热中子照相的MCNP数值模拟初步研究

中子照相的数值模拟在中子照相技术研究中有着重要的作用。介绍了MCNP程序在数值模拟中子照相中的应用，给出了样品的模拟图像结果，并利用MCNP模拟分析了散射中子对中子照相成像结果的影响，对以后的中子照相实验具有指导意义。     

60Co双投影辐射成像系统中的散射干扰模拟研究

~(60)Co双投影辐射成像系统,采用两个射线源,分别底置和侧置,通过一次扫描即可获得仰视和侧视两个方向的投影图像。但由于散射的存在,两个投影平面的成像会相互干扰。为降低两投影平面间的散射影响,提高该系统辐射图像的质量,首先基于蒙特卡罗方法建立仿真模型,并对空载时散射分布的模拟值与实验值进行对比,验证了模型的可靠性,然后模拟分析了不同投影平面间距、探测器周围布置不同屏蔽材料(钨、铅、钢和铝)以及屏蔽层位置和厚度对散射的影响。结果表明:当投影平面间距为40 cm并且屏蔽层厚度相同时,使用铅并将其布置在支撑架和侧视电离室之间时,对散射的屏蔽效果最好;投影平面间距分别为40 cm和80 cm时,散射可以分别降低至间距为10 cm时的55%和40%以下,安装2.5 cm的铅片可以将散射进一步降低80%。此研究为双投影系统针对散射校正的硬件改良和升级提供参考与指导。     

~(252)Cf源环境X光闪光照相光电接收系统的屏蔽

采用含硼聚乙烯、铅及铅玻璃等辐射屏蔽材料,设计和建立了252Cf源辐射环境下X光闪光照相光电接收系统的抗辐射综合屏蔽系统。理论分析和实验结果均表明,在屏蔽系统的综合屏蔽下,科研要求的典型辐射背景在CCD相机图像上形成的本底灰度要小于科学级相机的固有本底噪声一个量级,直接作用在CCD相机上的粒子和射线形成的白斑噪声比例约为1 pix/104pix,可以保证光电接收系统的图像采集质量。     

a-Si FPD在闪光X光照相中的应用可行性实验研究

非晶硅平板探测器(a-Si Flat Panel Detector)已在无损检测、医疗领域的连续式X光照相中得到了广泛应用,但在脉冲式(ns量级)闪光X光照相中的应用可行性及成像性能却未见报导.本文利用Varian公司生产的PaxScan 2520 HE型非晶硅平板探测器为接收系统,在450keV脉冲X光机上开展了闪光照相实验,考察了该探测器的应用可行性,测量了其探测灵敏度、像质计灵敏度、线扩散函数(LSF)及调制传递函数(MTF).实验结果表明该探测器可应用于闪光X光照相,且探测灵敏度较高,空间分辨率较好,是一种有前景的闪光X光照相图像接收系统.     

闪光层析成像散射修正计算研究

利用闪光照相成像系统底片信息,通过直接法对客体线性吸收系数空间分布进行重建,需能精确得到底片直穿量的信息,为解决这一问题必需对散射量进行修正。本文通过解析法推导出精确计算X光正向输运过程中未经碰撞与只经1次碰撞到达探测器处计数的理论公式,同时通过蒙特卡罗计算程序MCNP分析不同照相客体次级散射效应特性,对次级散射量进行数值修正,并由此编制快速光子正向计算程序SAT。与MCNP程序统计结果进行数值校验,结果表明,SAT程序能精确计算X光穿过照相客体到达探测器处的直穿量、散射量及总照射量计数,且计算速度远快于MCNP程序,这为闪光层析成像图像重建迭代扣除散射提供了快速精确的散射修正工具。     

中子照相∑定量测量方法

本文描述了一种利用中子照相进行定量分析的方法,介绍了该方法的基本原理,并应用粒子输运程序MCNP模拟分析样品内部散射中子随样品与探测器之间距离变化对中子成像的影响.为削弱散射中子对中子透射成像图像信息的影响,对穿透样品的厚度进行了定量分析.     

闪光照相X源光斑编码孔成像诊断可行性研究

X源光斑形状和强度分布是高能闪光照相图像反演时必需的定解条件,需高精度的测量,但目前缺少成熟的方法应用于成像时诊断对应的X源光斑。本文通过蒙特卡罗程序直接模拟及图像重建,验证了对有能量和角度分布、有散射等干扰的实际X源,由其编码图像可重建出源的强度空间分布信息,且具有较高精度。采用编码孔成像方法,在闪光照相实验的同时伴随X源光斑的特征检测是可行的,可提高闪光照相图像反演精度。     

定量中子数字成像系统的散射校正迭代算法研究

针对中子散射将直接影响在进行无损检测某种成分时的定量分析,研究分析了中子散射对定量中子数字成像系统图像降质原理,采用点扩展函数的叠加表征成像系统中子散射对图像降质的过程,并在构建中子成像点扩展函数模型的基础上,提出了对中子图像进行散射校正的空域迭代算法。研究结果表明,对于成分相同但几何形状不同的H2O样品,在样品到探测器之间距离不同情况下所得到的中子数字图像,在利用该算法进行散射校正后,影响成像质量的散射成分能较好得到抑制,所恢复出的样品厚度趋近于实际厚度。     

D-T中子源快中子照相准直屏蔽系统设计

减少散射中子对成像质量的影响是快中子照相的关键技术之一。本文采用MCNP程序完成了D-T中子源快中子照相准直屏蔽系统的设计,该设计具有很好的中子散射屏蔽效果,能够将实验室墙壁的散射中子数量降低一个量级,大大减少了成像的对比度失真;同时,能够增加约20%的中子源强,提高了成像速度。     

SPRR-300反应堆在线中子照相技术初步研究

简要描述了在线中子照相技术的特点,较详细阐述了SPRR－300反应堆在线中子照相中中子束的获取、转换屏的选择、微光成像系统和图像采集与处理系统的构成,并对在线中子照相实验结果进行了分析。     

Scatter reduction with energy-weighted acquisition

The theory of energy-weighted acquisition (EWA) in nuclear medicine imaging is outlined, and a system that implements EWA is described. EWA reduces the effects of scattered radiation by allowing events of all energies to contribute to image formation, processing each energy with its own short-range spatial filter. This approach implements short-range energy-dependent filtering with an image buffer called a weighted acquisition module, providing scatter reduction with controllable noise and resolution properties. The system's response to point sources and planar distributions of radioactivity embedded in radiation-scattering media have been measured. EWA is compared to conventional energy-window acquisition, showing that the EWA approach provides improved image contrast     

工业CT的散射研究

X射线散射的存在严重影响了工业CT系统的性能．物体内部密度锐变区域的散射将可能引起重建图像严重的高频伪影,从而影响缺陷的检测,理论计算了不同材料交界面处的X射线散射引起的投影数据偏差的强度及分布,并通过Monte Carlo方法作了验证．在此基础上,简单分析了散射引起的伪影的表现形式。     

散射对工业CT图像质量的影响

设计了能够模拟工业CT散射的仿真系统,研究了散射对重建图像质量的影响,并得出了该影响与被检测物体的吸收系数及被检测物体尺寸之间的关系.     

Scatter Considerations in Fan Beam Computerized Tomographic Systems

In fan beam computerized tomography, a crosschannel noise problem arises due to the scattering of photons from one path into another. This paper describes the effect of these scattered photons on the reconstructed image and gives an estimate of the quantities of scatter which are acceptable without serious degradation of the reconstruction.     

一种CT成像散射抑制方法

在X射线CT(Computed Tomography)成像尤其是锥束成像过程中,散射是不可忽略的影响因素。探测信号中散射信号的存在导致CT图像中的伪影。依据散射信号的特征属性,提出了一种散射的估计方法。该方法利用简单的解析公式估计散射,简单易操作。实际的实验数据验证了方法的有效性。     

散射大厅内中子导管屏蔽计算

中子导管将冷中子束从冷源引出至散射大厅,为保证大厅工作人员的安全,提供低本底实验环境,必须设计相应的屏蔽体进行屏蔽。在已有中子导管屏蔽体初步结构设计方案的条件下,联合McStas、MCNP,采用分段计算的方法对其进行了屏蔽计算,得到了散射大厅内中子导管周围不同位置处的辐射剂量率,验证了中子导管屏蔽体结构设计方案的有效性,为进一步开展工程设计提供了依据。     

Scatter correction techniques in 3D PET: a Monte Carlo evaluation

In this work, a Monte Carlo software package, PET-EGS, designed to simulate realistic PET clinical studies, was used to assess three different approaches to scatter correction in 3D PET: analytical (gaussian fitting technique), experimental (dual energy window technique) and probabilistic (Monte Carlo technique). Phantom and clinical studies were carried out by 3D PET and simulated by PET-EGS. A clinical study (¹⁸F-FDG brain study) was simulated assuming PET emission/transmission multiple-volume images as a voxelised source object describing the distribution of both the radioactivity and attenuation coefficients and accounting for out-of-field activity and media. The accuracy of PET-EGS in modelling the physical response of a 3D PET scanner was assessed by statistical comparison between measured and total (scatter+unscatter) simulated distributions (probability for the two distributions to be the same distribution: p>0.95). The accuracy of the scatter models, for each scatter correction technique, was evaluated on sinograms by statistical comparison between the estimated and the simulated scatter distributions (agreement <1 σ). The accuracy of scatter correction was evaluated on sinograms by comparison between scatter corrected and simulated unscatter distributions, proving a comparable accuracy of all the considered scatter correction techniques for brainlike distributed sources     

A Decay of Langmuir Wave Quanta in Their Scatter by Plasma Electrons

The concept of informativeness of nonlinear plasma scenario is clarified. Key principles are formulated for heightening the informativeness of plasma kinetic models. An analysis is performed of informative model of Langmuir wave scatter by electrons of unmagnetized homogeneous turbulent plasma. It is shown that for this process the former idea of wave quanta conservation is somewhat quantitatively consistent for wave spectra with sufficiently small characteristic wavevector k only ((kr_D)³ << m_e/m_i(kr_D)^{3}\ll {m_e/m_i}). For wave spectra with a larger value of characteristic wavevector ((kr_D)³ >~m_e/m_i (kr_D)^{3}\gtrsim {m_e/m_i}), the rate of wave quanta decay is shown to be comparable with the rate of wave spectral transfer.     

60Co治疗机散射校正因子的蒙特卡罗计算

利用蒙特卡罗程序MCNP模拟计算了⁶⁰Co治疗机的3种散射校正因子,并计算了总散射校正因子S_c,p与模体散射校正因子S_p随射野及深度的变化。计算结果表明：散射校正因子计算结果与测量结果符合较好;S_c,p与S_p随射野的增大而增大;射野大于10cm×10cm时,S_c,p与S_p有随着深度的增加而增大的总趋势;射野小于10cm×10cm时,S_c,p与S_p有随深度增加而减小的总趋势。因此,在计算处方剂量时不可忽略散射校正因子的影响。利用蒙特卡罗方法可建立1组准确和全面的散射校正因子,为放射治疗临床使用、质量保证和质量控制提供依据。     

On the Sensitivity and Application Possibilities of a Novel Compton Scatter Imaging System

A novel type of Compton scatter imaging system is described having the following features: maximum possible use is made of the available scatter radiation; and mechanical constraints permit millisecond 2-D scan times. The sensitivity of this type of scatter system is compared with that of transmission radiography, and it is found that the former is to be preferred for `radiologically thin' objects. An important medical example is the imaging in back scatter mode of radiation-sensitive organs lying near the surface of the body. Some industrial applications of the scatter technique also suggest themselves: e.g. for section imaging of `thin' sheets, plastic components, or back scatter imaging of surface regions of bulky objects, for which the conventional transmission technique gives inadequate penetration. Data correction for attenuation effects and multiple scatter radiation is illustrated using scatter images of objects drawn from the fields of medical and industrial radiology.