大孔径厚针孔成像数值模拟研究

为解决cm级尺寸射线源的空间分布诊断,本文提出大孔径厚针孔成像方法。采用Geant4软件编写蒙特卡罗程序模拟5、10和15 mm大孔径厚针孔的成像过程,采用Matlab软件实现逆滤波、Wiener滤波和Lucy-Richardson复原算法,以均方根误差σ作为复原效果评判标准,优化了Wiener滤波信噪比参数和Lucy-Richardson算法迭代次数参数。将复原算法应用于不同孔径厚针孔的图像复原,发现理想逆滤波算法会引起噪声放大;Wiener滤波算法图像复原解存在较严重的伪影;Lucy-Richardson算法可获得清晰的复原图像。结合Lucy-Richardson算法的5、10、15 mm大孔径厚针孔成像获取的图像的σ是0.5 mm孔径传统针孔成像结果的1.15、1.21、1.26倍。结果表明,结合图像复原算法的大孔径厚针孔成像技术能获得与传统针孔成像接近的源图像测量结果,初步验证了该方法具有可行性。     

高分辨micro-SPECT成像系统针孔准直器的优化设计

为了最大限度地减少γ射线在针孔准直器上的透射和散射效应,采用蒙特卡罗方法对单光子发射计算机断层(SPECT)成像系统使用的针孔准直器进行优化.模拟计算结果与实验测试得到的系统灵敏度和空间分辨率曲线相符,达到了高分辨小动物成像的要求.     

应用于高能伽马射线源图像诊断的厚针孔设计

在伽马射线辐射源图像诊断中,一般使用厚针孔将辐射源产生的伽马射线图像成像在射线图像探测器上.由于射线具有较强的穿透效应,针孔具有较厚的厚度,厚针孔成像系统的空间分辨、能通量、有效视场等测试要求相互制约,必须给出最优的厚针孔结构设计.根据厚针孔成像系统的特点,模拟计算厚针孔成像系统的基本过程.计算结果表明,在最优厚针孔结...     

高分辨率小动物SPECT成像系统的设计与性能评估

为实现低成本、高分辨率的小动物单光子发射断层成像(SPECT),设计了一种基于临床探测器和针孔准直器的小动物SPECT成像系统。综合考虑视野尺寸、空间分辨率和绝对探测效率等指标,分别给出了单针孔和七针孔准直器的两种最优化设计方案,并通过对比两种设计方案在全视野范围内的平均空间分辨率和探测效率,对成像性能进行了评估。结果表明,与单孔准直器相比采用七针孔准直器设计可同时获得更高的空间分辨率和探测效率。经蒙特卡罗模拟成像实验验证,该七针孔准直器在临床探测器上可达0.5mm级的超高空间分辨率,且成本远低于采用专用探测器的现有小动物SPECT系统。     

γ针孔照相测量放射性污染分布

介绍了一种测量设备γ放射性表面污染的新方法。利用γ射线的直线传播和与物质相互作用的特性，采用针孔成像原理，以＾２３８Ｕ作为针孔材料，通过模拟计算得到针孔的设计参数，并据此制作一视场角为６０°的针孔相机。利用此相机分别对γ污染源和污染设备成像，经计算机处理，将此二幅图像叠加起来，可以清楚地知识污染源在设备上的位置及其在相机处产生的剂量水平。该方法具有直观、清晰、工作人员受辐射剂量少的优点，测量结果可     

基于模拟点源测量的多针孔SPECT精确迭代重建

高分辨率单光子发射计算机断层成像（SPECT）是临床和小动物影像的重要技术之一。对SPECT的系统传输矩阵进行建模,并结合于迭代重建算法中,是提高系统空间分辨率的有效手段。本文基于实验室自主研发的多针孔小动物SPECT系统,研究了一种蒙特卡罗模拟和解析模型相结合的系统传输矩阵精确计算方法。该方法是基于多点源模拟的投影数据,结合数据再估计方法和高斯拟合方法提取点扩展函数（PSF）特征参数,并结合解析形式的系统物理模型,从而计算任意位置的精确PSF。通过对四点源模拟投影数据分别用纯解析方法和蒙特卡罗模拟与解析模型相结合的方法进行计算,并对所得的系统传输矩阵进行重建对比,验证了蒙特卡罗模拟与解析模型相结合方法对图像分辨率的改善效果。进一步比较了不同点源排列的重建效果,获得了优化的点源排布方案。相对于基于实验方法测量系统传输矩阵,本方法能以更合理的代价实现系统传输矩阵的精确建模,并能有效提高系统空间分辨率。     

基于约束的MLEM图像重建算法

在编码孔γ相机的二维辐射图像重建中,最大似然期望最大化法(maximum likelihood expectation maximization,MLEM)相较传统线性算法能更好地抑制噪声,恢复出高质量的图像,但其缺点是收敛速度慢,且在投影数据有较高统计涨落或含有较大噪声时,迭代次数过高会导致重建图像中噪声急剧增加,图像质量转而变差。本文基于直接解调法思想对MLEM算法进行优化,在MLEM迭代重建过程中加入先验物理约束条件,并将交叉相关法重建结果作为迭代初值。仿真结果表明,添加约束后的MLEM迭代收敛速度得到加快,重建图像的收敛性显著改善。MLEM算法中添加合理约束条件是提高其重建图像质量的一种有效方法。     

基于平板型闪烁晶体成像探测器位置刻度系统

对于通过准直方式成像的γ相机，因其闪烁体、位置灵敏光电倍增管（PSPMT）光阴极均具有一定的厚度，以及通过重心法计算位置的方式等原因，造成了γ相机在不对所成图像进行修正的情况下，这些γ图像向中心压缩，即出现边沿压缩效应。为减少和修正边沿压缩效应，本文设计了一套刻度系统。该系统采用逐点扫描探测器表面的办法，获得平板型探测器有效区域内所扫点位置与探测器响应位置的原始信息，得到进入探测器的光子的真实位置与γ相机电路输出信息之间的关系映射，形成用于还原每个进入γ相机有效区域内γ光子的实际位置的查找表，从而有效解决了γ相机成像时无法避免的边沿压缩效应造成的图像畸变。该系统在刻度过程中无需人工干预，自动生成位置查找表，为γ相机小孔成像的图像还原或编码孔成像的图像重建提供了必要条件。     

旋转镜像NLM约束稀疏角度CT重建

提出了基于旋转镜像变换的自适应非局部平均算法,用于稀疏角度CT重建。设计基于相似窗旋转镜像变换的相似性测度以避免图像边缘模糊;并根据像素梯度及重建迭代次数自适应调整滤波参数,有效保护图像结构信息。新算法主要包含三个步骤:代数重建、非负修正及RAMNLM滤波,循环执行以上步骤直至满足迭代收敛准则。以Shepp-Logan体模进行仿真重建结果表明,新算法可有效抑制噪声并恢复图像细节,显著提高图像质量。     

MC法模拟多路平行束准直器对比度

采用MC法模拟设计了用于SPECT成像的多路平行束(MPB)准直器,将MPB系统重建的模具图像与平行束(PB)系统重建图像进行对比。用OS-EM迭代算法重建图像并计算对比度、信噪比、归一化标准偏差三个指标用于评价图像质量。结果表明:随着迭代次数增加,MPB系统重建图像中热源的对比度高于PB系统,对于小半径热源这个差值更明显,但是冷源的对比度低于PB系统;两个系统有可比较的重建图像收敛速度,随着迭代次数增加,MPB系统的噪声特性略差于PB系统。     

OSEM算法在编码板成像中的应用

为提高γ辐射编码成像中MLEM算法在图像重建时的速度及质量,论文基于核医学ECT图像重建中的OSEM算法思想在图像重建时对编码成像所得投影数据进行分组,并采用精细采样平衡相关法的重建结果作为初值进行迭代。通过实验对该算法进行验证。实验表明:OSEM算法可以有效提高收敛速度,改善重建图像质量。在γ辐射编码成像中采用OSEM算法是有效且可行的。     

An analytical simulation technique for cone-beam CT and pinhole SPECT

This study was aimed at developing an efficient simulation technique with an ordinary PC.The work involved derivation of mathematical operators,analytic phantom generations,and effective analytical projectors developing for cone-beam CT and pinhole SPECT imaging.The computer simulations based on the analytical projectors were developed by ray-tracing method for cone-beam CT and voxel-driven method for pinhole SPECT of degrading blurring.The 3D Shepp-Logan,Jaszczak and Defrise phantoms were used for simulation evaluations and image reconstructions.The reconstructed phantom images were of good accuracy with the phantoms.The results showed that the analytical simulation technique is an efficient tool for studying cone-beam CT and pinhole SPECT imaging.     

X射线源针孔成像方法可行性研究

为检验利用针孔成像对X射线源进行研究的可行性，对X射线针孔成像系统进行优化设计，搭建了1套X射线源针孔成像装置。从理论上对X射线源的位置和强度发生变化时图像的相应变化进行了计算和预估，并用实验加以验证。结果表明，像斑中心位置的移动和像斑灰度值的变化分别与X射线源位置和强度的变化在一定范围内存在线性关系，由此证明了利用针孔成像对X射线源进行空间定位及定量研究的可行性。本文结果对X射线源针孔成像的系统设计具有参考意义。     

SPECT/CT活度定量中校准体模和重建方法的选择

本文探讨了SPECT/CT校准体模和重建方法的选择,以用于临床SPECT/CT的准确活度定量。分别对活度已知的点源、大圆柱、小圆柱3种校准体模进行SPECT/CT扫描,然后对每个断层数据依次采用4种重建方法（FBP、OSEM、OSEM+AC、OSEM+AC+SC）进行重建,并据此计算校准因子。将小圆柱作为活度定量的对象进行定量误差计算。OSEM+AC和OSEM+AC+SC的平均相对定量误差均小于10%,FBP和OSEM的平均相对定量误差均大于20%。OSEM+AC的平均相对定量误差在采用小圆柱做校准体模时与OSEM+AC+SC无显著差异,而在采用大圆柱和点源时,分别较OSEM+AC+SC的小3.38%和6.48%（P<0.05）。3种体模的对比中,采用OSEM+AC时差异不显著,采用其他重建方法时差异显著。采用OSEM+AC+SC时,小圆柱作校准体模的定量误差较大圆柱和点源的结果分别小3.92%和6.42%（P<0.05）。结果表明,基于CT和迭代算法的衰减校正可有效提升活度定量准确性,但散射校正的效果不佳;校准体模应尽可能与目标定量对象相似。合理选择校准体模和重建方法,可有效提升SPECT/CT的活度定量准确性。     

Lead and tungsten pinhole inserts for I-131 SPECT tumor imaging:experimental measurements and photon transport simulations

The potential use of lead and tungsten pinhole inserts for high-resolution SPECT imaging of intratumor activity in I-131 radioimmunotherapy was investigated using experimental point source measurements and photon transport simulations. I-131 imaging is challenging because the primary photon emission is at 364 keV and penetration through the insert near the pinhole aperture is significant. Point source response functions (PSRF's) for lead (Pb) and tungsten (W) pinhole inserts were measured experimentally. These response functions were simulated using a photon transport computer code that modeled the primary emission at 364 keV and secondary emissions at 284, 637, and 723 keV. Scatter within the pinhole insert, camera shielding, and scintillation crystal was modeled. There was good agreement between the experimental and simulated PSRF's. Simulated point source response functions for geometrically identical Pb and W pinhole inserts were narrower for the W insert due to reduced penetration. SPECT pinhole imaging with these inserts was simulated for 3-cm-diameter tumors with a central core and 3-5-mm-thick shells. For one set of simulations there was no core activity, and for a second set the shell:core activity concentration ratio was 5:1. In both cases, the tumor shells were better resolved with the W insect. As a result, shell:core activity ratios were more accurate and contrast was improved with the use of the W pinhole insert. This study suggests that W inserts have potential advantages over Pb inserts for high-resolution I-131 pinhole imaging     

A high-resolution small animal SPECT system developed at Tsinghua

An SPECT system dedicated to small animal imaging shall be of a millimeter spatial resolution or even better.This study was aimed at achieving 0.5-mm spatial resolution for a small animal SPECT system at low cost.It was developed from a single-head clinical SPECT scanner,with a seven-pinhole collimator and a four-degree-offreedom motion control stage.Several key techniques were developed,including high-resolution image reconstruction algorithm,high accuracy geometrical calibration method,and optimized system matrix derivation scheme.The system matrix was derived from Monte-Carlo simulation and de-noised by fitting each point spread function to a two dimensional Gaussian function.Experiments of point source and ultra micro hot rod phantom were conducted.With a spatial resolution of 0.5-0.6 mm,this system provides a practical way for low-cost high-resolution animal imaging on a clinic SPECT system.     

High-resolution SPECT for small-animal imaging

1Introduction The rapid advance in molecular biology and the increasing use of transgenic mice as models of human biology and diseases have led to significant interests in the development of noninvasive,high-resolution,in vivo imaging techniques for small animals[1-4].Medi-cal imaging techniques including ultrasound(US),computed tomography(CT),magnetic resonance im-aging(MRI),positron emission tomography(PET),and single-photon emission computed tomography(SPECT),are contributing to this …     

Quantification of the Multiplexing Effects in Multi-Pinhole Small Animal SPECT: A Simulation Study

Our goal is to study the trade-off between image degradation and improved detection efficiency and resolution from allowing multiplexing in multi-pinhole (MPH) SPECT, and to determine the optimal pinhole number for MPH design. We used an analytical 3D MPH projector and two digitized phantoms: the mouse whole body (MOBY) phantom and a hot sphere phantom to generate noise-free and noisy projections, simulating pinhole collimators fitted with pre-studied pinhole patterns. We performed three schemes to achieve different degrees of multiplexing: 1. Fixed magnification and detection efficiency; 2. Fixed detection efficiency and changed magnification; 3. Fixed magnification and changed detection efficiency. We generated various noisy data sets by simulating Poisson noise using differently scaled noise-free projections and obtained 20 noise realizations for each setting. All datasets were reconstructed using 3D MPH ML-EM reconstruction method. We analyzed the quantitative accuracy by the normalized-mean-square-error. We evaluated the image contrast for the hot sphere phantom simulation, and also the image noise by the average normalized-standard-deviation of certain pixels for different degrees of multiplexing. Generally, no apparent artifacts were observed in the reconstructed images, illustrating the effectiveness of reconstructions. Bias increased for increased degree of multiplexing. Contrast was not significantly affected by multiplexing in the specific simulation scheme (1). Scheme (2) showed that excessive multiplexing to improve image resolution would not improve the overall trade-off of bias and noise compared to no multiplexing. However, scheme (3) showed that when comparing to no multiplexing, the trade-off improved initially with increased multiplexing by allowing more number of pinholes to improve detection efficiency. The trade-off reached a maximum and decreased with further multiplexing due to image degradation from increased bias. The optimal pinhole number was 7 for a compact camera with size of 12 cm × 12 cm and 9 for a standard gamma camera with size of 40 cm × 40 cm in this scheme. We conclude that the gains in improved detection efficiency and resolution by increased multiplexing are offset by increased image degradations. All the aforementioned factors must be considered in the optimum MPH collimator design for small animal SPECT imaging.