蒙特卡罗剂量分布去噪中三维高斯和Savitzky-Golay滤波器的改进与混合

对模拟粒子轨迹数较少模拟时间较短的蒙特卡罗粗糙剂量分布进行三维滤波,可以加速其收敛速度.结合蒙特卡罗剂量分布特征,改进三维高斯和Savitzky-Golay滤波器,建立三维混合滤波方法,并比较并联和级联两种基本混合方式.根据卷积性质,提出用等效卷积核简化混合滤波器结构的方法.结果表明,改进后的高斯和Savitzky-Golay滤波器的整体去噪效果得以增强,混合滤波器进一步降低滤波结果的局部误差,两种混合滤波器都能够大幅度抑制MC粗糙剂量分布中的噪声,级联混合滤波器降噪效果略优于并联混合滤波器.     

基于切伦科夫辐射的核素治疗剂量测量可行性分析

针对现有核素治疗中内照射剂量测量缺乏简单、高效方法的问题,基于内照射剂量与切伦科夫辐射之间的关系,提出一种基于切伦科夫辐射的核素治疗内照射剂量测量的新方法。利用蒙特卡罗计算程序Geant4,模拟放射性核素131I在水体模型和甲状腺模型中产生切伦科夫辐射与剂量沉积的分布情况,并定量分析切伦科夫光子数与剂量之间的关系。计算结果表明:在水体模的半径方向上切伦科夫光子数与剂量之间有着相同的变化趋势,且两者有着相同的二维分布规律;核素131I在介质中产生的切伦科夫光子数与剂量两者之间存在一定的线性关系,且这种线性关系与核素的分布情况无关。研究结果证实,将这种放射性核素在介质中产生的切伦科夫辐射应用于内照射剂量学具有非常大的研究潜力和价值。     

血液中Na活化放射性测量的蒙特卡罗模拟

针对超临界事故中人体受到中子辐照后感生的24Na活度测量,采用MCNP软件建立蒙特卡罗模拟模型,分别模拟不同类型NaI探测器对24Na衰变的两条射线全能峰的探测效率和塑料闪烁体探测器对24Na衰变的射线总计数的探测器效率。模拟结果表明:井型NaI探测器与圆柱型相比,24Na衰变的1.38 MeV和2.76 MeV 射线全能峰探测效率分别提高了4.30倍和4.11倍;塑料闪烁体探测器对24Na衰变的射线的探测效率是NaI探测器对24Na 射线的探测效率的1.72倍;同时粗略计算了探测器计数与人体所受中子辐照剂量之间的关系。     

快速高能质子照相程序QMCPrad的研制

为了快速模拟高能质子照相过程,利用蒙特卡罗（MC）技术和技巧编写了程序QMCPrad,并开展了QMCPrad与MCNPX的对比计算以及实验E955的模拟验证工作。在模型选取中,修改核反应过程权重的隐式俘获模式和一次抽样模拟穿透客体的多次库仑散射的浓缩历史方式都节约了大量的计算时间。与MCNPX相比,QMCPrad具备模拟质子在磁透镜中输运过程的能力。QMCPrad与MCNPX、实验E955的对比及验证结果表明,QMCPrad能够准确定量地模拟高能质子照相,为质子照相实验设计提供工具。     

基于TPS和蒙特卡罗计算的12C肿瘤放射治疗In-beam PET剂量监测研究

In-beam PET成像是碳离子放射治疗剂量监测的有效手段,可以对碳离子放疗过程的物理剂量分布和生物剂量分布进行实时监测。结合放射治疗计划系统(TPS)和蒙特卡罗(MC)模拟分别对静态均匀水模体和腹部肿瘤CT图像进行治疗计划设计、MC计算和PET成像,比较TPS肿瘤靶区剂量分布、MC模拟剂量分布和PET成像三者之间的一致性。TPS和MC模拟中相对生物学效应(RBE)的计算均采用线性二次模型(LQ)。研究结果显示,TPS和MC计算的静态均匀水模体、单野治疗腹部肿瘤的物理剂量、RBE加权剂量在SOBP区域的平均误差均在0.5%和2%以内。碳离子束流能量为120~400 MeV/u时,束流方向剂量深度分布与PET成像在SOBP区域的位置差异均在8 mm以内。In-beam PET可作为碳离子放射治疗中位置验证和剂量验证的有效手段。     

基于MCPT解法器库的专用辐射探测模拟软件研发与应用北大核心CSCD

甲状腺内^(131)I放射性活度与辐射探测结果的比例关系与甲状腺几何尺寸、探测距离等因素相关,是估算甲状腺内^(131)I含量与其可能造成的辐照损伤的关键参数。基于MCPT辐射输运数值模拟算法器库开发了用于开展NaI探测器伽马辐射测量模拟的应用程序,进而建立了多组具有不同容积的甲状腺型容器和不同探测距离的物理模型,最终通过蒙特卡罗数值计算得到了不同测量状态下探测器的探测效率。在甲状腺型容器与探测器距离较远时,数值模拟给出的结果与理论计算结果一致,证明此应用程序可用于定量分析NaI的探测效率。数值模拟结果表明,小距离模型的结果受甲状腺样容器的大小和距离的显著影响,模拟给出的探测效率表为开展深入细致的实验研究奠定了基础。     

特征γ射线谱分析的蒙特卡罗模拟技术

蒙特卡罗方法(MC)是模拟核探测问题的理想方法,用中子照射客体,中子诱发产生非弹γ和俘获γ,通过特征γ射线能谱和时间谱分析,确定客体核素组成和重量百分比.本文基于非弹γ和俘获γ时间门测量技术,给出了脉冲源发射下探测器响应计数公式.在中子与核作用产生次级光子方面,采用期望值估计(expect value estimator,EVE)产光.为了避免大量小权光子模拟带来的计算存储量增加,设计了EVE产光与直接估计(direct estimator,DE)产光耦合.仅增加少量计算时间,便实现了特征γ射线解谱.数值模拟在自主MC软件JMCT上开展,计算结果初步验证了方法的正确有效性.     

推广的液滴模型对超重核288115及其α衰变链性质的研究

运用推广的液滴模型来确定新核素²⁸⁸115及其α衰变链上核的衰变位垒, 采用量子力学中处理α衰变的WKB方法, 对该链上各原子核的α衰变半寿命进行了研究．计算结果表明推广的液滴模型结合WKB方法可以很好地在超重区符合α衰变半寿命的实验值. 同时把推广的液滴模型的计算结果和采用密度相关的M3Y微观核力的结果做了详细的比较, 宏观模型(推广的液滴模型)和微观模型(密度相关的M3Y微观核力)计算的α衰变半寿命以及实验值三者之间的符合是对新核素²⁸⁸115及其α衰变链上核半寿命很好的检验.     

方阱链流体在固液界面分布的密度泛函理论研究

应用Yethiraj的加权密度近似泛函理论研究平板狭缝中方阱链流体的密度分布，系统的Helm holtz自由能泛函分为理想气体的贡献利剩余贡献两部分，其中剩余贡献部分分别采用刘洪 来等人建立的基于空穴相关函数的方阱链流体状态方程和Gil-Villegas等人提出的统计缔合 流体理论状态方程(SAFT-VR)结合简单加权密度近似计算.考察了不同链长、温度、系统密度 和壁面吸引强度下平板狭缝中方阱链流体的密度分布，并与Monte Carlo(MC)模拟结果进行 了比较.结果表明采用不同的状态方程对密度分布的计算有明显的影响，对于受限于硬壁狭 缝中的方阱链流体，温度和密度比较高时，两种状态方程计算的结果均与MC模拟符合得比较 好，在低温和低密度下效果变差，SAFT-VR方程的计算结果更接近于MC模拟结果.对于受限于 方阱壁狭缝中的方阱链流体，由于系统密度分布的非均匀性加强，采用两种状态方程计算的 结果均与MC模拟结果有一定偏差，寻找更合适的权重函数是进一步改进的关键. 关键词： 密度泛函理论 非均匀流体 密度分布 固液界面 方阱链     

在复合核30P中与同位旋有关的能级衰变宽度的计算

在高激发复合核中同位旋自由度是否守恒给出复合核中库仑相互作用信息及其对复合核衰变的影响.用H.A.Weidenmuller的统计理论可以算出复合核³⁰P中与同位旋有关的能级衰变宽度.它与Hauser-Feshbach公式的主要不同之处在于对穿透因子和总衰变道数的处理中加进了同位旋耦合,并考虑了与同位旋有关的能级密度.     

Influence of thyroid volume reduction on absorbed dose in 131I therapy studied by using Geant4 Monte Carlo simulation

A simulation study has been performed to quantify the effect of volume reduction on the thyroid absorbed dose per decay and to investigate the variation of energy deposition per decay due to β- and γ-activity of 131I with volume/mass of thyroid, for water, ICRP- and ICRU-soft tissue taken as thyroid material. A Monte Carlo model of the thyroid, in the Geant4 radiation transport simulation toolkit was constructed to compute the β- and γ-absorbed dose in the simulated thyroid phantom for various values of its volume. The effect of the size and shape of the thyroid on energy deposition per decay has also been studied by using spherical, ellipsoidal and cylindrical models for the thyroid and varying its volume in 1-25 cm3 range. The relative differences of Geant4 results for different models with each other and MCNP results lie well below 1.870%. The maximum relative difference among the Geant4 estimated results for water with ICRP and ICRU soft tissues is not more than 0.225%. S-values for ellipsoidal, spherical and cylindrical thyroid models were estimated and the relative difference with published results lies within 3.095%. The absorbed fraction values for beta particles show a good agreement with published values within 2.105% deviation. The Geant4 based simulation results of absorbed fractions for gammas again show a good agreement with the corresponding MCNP and EGS4 results （±6.667%） but have 29.032% higher values than that of MIRD calculated values. Consistent with previous studies, the reduction of the thyroid volume is found to have a substantial effect on the absorbed dose. Geant4 simulations confirm dose dependence on the volume/mass of thyroid in agreement with MCNP and EGS4 computed values but are substantially different from MIRD8 data. Therefore, inclusion of size/mass dependence is indicated for 131I radiotherapy of the thyroid.     

X射线在介质中的剂量分布计算

研究了放射治疗中X射线在介质中的输运过程, 编程实现了基于蒙特卡罗方法的剂量计算. 并在便于图形处理的软件Matlab中对光子输运结果进行了可视化处理. 对X射线在均匀介质和非均匀介质中的蒙特卡罗模拟结果与实测结果、其他蒙特卡罗软件模拟结果进行了比较, 结果符合较好. 实验结果表明该方法既可以获得很快的仿真速度, 又能得到精确直观的剂量计算结果, 为提高放射治疗水平具有重要的指导意义和应用价值.     

TLD occupational dose distribution study in nuclear medicine

The large and growing number of patients undergoing nuclear medicine procedures and workers involved in this practice requires continued efforts to improve the quality of diagnosis and to reduce the radiological risk associated. In Brazil, external individual monitoring with a dosemeter located on the thorax is compulsory for all workers in controlled areas. Extremity dosemeters are recommended in activities where hand dose can be much higher than on the thorax. This is typically the case for nuclear medicine procedures, but extremity dosemeters are not regularly used in Brazil. With the aim to study the occupational dose distribution in the nuclear medicine staff during their tasks of preparation and injection of radionuclides, Harshaw TLD chips of LiF:Mg,Cu,P (TLD-100H) were used for individual monitoring on different parts of the worker body. As expected, all doses measured on the thorax were much lower than on the hands. For both Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), the dose to the fingers during preparation is about two orders of magnitude higher than in the thorax, and in the injection procedure, one order higher. The doses received by the workers' fingers varied widely depending on how they hold the syringe, but fingertips always received higher doses. The dose values measured in the eye lens were 200% higher than the one measured on the thorax.     

Absorbed dose estimations of 131I for critical organs using the GEANT4 Monte Carlo simulation code

The aim of this study is to compare the absorbed doses of critical organs of ¹³¹I using the MIRD (Medical Internal Radiation Dose) with the corresponding predictions made by GEANT4 simulations. S-values (mean absorbed dose rate per unit activity) and energy deposition per decay for critical organs of ¹³¹I for various ages, using standard cylindrical phantom comprising water and ICRP soft-tissue material, have also been estimated. In this study the effect of volume reduction of thyroid, during radiation therapy, on the calculation of absorbed dose is also being estimated using GEANT4. Photon specific energy deposition in the other organs of the neck, due to ¹³¹I decay in the thyroid organ, has also been estimated. The maximum relative difference of MIRD with the GEANT4 simulated results is 5.64% for an adult's critical organs of ¹³¹I. Excellent agreement was found between the results of water and ICRP soft tissue using the cylindrical model. S-values are tabulated for critical organs of ¹³¹I, using 1, 5, 10, 15 and 18 years (adults) individuals. S-values for a cylindrical thyroid of different sizes, having 3.07% relative differences of GEANT4 with Siegel & Stabin results. Comparison of the experimentally measured values at 0.5 and 1 m away from neck of the ionization chamber with GEANT4 based Monte Carlo simulations results show good agreement. This study shows that GEANT4 code is an important tool for the internal dosimetry calculations.     

Measurement of doses to the extremities of nuclear medicine staff

Medical uses of ionizing radiation now represent>95% of all man-made radiation exposure, and is the largest single radiation source after natural background radiation. Therefore, it is important to quantify the amount of radiation received by occupational individuals to optimize the working conditions for staff, and further, to compare doses in different departments to ensure compatibility with the recommended standards. For some groups working with unsealed sources in nuclear medicine units, the hands are more heavily exposed to ionizing radiation than the rest of the body. A personal dosimetry service runs extensively in Egypt. But doses to extremities have not been measured to a wide extent. The purpose of this study was to investigate the equivalent radiation doses to the fingers for five different nuclear medicine staff occupational groups for which heavy irradiation of the hands was suspected. Finger doses were measured for (1) nuclear medicine physicians, (2) technologists, (3) nurses and (4) physicists. The fifth group contains three technicians handling ¹³¹I, while the others handled ^99mTc. Each staff member working with the radioactive material wore two thermoluminescent dosimeters (TLDs) during the whole testing period, which lasted from 1 to 4 weeks. Staff performed their work on a regular basis throughout the month, and mean annual doses were calculated for these groups. Results showed that the mean equivalent doses to the fingers of technologist, nurse and physicist groups were 30.24±14.5, 30.37±17.5 and 16.3±7.7 μSv/GBq, respectively. Equivalent doses for the physicians could not be calculated per unit of activity because they did not handle the radiopharmaceuticals directly. Their doses were reported in millisieverts (mSv) that accumulated in one week. Similarly, the dose to the fingers of individuals in Group 5 was estimated to be 126.13±38.2 μSv/GBq. The maximum average finger dose, in this study, was noted in the technologists who handled therapeutic ¹³¹I (2.5 mSv). In conclusion, the maximum expected annual dose to extremities is less than the annual limit (500 mSv/y).     

放射源辐射场Monte Carlo模拟计算

选取Pu-238为放射源，采用简化的级联衰变链对其进行源项分析，得到放射源各衰变子体原子数及放射性活度的变化曲线，在此基础上，采用MCNP程序，对放射源进行了精确建模，对放射源辐射场及其有效屏蔽问题进行了模拟计算，得到放射源周围中子、能谱及辐射场分布，辐射场计算值与参考实验剂量吻合较好，屏蔽体的采用可有效降低放射源的辐射剂量，其中，剂量降低为裸源的0.1%左右，中子剂量降低为裸源的13%～17%。     

Self‐Assemble Polymeric Nanoparticle with GSH Exhaustion for SPECT Imaging–Guided Enhanced Radioisotope Therapy

Exploiting biocompatible nanomaterials for cancer theranostics has attracted great attention in recent years. Herein, a multifunctional self‐assembled nanoparticle based on a biocompatible polymer that contains 3‐(4‐hydroxyphenyl) propionic acid N‐hydroxysuccinimide ester (HOPA) for radiolabeling and piperlongumine (PL) for exhausting endogenous glutathione (GSH) (HOPA‐C18PMH‐PEG/PL) is successfully synthesized. With radionuclide ¹²⁵I labeling, SPECT imaging shows high tumor uptake of HOPA‐C18PMH‐PEG/PL after intravenous injection. The in vitro and in vivo combined radioisotope therapy (RIT) and chemotherapy using ¹³¹I‐labeled HOPA‐C18PMH‐PEG/PL is then carried out, achieving synergistic antitumor effect. This is because the reactive oxygen species (ROS) level in the tumor sites of mice treated with ¹³¹I‐labeled HOPA‐C18PMH‐PEG/PL is increased after the exhaustion of GSH by PL. Additionally, such a strategy (exhausting GSH and increasing ROS) induces no obvious toxicity to normal tissue. Therefore, as‐made polymeric nanoparticles exhibit multifunctional properties for SPECT imaging–guided combined RIT and chemotherapy in one system. This finding will further promote polymeric nanoparticle–based RIT of cancer and is expected to be used for future clinical transformation.