球形石墨空腔电离室壁修正因子的Monte-Carlo模拟和实验测量

采用Monte-Carlo模拟和实验测量两种方法对30cm³球形石墨空腔电离室在⁶⁰Coγ射线下的壁修因子Kwall进行研究。实验和模拟计算结果的比较表明，传统的线性外推法确定的壁修正能基准组的30cm³和50cm³球形石墨空腔电离室在⁶⁰Coγ射线下的相对电离电流和壁修正因子进行了Monte-Carlo模拟计算，计算的相对电离电流值与NIST发布的实验测量值在0.12%内吻合。     

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     

Evaluation of the effect of scatter correction on lesion detectionin hepatic SPECT imaging

The effect of scatter correction on the accuracy of lesion detection in single-photon emission computed tomography (SPECT) imaging requires analysis of observer performance. Experiments were designed to evaluate the class of correction methods that subtract counts. Simulations were used to approximate liver imaging with labeled antibodies. The lesion was a 2.5-cm-diameter, spherical, “cold” tumor. Ramp-filtered backprojection and noniterative Chang attenuation compensation were used to approximate clinical practice. Perfect scatter rejection, defined as images containing only primary (nonscattered) photons, was selected as the ideal case. These images were compared with uncorrected images for conditions of both low- and high-scatter fractions (SF). The dual photopeak window (DPW) method was also tested to evaluate a practical subtraction correction. Receiver operating characteristic (ROC) experiments were conducted under signal-known-exactly (SKE) conditions, using the area under the curve as the index of accuracy. A statistically significant difference in detection was found only in a few experiments when scatter rejection was compared with no correction. The results suggest that scatter correction does not necessarily assure improved detection accuracy at a statistically significant level. However, corrections that produce conditions similar to ideal scatter rejection may offer such improvement in detection, particularly for cases of high SF's     

Reconstruction method for fluorescent X-ray computed tomography byleast-squares method using singular value decomposition

We describe a new attenuation correction method for fluorescent X-ray computed tomography (FXCT) applied to image nonradioactive contrast materials in vivo. The principle of the FXCT imaging is that of computed tomography of the first generation. Using monochromatized synchrotron radiation from the BLNE-5A bending-magnet beam line of Tristan Accumulation Ring in KEK, Japan, we studied phantoms with the FXCT method, and we succeeded in delineating a 4-mm-diameter channel filled with a 500 μg I/ml iodine solution in a 20-mm-diameter acrylic cylindrical phantom. However, to detect smaller iodine concentrations, attenuation correction is needed. We present a correction method based on the equation representing the measurement process. The discretized equation system is solved by the least-squares method using the singular value decomposition. The attenuation correction method is applied to the projections by the Monte Carlo simulation and the experiment to confirm its effectiveness     

Towards Quantification of Functional Breast Images Using Dedicated SPECT With Non-Traditional Acquisition Trajectories

Quantification of radiotracer uptake in breast lesions can provide valuable information to physicians in deciding patient care or determining treatment efficacy. Physical processes (e.g., scatter, attenuation), detector/collimator characteristics, sampling and acquisition trajectories, and reconstruction artifacts contribute to an incorrect measurement of absolute tracer activity and distribution. For these experiments, a cylinder with three syringes of varying radioactivity concentration, and a fillable 800 mL breast with two lesion phantoms containing aqueous (99m)Tc pertechnetate were imaged using the SPECT sub-system of the dual-modality SPECT-CT dedicated breast scanner. SPECT images were collected using a compact CZT camera with various 3D acquisitions including vertical axis of rotation, 30° tilted, and complex sinusoidal trajectories. Different energy windows around the photopeak were quantitatively compared, along with appropriate scatter energy windows, to determine the best quantification accuracy after attenuation and dual-window scatter correction. Measured activity concentrations in the reconstructed images for syringes with greater than 10 μCi /mL corresponded to within 10% of the actual dose calibrator measured activity concentration for ±4% and ±8% photopeak energy windows. The same energy windows yielded lesion quantification results within 10% in the breast phantom as well. Results for the more complete complex sinsusoidal trajectory are similar to the simple vertical axis acquisition, and additionally allows both anterior chest wall sampling, no image distortion, and reasonably accurate quantification.     

Ultrasonic study of UO2: effects of porosity and grain size on ultrasonic attenuation and velocities

Ultrasonic techniques applied to nuclear fuel characterisation are developed in our group since 1996. Before applying our methods to irradiated fuel, we are searching sensitive parameters which could give interesting information. That is the reason why only results concerning non-irradiated UO₂ are presented. This paper mainly deals with the investigation of a relevant acoustic parameter: the attenuation. Indeed, the ultrasonic attenuation in UO₂ as a function of the operating ultrasonic frequency has been measured on samples with various microstructures: variable fraction volume porosity (1–6%) and grain size (10–90 μm). Using a 15 MHz operating frequency, no attenuation has been observed. With frequencies around 40 MHz, we show that the measured ultrasonic attenuation is only sensitive to grain size (no effect of porosity has been observed). On the contrary, the ultrasonic velocities (which are very sensitive to porosity) are not affected by the sizes of the grains. These reversed and non-correlated effects constitute an interesting tool for UO₂ study because two aspects of the microstructure can be studied separately with ultrasonic waves.     

土壤湿度对现场γ能谱测量的影响修正

现场γ能谱测量在地质勘查和电离辐射环境测量中是一种高效、准确、经济的方法。针对γ能谱在现场测量过程中的主要影响因素--土壤湿度,根据γ射线在土壤中的衰减规律,建立数值模型,得到不同物质的质量衰减系数和线衰减系数,计算不同土壤湿度的修正系数。同时利用四道γ能谱仪对同一地点不同土壤湿度情况下土壤中铀、钍、钾含量进行了实际测量,根据计算的修正系数得到的修正数值与HPGe谱仪测量的结果进行了比较。结果表明,现场γ能谱测量结果通过数值模拟计算修正能有效降低土壤湿度的影响。     

计数法测定脏器容积

利用三组模型研究ＳＰＥＣＴ脏器功能容积测定中衰减校正系数、放射性活度、容积大小、放射性缺损区和阀值对容积测定的影响。结果表明,在ＳＰＥＣＴ图像重建过程中,不进行衰减校正测得的容积明显高于模型实际容积,测得的容积随校正系数的增大而减小。当校正系数为０．１时,测得的容积与模型实际容积最接近。放射性活度在３７－１８５ｋＢｑ范围内,测得的容积没有明显差别（Ｐ〈０．０５）。阀值增加,测得容积减小;当阀值为４     

Compton Scattering in Clinical PET/CT With High Resolution Half Ring PET Insert Device

The integration of a high resolution PET insert into a conventional PET system can significantly improve the resolution and the contrast of its images within a reduced imaging field of view. For the rest of the scanner imaging field of view, the insert is a highly attenuating and scattering media. In order to use all available coincidence events (including coincidences between 2 detectors in the original scanner, namely the scanner-scanner coincidences), appropriate scatter and attenuation corrections have to be implemented. In this work, we conducted a series of Monte Carlo simulations to estimate the composition of the scattering background and the importance of the scatter correction. We implemented and tested the Single Scatter Simulation (SSS) algorithm for a hypothetical system and show good agreement between the estimated scatter using SSS and Monte Carlo simulated scatter contribution. We further applied the SSS to estimate scatter contribution from an existing prototype PET insert for a clinical PET/CT scanner. The results demonstrated the applicability of SSS to estimate the scatter contribution within a clinical PET/CT system even when there is a high resolution half ring PET insert device in its imaging field of view.     

Evaluation of low energy threshold settings for PVI PET systems

The authors studied the effects of adjusting the low level energy discriminator (LLD) for positron volume imaging (PVI) mode acquisitions using a GE Advance PET system. NEMA scatter fraction and count loss measurements were performed for a 20 cm right circular cylinder at LLD between 300 keV and 450 keV. From these data, noise equivalent count rate (NECR) curves were calculated to estimate effects of LLD on scans of head sized objects. To evaluate the effects of LLD on whole-body image quality, contrast-to-noise ratio (CNR) values were obtained from images processed without attenuation or scatter correction. CNR data were acquired for 1.6 cm and 1.0 cm spherical lesions within the liver region of a Data Spectrum torso phantom scanned with arms, and activity outside the field of view. The results indicate that the NECR for a head-sized object is fairly insensitive to LLD differences between 300 keV and 425 keV for activity concentrations up to 0.4 μCi/cc. The NECR improved slightly for higher concentrations with 375 keV相似文献   

Evaluation of simulation-based scatter correction for 3-D PETcardiac imaging

Quantitative imaging of the human thorax poses one of the most difficult challenges for three-dimensional (3-D) (septaless) positron emission tomography (PET), due to the strong attenuation of the annihilation radiation and the large contribution of scattered photons to the data. In [¹⁸F] fluorodeoxyglucose (FDG) studies of the heart with the patient's arms in the field of view, the contribution of scattered events can exceed 50% of the total detected coincidences. Accurate correction for this scatter component is necessary for meaningful quantitative image analysis and tracer kinetic modeling. For this reason, the authors have implemented a single-scatter simulation technique for scatter correction in positron volume imaging. Here, they describe this algorithm and present scatter correction results from human and chest phantom studies     

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.     

一种工业CT散射校正的新模型

建立了第三代CT扫描过程中理想的散射模型；针对^60Co集装箱（大型客体）CT检测系统的特点，得到了简化模型；通过实验测得模型参数，并验证了基于这种简化模型的散射校正对匀质圆盘客体的图像重建效果。     

X射线凸度仪中散射信号的物理模型与解析计算研究

在各种凸度检测技术中,射线法能够适应恶劣的生产环境、测量精度高等一些优点使它成为研究的主流方向[1]。散射是影响射线法凸度检测精度的一个重要因素,关于散射的研究是该技术研发的重要环节,也是研究的难点[1]。通过建立物理模型,使用解析法进行散射校正,与Monte Carlo模拟结果对比,得到了理想的结果。     

Incoherent-scatter computed tomography with monochromaticsynchrotron x ray: feasibility of multi-CT imaging system forsimultaneous measurement-of fluorescent and incoherent scatter x rays

We describe a new system of incoherent scatter computed tomography (ISCT) using monochromatic synchrotron X rays, and we discuss its potential to be used in in vivo imaging for medical use. The system operates on the basis of computed tomography (CT) of the first generation. The reconstruction method for ISCT uses the least squares method with singular value decomposition. The research was carried out at the BLNE-5A bending magnet beam line of the Tristan Accumulation Ring in KEK, Japan. An acrylic cylindrical phantom of 20-mm diameter containing a cross-shaped channel was imaged. The channel was filled with a diluted iodine solution with a concentration of 200 μgI/ml. Spectra obtained with the system's high purity germanium (HPGe) detector separated the incoherent X-ray line from the other notable peaks, i.e., the iK_α and K_β1 X-ray fluorescent lines and the coherent scattering peak. CT images were reconstructed from projections generated by integrating the counts In the energy window centering around the incoherent scattering peak and whose width was approximately 2 keV. The reconstruction routine employed an X-ray attenuation correction algorithm. The resulting image showed more homogeneity than one without the attenuation correction     

分段γ扫描自吸收校正法分析残渣和废物中的铀,钚含量

叙述了用于非破坏分析残渣和废物中的铀、钚含量的分段γ扫描自吸收校正法（简称分段γ扫描法）。研究了分段γ扫描测量核工厂实物盘点中含铀、钚各类非均匀样品的刻度方法，分析了各种测量条件对γ自吸收校正及测量结果的影响。本方法与化学破坏分析法进行了比对分析，结果表明分段γ扫描自吸收校正法与化学破坏分析法的偏差为－３．６％，在分析误差范围内符合得很好。本分析方法成功地应用于核工厂实物盘点现场分析，测量了４类中低密度含铀、钚的残渣和废物样品，测量结果的不确定度为５％。     

Attenuation correction of polychromatic L-shell X-ray fluorescence computed tomography imaging

Polychromatic L-shell X-ray fluorescence computed tomography is a promising imaging technique to explore the element distribution of the sample. However, the image quality of large samples would be decreased dramatically due to the present of the self-absorption. In this investigation, an attenuation correction algorithm based on theories related to X-ray fluorescence was proposed. With this method, the attenuation coefficients at the incident energies were expressed as a function of known X-ray energies and unknown platinum concentrations. Then the attenuation coefficients were calculated based on the theories and added in the contribution value of the pixel in Maximum Likelihood Expectation Maximization reconstruction method. The element distributions can be got through continuous iterations. Finally, the feasibility of this method was tested by Monte Carlo simulation. The results indicated that, for a 4-mm diameter phantom containing an object of 2 mm in diameter with 0.10 wt% platinum solutions, the CNR of reconstructed images increased from 53.5 to 203.8 after attenuation correction while the relative error decreases from 97% to 2%.     

A beam stop based correction procedure for high spatial frequency scatter in industrial cone-beam X-ray CT

In the energy range of industrial cone-beam CT (a 450 kV X-ray tube is used), the detector scatter (veiling glare) turned out to be the most important source of secondary radiation for a digital imaging system working in the indirect conversion mode. It has been shown that the detector scatter drives the shape of the total image scatter. As a consequence, the latter may be characterized by an important high spatial frequency content questioning the hypothesis, frequently invoked in the scatter correction techniques, of a slowly varying scatter. We propose a beam stop array (BSA) based method for the evaluation/subtraction of the image scatter. The technique has the potential of simultaneously manage all sources of scattering radiation. It can be used in the high spatial frequency scatter case at the condition of opportunely selecting the BSA effective sampling step. It has been shown that the BSA approach can be also successfully combined with a beam hardening correction scheme based on the linearisation method. The work we present is supported by experimental measurements performed with two test objects at two different beam qualities: (i) 200 kV + 2.5 mm of Cu and (ii) 400 kV + 4 mm of Pb and 1 mm of Cd. The detector scatter represents about 36% and 65% of the total white field image for the two investigated beam qualities, respectively.     

基于实验与模拟相结合的气体样品(n,α)反应截面测量方法研究

建立了一套利用屏栅电离室测量快中子诱发气体样品(n,α)反应截面的方法。针对气体样品测量中存在的样品核数确定、中子注量测量、待测事件挑选等问题,本方法首先利用模拟计算结果选择合适的气体样品有效区域,使得事件区尽量无本底覆盖且易于统计;再利用中子准直器和阴极、阳极信号时间信息,分离并统计样品有效区域产生的事件数;最后利用实验测量的238U(n,f)裂变计数和SuperMC模拟结果,得到有效区域内的中子注量。利用该方法在中子能量为471、487、500、512、529和545 MeV的能点系统地测量了14N(n,α0)反应的相对截面,结果与ENDF/B Ⅷ0符合良好。     

A New Method to Make Gamma-ray Self-absorption Correction

This paper is devoted to discuss a new method to directly extract the information of the geometric self-absorption correction through the measurement of characteristic ? radiation emitted spontaneously from nuclear fissile material. The numerical simulation tests show that this method can extract the purely original information needed for nondestructive assay method by the ?-ray spectra to be measured, even though the geometric shape of the sample and materials between sample and detector are not known in advance.