Monte-Carlo simulation of pinhole collimator of a small field of view gamma camera for small animal imaging

Needs in scintimammography applications,especially for small animal cardiac imaging,lead to develop a small field of view,high spatial resolution gamma camera with a pinhole collimator.However the ideal pinhole collimator must keep a compromise between spatial resolution and sensitivity.In order to design a pinhole collimator with an optimized sensitivity and spatial resolution,the spatial resolution and the geometric sensitivity response as a function of the source to collimator distance has been obtained by means of Monte-Carlo simulation for a small field of view gamma camera with a pinhole collimator of various-hole diameters.The results show that the camera with pinhole of 1 mm,1.5 mm and 2 mm diameter has respectively spatial resolution of 1.5 mm,2.25 mm and 3 mm and geometric sensitivity of 0.016%,0.022% and 0.036%,while the source to collimator distance is 3 cm.We chose the pinhole collimator with hole diameter size of 1.2 mm for our the gamma camera designed based on the wade-off between sensitivity and resolution.     

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.     

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 …     

Development of a low-energy high resolution X-ray camera for high-energy gamma photon background environments

Although a high-energy gamma camera can obtain images of ¹³⁷Cs distribution by detecting the 662-keV gamma photons, its spatial resolution is reduced because high-energy gamma photons penetrate the edge of the pinhole collimator. To solve this problem, we developed a low-energy X-ray camera that detects the characteristic X-ray photons (32–37 keV) that are emitted from ¹³⁷Cs to obtain high resolution images. We used a 45 × 45 × 1-mm-thick NaI(Tl) scintillator that was encapsulated in 0.1-mm-thick aluminum and optically coupled to a 2-inch square, position sensitive photomultiplier tube (Hamamatsu Photonics, PSPMT:H12700 MOD) as an imaging detector. The imaging detector was encased in a 2-cm-thick tungsten alloy container and a pinhole collimator was attached to its camera head. The spatial resolution and sensitivity were ～5 mm full-width at half-maximum and ～0.6 cps/MBq for the 1.5-mm pinhole collimator 10 cm from the collimator surface, respectively. We administered 5 MBq of ¹³⁷Cs to a soybean seedling, imaged the distribution of radionuclides for six hours, and successfully obtained a high resolution image of it with our developed X-ray camera. We believe our camera will be a powerful tool for such ¹³⁷Cs imaging in plants.     

高能γ光子成像针孔准直器的解析设计方法

在对高辐射环境进行射线源分布成像时,需要为γ相机设计专门的用于高能光子成像的针孔准直器。为了简化针孔准直器的设计过程,使用几个新的解析公式来描述准直器各参数与性能间的关系。首先,采用解析方法推导了更通用的有效孔径公式、角分辨率公式和几何相对效率公式;然后,与采用蒙特卡罗方法模拟计算得到的数值进行比较。结果表明,解析方法提供的计算结果在一定范围内准确可靠。因此,本工作使用的针孔准直器解析设计方法简单方便,物理图像清晰,具有一定的实用价值。     

区域γ辐射定向监测仪准直器的设计

为区域γ辐射分布定向监测实验相机系统设计了一个针孔准直器。应用针孔准直器的解析公式,根据探测器内在分辨率和实验相机系统的预期分辨率,考虑探测器尺寸,确定了相机的焦距及准直器的张角;然后根据预期角分辨率计算有效孔径,得出孔径和孔深的范围;再利用蒙特卡罗方法模拟面源入射,在各种孔径和孔深组合中,挑选出透射和散射光子份额最小的组为最优组,对应孔径和孔深参数即为最优参数。     

X射线源针孔成像系统优化设计研究

为提高X射线源针孔成像系统的性能,对成像能区为10~100 keV的X射线源针孔成像系统进行了优化设计研究。综合应用了理论分析和蒙特卡罗模拟的方法,首先根据X射线波长、准直器角响应和X射线穿透效应对针孔成像的不同影响结果设计了可有效控制成像分辨率和成像面积变化的船底型准直器,随后用蒙特卡罗方法对使用该准直器的针孔成像系统进行了模拟验证。结果表明,对于100 keV以下的X射线,经船底型准直器后,成像的空间分辨率和亮斑亮度较稳定,能得到相对准确的X射线源定位、定量信息。     

Phantom experiments on a PSAPD-based compact gamma camera with submillimeter spatial resolution for small animal SPECT

We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for the application of small animal single photon emission computed tomography (SPECT). The silicon PSAPD with a two-dimensional resistive layer and four readout channels is implemented as a gamma ray detector to record the energy and position of radiation events from a radionuclide source. A 2 mm thick monolithic CsI:Tl scintillator is optically coupled to a PSAPD with a 8mm×8mm active area, providing submillimeter intrinsic spatial resolution, high energy resolution (16% full-width half maximum at 140 keV) and high gain. A mouse heart phantom filled with an aqueous solution of 370 MBq (99m)Tc-pertechnetate (140 keV) was imaged using the PSAPD detector module and a tungsten knife-edge pinhole collimator with a 0.5 mm diameter aperture. The PSAPD detector module was cooled with cold nitrogen gas to suppress dark current shot noise. For each projection image of the mouse heart phantom, a rotated diagonal readout algorithm was used to calculate the position of radiation events and correct for pincushion distortion. The reconstructed image of the mouse heart phantom demonstrated reproducible image quality with submillimeter spatial resolution (0.7 mm), showing the feasibility of using the compact PSAPD-based gamma camera for a small animal SPECT system.     

被动法γ光子针孔成像测量Pu部件对称性的数值模拟分析

采用Monte-Carlo数值模拟方法,理论分析Pu部件在不同针孔装置下的γ光子辐射场分布,论证了被动法γ光子针孔成像用于测量Pu部件对称性的可行性。数值模拟分析显示：采用优化的针孔装置,可提高成像质量;针孔装置特征、物距、像距属于敏感信息,对这些因素应有限制措施,以防止敏感信息泄漏。     

基于LaBr3(Ce)晶体耦合SiPM阵列的可变角分辨率伽玛相机的设计与研制

编码孔径成像技术由于探测效率高、信噪比高、角分辨率好、成像质量稳定可靠等优点而广泛应用于核安全、核设施的去污及退役的测量、核医学等领域。建立通过改变编码准直器和探测器之间距离进而实现可变角分辨的伽玛成像系统。整个成像系统主要由编码准直器、位置灵敏探测(position sensitive detector, PSD)、数据采集卡以及图像重建系统组成。该成像系统的编码准直器采用修正均匀冗余阵列(modified uniformly redundant array, MURA)编码方式,为了保障对较高能量射线的探测能力,编码准直器的材料采用含钨量90%的钨铜合金,PSD通过LaBr₃(Ce)晶体耦合SiPM阵列组成,重建算法采用的是直接互卷积算法,快速高效。测试结果显示,整个位置灵敏探测器的平均能量分辨率为4.96%(662 keV);该辐射成像系统可以准确地对Am-241、Cs-137、Co-60进行清晰成像,并通过改变编码准直器和探测器之间的距离成功分辨出两个Cs-137点源的位置。     

Development of a Small-Aperture Slit System for a High Collimator Ratio at the Thermal Neutron Radiography Facility in JRR-3

A small-aperture slit system has been developed and installed to enhance the collimator ratio (“L/D”) of the thermal neutron radiography facility (TNRF) in JRR-3. The degree of unsharpness on neutron images is reduced by increasing the L/D. The small-aperture slit system increased the L/D by creating a small aperture size (“D”). Image sharpness improved when the aperture size was reduced to below 10mm by 10mm in the TNRF. On the other hand, there was almost no difference in unsharpness on images obtained above 10mm by 10mm in aperture size. These results indicate that an aperture size of less than 10mm by 10mm should be used for high-spatial-resolution imaging at the TNRF. The beam area of the small-aperture slits was relatively small in comparison with that of a conventional large collimator, though gradually increasing with increasing aperture size. Even with an aperture size of 5mm by 5 mm, the practical beam area for imaging examinations corresponded to around 25mm by 20 mm, which is enough area to carry out high-spatial-resolution imaging.     

多平行束准直器模拟研究

在平行束准直器中引用多通道技术设计了三通道的多平行束（Multi-parallel Beam,MPB）准直器。计算机模拟表明,MPB准直器在保持较高的空间分辨率下,有效地改善了系统的灵敏度。放射药物匀均分布的圆柱数字模具和Jaszczak数字模具（phantom）模拟表明,随重叠比例的增加,未出现重建环状伪影;虽然靠近准直器表面空间分辨率有所下降,但在FOV（Field of View）中心区域空间分辨率近似等同于平行束准直器,重建图像的匀均度有了改善。投影数据的重叠表明,MPB的投影有放大作用。该研究为高成像质量的临床SPECT成像提供一种新的准直器设计方法。     

Point spread functions for a small gamma camera with pinhole collimator

A set of point spread functions (PSF) has been obtained by means of Monte-Carlo simulation for a small gamma camera with a pinhole collimator of various hole diameters. The FOV (field of view) of the camera is expended from 45 mm to 70 mm in diameter. The position dependence of the variances of PSF is presented, and the acceptance for the 140 keV gamma rays is explored. A phantom of 70 mm in diameter was experimentally imaged in the camera with effective FOV of only 45 mm in diameter.     

A new multi-parallel-beam collimators with improved sensitivity for SPECT

In this paper, a new design of multi-parallel-beam (MPB) collimators with projection multiplexing is proposed. In the MPB system, two different oblique parallel channels are introduced in a conventional parallel-beam collimator. The sensitivity of the single photon emission computed tomography (SPECT) system is improved by allowing projection overlapping. Comparative simulation studies were performed in the MPB collimators, general purpose parallel-beam (GPPB) collimators and high sensitivity parallel-beam (HSPB) collimators. In the simulation, attenuation, scattering and the impact of detector response were neglected. Simulation results show that the sensitivity is improved for the MPB collimator comparing with parallel-beam collimator. The behavior of spatial resolution is only different near the front face of the collimators and approaches that of the GPPB with increasing depth. Proper pre-filtering is helpful for the image reconstruction in the MPB collimators. Comparing with the HSPB collimator, the MPB can achieve a similar sensitivity and better resolution. The simulation ot the U87 cells, and their expression levels were higher in the 10 Gy group than in the 0 Gy group. The differential gene expression in DCX-U87 cells before and after radiation is helpful for future investigations into the mechanisms of radiation therapy in neurogliocytoma cells.     

Characteristics of parallel-hole and pinhole collimators for nuclear medicine imaging

In this paper,the spatial resolution and geometry sensitivity of parallel- and pin-hole collimators in compact gamma camera are presented,and quantitatively compared by GEANT4 code in Monte-Carlo library.The results show that the geometry sensitivity for pinhole collimator rapidly drops with increasing the gamma source-to- collimator distance(SCD);and for parallel-hole collimator,mildly.Meanwhile,the spatial resolution for pin-hole collimator mildly deteriorates;and for parallel-hole collimator,severely.The pin-hole collimator for close imaging objects has higher geometry sensitivity than parallel-hole collimator.Our findings are helpful for setting compact gamma camera collimators in nuclear medicine imaging.     

Aperture edge scattering in focused MeV ion beam lithography and nuclear microscopy: An application for the GEANT4 toolkit

Collimators are widely used to define MeV ion beams. Recent studies have shown the capability of combinations of collimators and lenses to define beams of MeV ions with sub-100 nanometre dimensions. Such nanometre beams have potential applications in MeV ion beam lithography, which is the only maskless technique capable of producing extremely high aspect ratio micro- and nano-structrures, as well as in high resolution MeV ion beam based imaging. The ion scattering from the collimator-edges can be a resolution restricting factor in these applications. Scattering processes at edges are difficult to study using conventional simulation codes because of the complicated geometry. In this work we overview the possibilities and pitfalls in using the GEANT4 toolkit as a simulation tool for studying the behaviour of ions impinging onto, or in close proximity to aperture edges. Results from simulations of scattering of 2 MeV protons incident on a 5 mm in diameter cylindrical beam defining tungsten carbide (WC) aperture blade, such as used in nuclear micro- and nanobeams as object and scraper apertures, are presented. The bubble formation at the aperture edges due to the implantation of H was first believed to cause edge roughening and hence the beam quality degradation. The concentration profile of implanted H in a WC cylinder shows that the bubble formation is not likely to be the edge roughening factor, but rather other mechanisms of beam induced topographical changes such as ion beam induced morphological changes and cracking of hydrocarbons.     

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.     

用于闪烁薄膜探测器灵敏度标定的中子屏蔽体设计

为标定薄膜厚度大于0.1mm的闪烁薄膜探测器灵敏度,采用MCNP程序建模优化设计了适用于在中国原子能科学研究院放射性计量测试部的5SDH-2串列加速器上进行实验的中子屏蔽体。实验表明,该屏蔽体可将偏离通道的中子注量减弱到通道中子注量的十分之一以下,将本底信号抑制在较光电倍增管暗电流略低的水平上,对于薄膜厚度大于0.1mm的探测器,可使其信噪比大于1∶1。计算表明,准直孔的散射对探测器测量灵敏度的影响不超过5％。     

Optimization study of some neutron imaging parameters in a reactor based neutron radiography facility

Neutron imaging technique can be used as a means of material Non-Destructive testing. One of common neutron sources for neutron radiography is nuclear research reactor. In this work, several neutron imaging parameters such as aperture distance and the radiography plane location from the neutron source as well as the aperture diameter have been computationally optimized to deliver a proposed neutron beam. According to the results, the aperture diameter of 3.5–4 cm which was located at 55–85 cm from the outer layer of reactor core and the position of image plane at 300–400 cm fulfills delivering of the suitable neutron flux and other required conditions. W, Fe and Pb walls with an identified length formed the convergent-divergent collimator and shielded the neutron and gamma out of beam path. Bi and Fluental filters with an optimal dimension were used to efficiently improve the neutron beam profile at a sample position.     

断层屏蔽SPECT成像法探讨

本文以空间积分原理为基础,以逐渐逼近法为图像重建的核心,探讨了可实现断层屏蔽的SPECT成像的途径.与传统的SPECT成像方法比较,它具有高的灵敏度和数据获取速度快的特点.由于无须用机械装置将γ照相机旋转,消除了运动带来的误差,提高了成像的质量.本文用线性方程组的松弛解法对无散射和无噪音的二维空间的模拟数据进行了验算,证明逐渐逼近法能逐渐逼近到放射性分布区域,并能基本给出分布形式.