. It is found that the spectrum of the core is steeper than the inner and outer cone.     

Sinusoidal fringe projection system based on compact and non-mechanical scanning low-coherence Michelson interferometer for three-dimensional shape measurement

We report a sinusoidal fringe projection system based on superluminiscent diode (SLD) as a broad-band light source in conjunction with an acousto-optic tunable filter (AOTF) as frequency tuning device for three-dimensional shape measurement. The present system is based on a compact low-coherence Michelson interferometer system. The conventional interferometric system was modified in which one side of the beam splitter was coated with aluminum oxide which is used as reference mirror. With this modified version, interference fringes can easily be obtained by simply placing the external mirror in contact on the other side of beam splitter. Sinusoidal fringes with multiple spatial-carrier frequency can be generated in real-time using the present system by means of changing the radio-frequency signal to AOTF electronically without mechanically moving any component in the system. The present system was tested by projecting the sinusoidal fringes on a step-like object and 3D shape of the object was reconstructed using Fourier transform fringe analysis technique. The main advantages of the proposed system are completely non-mechanical scanning, easy to align, high stability because of its nearly common-path geometry and compactness.     

空间频率对透射式全息图衍射率的影响

为研究干涉条纹空间频率与透射式全息图衍射率的关系,在相同条件下,拍摄以平面波、发散的和会聚的球面波分别作参考光、物光的透射式全息图,通过实验测量其衍射效率,同时计算了干涉条纹的空间频率。然后以会聚的球面波为物光,平面波为参考光,分别改变入射角和球面波会聚点到记录面的距离制作另外4块透射式全息图,并计算了它们的空间频率,测量了其衍射效率。结果表明：在相同条件下制作透射式全息图时,衍射效率只由空间频率决定,与使用何种参考光和物光无关,空间频率仅随参考光和物光的夹角变化而变化。     

An image processing algorithm for single-photon fluorescent tomography under conditions of strong scattering

An algorithm for digital processing of images recorded by means of scanning laser fluorescent tomography is developed. Images of the test object fabricated from a polymethylmethacrylate matrix with rhodamin 6G put in water solution of dry milk modeling a brain tissue are processed. The algorithm allows not only the contours of the object distorted because of strong scattering in biological tissues but also a finer object structure to be reconstructed. In addition, the algorithm enables three-dimensional object structure to be reconstructed for the known distribution of dye concentration in the object, which is also demonstrated in the present paper. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 40–46, April, 2007.     

Efficient Algorithm for Generating Maxwell Random Variables

A fast, easily implemented and high efficiency algorithm is derived for sampling from the Maxwell distribution. The algorithm is derived from the rejection-acceptance sampling method using the simple exponential decay function as an envelope function for the Maxwell distribution. The derived algorithm requires less number of random numbers per iteration, consumes less number of random numbers per sample and requires less expensive computation functions than the direct and Johnk’s algorithms. The speed of the proposed algorithm is about 1.6 times that of the direct algorithm and is about 1.5 times that of Johnk’s algorithm. Since the proposed algorithm for sampling from Maxwell distribution verified high efficiency and speed, Watt random variables can be generated by transforming Maxwell random variables generated by the proposed algorithm. The speed of generating Watt random variables using the proposed algorithm is about 1.1 times that generated from Kalos’s algorithm.     

Recent results at LULI on fast electron transport with and without guiding cone in the context of fast ignitor

We present experimental and numerical results obtained at LULI (Laboratoire pour l’Utilisation des Lasers intenses) on propagation and energy deposition of laser-generated fast electrons into conical targets. The experimental measurements were performed by means of several diagnostics in order to assess the predicted benefit of conical targets over standard planar ones. Various configurations have been tried, regarding the laser parameters with the aim of optimizing the laser-to-target coupling. Our best results have been obtained when the laser was frequency-doubled at 0.53 μm, corresponding to interaction conditions without laser pedestal due to the ASE (Amplified Spontaneous Emission). Our data pinpoint the detrimental influence of the pre-plasma generated by the laser pedestal at 1.057 μm, whose confinement is enhanced in conical geometry as evidenced by shadowgraphic measurements which is also confirmed by 2D Cu-Ka transverse images obtained from Cu cones. The consequence is the filling of the cone, preventing the laser beam from efficiently reaching the cone tip. These experimental results are compared to 2D PIC simulations modeling of the laser-cone interaction.     

Generation of three-dimensional optical structures by dynamic holograms displayed on a twisted nematic liquid crystal display

Reconstruction of computer generated holograms (CGHs) addressed on a spatial light modulator (SLM) is an effective way to dynamically generate designed light field distributions. Based on the classic Gerchberg–Saxton (GS) algorithm, we proposed a technique, which can greatly reduce the computation cost to about 60 % in calculating CGHs for three-dimensional (3D) structures but with little degradation of reconstructed light field compared with the classic GS algorithm. The CGHs calculated by our method were displayed on a twisted nematic liquid crystal display, working as a phase-only-modulation SLM, and 3D structures of optical fields, e.g., 3D array of optical traps and vortices, were reconstructed with high efficiency and high quality. Besides, the possibility for 3D holographic display or projection was also demonstrated with this algorithm by reconstruction several images simultaneously in distinct axial planes.     

Spectrum reconstruction of a nanosecond electron beam from the data on its extinction in thin foils

A possibility of reconstructing the electron beam spectrum from the dependence of beam extinction on foil thickness is demonstrated. The electron energy distributions are calculated using Tikhonov’s regularization of the Fredholm integral equation with the least number of a priori assumptions on the beam spectrum shape. The spectra experimentally obtained in a vacuum diode loaded by nanosecond voltage pulses are reconstructed and analyzed.     

A ray-based algorithm for multi-dimensional linear conversion

A numerical algorithm is proposed for connecting the incoming and outgoing wave fields in studies of linear conversion. This is the first such ray-based algorithm for wave conversion in multiple spatial dimensions. It is demonstrated that, aside from the overall phase of the coupling, one can directly evaluate all quantities needed for the connection coefficients from the ray geometry. The ray dynamics is generated using the determinant of the dispersion matrix as the Hamiltonian. Using information available while following an incoming ray, the algorithm automatically detects that the ray has entered a conversion region, evaluates the transmission and conversion coefficients, and launches the transmitted ray. The algorithm does not require any prior knowledge of the geometry of the conversion region. The algorithm is illustrated using a two-dimensional toroidal model with resonant conversion from a magnetosonic to an ion-hybrid wave.     

Generation of TE- and TH-polarized Bessel beams using one-dimensional photonic crystal

Based on the matrix method, a theory of propagation of TE- and TH-polarized Bessel light beams (BLBs) in a one-dimensional photonic crystal (1DPC) is developed. The transmission through a 1DPC (with and without a defect impurity) of a quasi-circularly-polarized incident Bessel beam generated by an axicon from a circularly-polarized Gaussian beam has been calculated and analyzed. Also a solution of the problem on the transmission of BLBs through crystalline plate (layer of a uniaxial crystal with the orientation of the optical axis orthogonally to its interfaces) and reflection from it has been presented.Based on this, a new method of formation of TE- and TH-polarized Bessel light beams has been proposed. It has been shown that it is possible to control this process by changing the cone angle of an incident Bessel light beam. The effect of generation of a coherent superposition of two Bessel beams with different cone angles in the case of a high birefringence of defect layer has been predicted theoretically.