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1.
改进CT技术的最终目标是用较低的辐射剂量重建出更高质量的图像以降低患癌症的风险。近年来受压缩感知理论的启发,减少投影角度重建一直是减少辐射量的一个热门课题。但是,当辐射剂量固定,减少投影角度并不总是意味着更好的图像质量。本文研究固定辐射剂量下图像质量和扫描角度数目的关系。数值实验表明对于固定的辐射剂量,起初图像质量随着扫描角度数目的增加而提高,但是当扫描角度数目足够大之后图像质量反而下降了。在等角全扫描模式下,对于我们测试的辐射剂量和图像,产生最佳图像质量的最佳扫描角度数目是300,这对于实际的应用具有借鉴意义。 相似文献
2.
合成层析成像可由有限几个投影数据重建三维物体,与传统CT相比,该方法所需的射线剂量少得多.由于投影数据不完备,导致的重建图像伪影是合成层析成像的一个主要问题.本文提出一种对感兴趣区域进行合成层析成像方法,和其他方法相比可以得到更好的重建图像质量.该方法首先对目标进行低分辨率全局扫描,然后对感兴趣区域进行高分辨率局部扫描,最后,利用两次扫描的投影数据集合重建感兴趣区域的图像.数值仿真结果表明,用本方法重建感兴趣区域图像,要明显好于没有使用全局扫描信息的方法. 相似文献
3.
目的：研究总结ECG门控自动毫安调制技术在MSCT冠状动脉成像中的应用价值,探讨心脏CT低剂量技术的初步经验和方法。方法：患者60例,其中30例随机使用固定毫安扫描,另30例使用ECG门控自动毫安调制技术扫描。以64排螺旋CT行冠状动脉成像,扫描前进行心率控制措施,使用后门控数据采集、容积再现（VR）、最大密度投影（MIP）、曲面重建（CPR）;以MIP图像评价冠状动脉管腔;以VR图像5级记分法评价图像质量。结果：60例患者均成功获得冠状动脉影像。自动毫安组较固定毫安组平均剂量降低10.46%,而图像质量两组一致,无统计学差异。ECG门控自动毫安技术在保证图像质量的前提下能有效降低患者平均辐射剂量。结论：ECG门控自动毫安技术能有效降低患者射线剂量,只要准备措施有效及方法得当,就能够用低剂量完成64排MSCT冠状动脉检查,并可获得较满意的效果。 相似文献
4.
一种基于数据外插改进的ART迭代算法 总被引：1，自引：0，他引：1
本文针对有限角度的投影数据的CT图像重建问题,提出了一种基于数据外插改进的ART算法.该算法的基本思想足运用已知角度的投影数据来补全未知角度的投影数据,再用ART算法进行图像重建.最后用模拟的投影数据进行了重建图像的数值实验.实验结果表明该算法不但提高了重建图像质量,同时也提高了图像达代的收敛速度. 相似文献
5.
图像重建算法FBP与OSEM在工业CT应用中的对比研究 总被引：1，自引：0，他引：1
滤波反投影法(FBP)与有序子集最大期望值法(OSEM)是图像重建中最为常用的两种方法,本文研究了这两种重建方法在工业CT中的应用,就不同的探测器数目、不同的旋转投影角度取样数、不同的噪声水平,对两种方法的重建效果进行对比与分析,结果表明在某些条件下迭代法OSEM重建图像质量明显好于解析法FBP。 相似文献
6.
针对目前CT的X射线电离辐射危害,提出了一种基于低剂量CT投影数据进行稀疏角度重建的方法来降低辐射剂量。本方法首先对低剂量CT的投影数据采用惩罚加权最小二乘(PWLS)方法进行去噪处理,然后对去噪后的投影数据进行稀疏角度的CT图像重建。对Shepp-Logan模体仿真实验及真实实验数据重建结果表明:本文提出的基于低剂量CT的稀疏重建方法可有效地抑制重建图像噪声和条形伪影,实现稀疏角度的低剂量CT图像重建。 相似文献
7.
在图像重建中,Landweber迭代算法是图像重建算法中的重要方法.本文将针对Landweber分块迭代算法中松弛参数的选取进行研究.在重建过程中采取对投影矩阵按投影角度分块的方法,选取特定的松弛参数.通过数值实验得出结论:对于按角度分块的块迭代算法,松弛参数选取为λ乘以块矩阵与其共轭转置矩阵乘积的最大特征值分之一,当采集完全投影数据,且λ接近(1/6)～(1/7)时效果最好.另外,本文按角度分块的做法和松弛参数的选取方法对于有限角度图像重建问题也是可行的. 相似文献
8.
目的:研究i Dose4迭代重建算法不同迭代水平在人类获得性免疫缺陷综合征(AIDS)合并耶氏肺孢子菌肺炎(PJP)患者胸部低剂量CT扫描中的效能对比。方法:应用飞利浦iCT对16例AIDS合并PJP患者分别行常规剂量和低剂量胸部CT平扫。常规剂量组的DRI指数设置为30;低剂量组的DRI指数设置为10。管电压均采用120 k V。常规剂量组采用FPB重建算法,低剂量组采用iDose4(L2,L4,L6)重建算法。测量不同扫描方案下图像的客观噪声值(SD),记录不同扫描方案下CT容积剂量指数(CTDI vol)、剂量长度乘积(DLP)并计算有效剂量(ED)。比较不同扫描方案下的辐射剂量、图像的客观指标(图像噪声)及主观指标(图像质量主观评分)的差异。结果:低剂量扫描条件下,LD_iDose4_L2组、LD_iDose4_L4组及LD_iDose4_L6图像噪声值分别为:12.41、10.13和8.01,LD_iDose4_L6组的图像噪声值与SD_FPB组(8.51)相比,差异无统计学意义。肺窗视图下,LD-iDose4_L2组、LD-iDose4_L4组CT图像质量主观评分均在3分以上,满足了诊断需求,以LD_iDose4_L2组显示最好。纵隔窗视图下,低剂量组各组主观评分均在3分以上,均满足了诊断需求,其中以LD_iDose4_L6组得分最高。低剂量组与常规组的ED值大小分别为6.18和2.28,差异均具有统计学意义(P<0.05),低剂量组的有效剂量较常规剂量组降低了63.11%。结论:iDose4迭代重建在AIDS合并PJP患者胸部CT病变及解剖结构的显示上具有明显的优势,能够在大幅降低辐射剂量的情况下得到满足诊断需要的CT图像。低剂量扫描条件下,iDose 4迭代重建迭代水平级别越高,降噪的能力越强,图像噪声值越小。但就图像总体质量而言,肺窗视图下,图像质量以LD_iDose4_L2组显示最好;纵隔窗视图下,图像质量以LD_iDose4_L6组显示最好。 相似文献
9.
在传统的CT系统中,系统的硬件成本和计算量都是非常巨大的.本文深入分析了一种利用偏移放置的面阵探测器的锥束CT系统,在这种系统中X射线束仅仅覆盖被扫描物体的一半体积,投影数据在探测器方向上是截断的,探测器尺寸和投影数据量都减少为传统CT系统的一半.在这种扫描方式下,现有的重建方法是首先利用重排算法获得180度范围内的平行束投影数据,然后再利用滤波反投影(FBP)算法重建出物体的三维图像.但是重排算法不可避免地会引入误差,降低重建图像的空间分辨率.本文提出了一种反投影滤波(BPF)形式的直接反投影重建方法,该方法不需要对投影数据重排,直接反投影滤波重建出最终的图像.因此,该算法在数学上更简洁,计算速度更快,能够更多地保留重建图像的高频信息.最后,数值模拟实验结果验证了该系统和重建算法能够获得高质量的CT图像. 相似文献
10.
针对有限角度扫描的CT重建,提出一种基于模型融合的CT迭代重建方法。模型来源于患者的早期Dicom图像。对扫描角度有限的投影数据,用统计迭代算法进行初步重建,得到预重建图像;将预重建图像与模型进行融合,得到融合图像;然后再次投影,补全原始投影缺失的部分,根据补全的投影数据重建出中间结果,之后重复投影、融合、重建过程直到满足终止条件。仿真实验表明,该算法能完整重建整个目标,在有效保留原目标特征的同时提高了小角度投影数据重建的质量。 相似文献
11.
12.
M. N. French R. L. Bras W. F. Krajewski 《Stochastic Environmental Research and Risk Assessment (SERRA)》1992,6(1):27-45
A procedure for short-term rainfall forecasting in real-time is developed and a study of the role of sampling on forecast ability is conducted. Ground level rainfall fields are forecasted using a stochastic space-time rainfall model in state-space form. Updating of the rainfall field in real-time is accomplished using a distributed parameter Kalman filter to optimally combine measurement information and forecast model estimates. The influence of sampling density on forecast accuracy is evaluated using a series of a simulated rainfall events generated with the same stochastic rainfall model. Sampling was conducted at five different network spatial densities. The results quantify the influence of sampling network density on real-time rainfall field forecasting. Statistical analyses of the rainfall field residuals illustrate improvement in one hour lead time forecasts at higher measurement densities. 相似文献
14.
《地震工程与工程振动(英文版)》2014,(4)
正This journal is established by the Institute of Engineering Mechanics(IEM),China Earthquake Administration,to promote scientific exchange between Chinese and foreign scientists and engineers so as to improve the theory and practice of earthquake hazards mitigation,preparedness,and recovery.To accomplish this purpose,the journal aims to attract a balanced number of papers between Chinese and 相似文献
15.
O. Martins 《水文研究》1988,2(1):19-29
Water quality analyses for the Niger River for the 1980/81 hydrological year are presented. The samples were collected from the main river at Lokoja, and from two main tributaries, the Kaduna and the Benue Rivers. Different water types were distinguished by the concentrations of major ions. The type Ca > Na > Mg > K - HCO3 > SO4 > Cl was represented at all stations during at least part of the year. Chloride was found to dominate the sulphate ion in the Kaduna and Niger, while the Benue maintained a higher concentration of sulphate relative to chloride all year round. Distinct patterns of seasonal variation in the ion concentrations were observed, particularly for the samples collected at Lokoja. Low ion concentrations were prominent during periods of high discharge, while low flow periods coincided with high dissolved ion concentrations. The contribution of rainwater to the total dissolved solids in the river waters was assessed indirectly using rainwater chemistry data from the Gulf of Guinea. The estimated rainwater contribution to the Lower Niger amounts to 5.15 mg 1?1. Geochemical weathering calculations involving reactions of the four major minerals of granitic rocks - anorthite, biotite, albite, and K-feldspar - with carbon dioxide and water, can account for the average water composition of the Lower Niger. The proportion of the ionic components was also related to the occurrence of the respective element in the minerals. 相似文献
16.
Historical earthquakes noted in the written records of the South China region, including Hong Kong, are not well delineated along identified prominent fault sources. Despite the lack of any definitive, localised trend in the spatial distribution of seismic activity in the region, there does appear to be some major disparity in the seismic activity rates (especially for large magnitude earthquakes) between the near-field and the far-field regions of Hong Kong. Despite this observation, previous studies of the regional seismic activity and seismic ground motion hazard (the latter using a probabilistic seismic hazard assessment, PSHA) have considered very broad source zone regions, in which uniform levels of seismic activity have been assumed. The present paper further scrutinises this broad source zone (BSZ) approach by adopting a novel expanding circular disc (ECD) method to determine the rates of earthquake recurrence. Such a method is intended to counter-check previously developed models by determining earthquake scenario events in terms of magnitude–distance (M–R) pairs or combinations, having defined values of average return period. Unlike the BSZ approach, the ECD method specifically accounts for the supposed variations in the seismic activity rates between events in the near-field and the far-field of Hong Kong.The form of the developed method is particularly suited to the determination of design-level earthquake ground motions for bedrock sites, since it assumes a directionally-independent attenuation model as described in the companion paper. It is found that, whilst the BSZ approach may indicate the overall average levels of hazard that are representative of the South China region as a whole, it does not capture the large disparity in seismic activity rates between near-field and far-field events. This important feature is expected to have a significant impact upon engineering assessments of the seismic safety of structures in Hong Kong and elsewhere in the South China region. For example, it is found that for events with M≥6, the seismic activity rate (normalised by time and area) in the very far-field is around 3.5 times larger than in the near-field and medium-field of Hong Kong. The resulting design M-R combinations, covering a range of return periods from 70 to 2500 years, are limited, for very long return periods and for distant events, by the maximum credible earthquake (MCE) magnitude. Intensive research to determine this seismic hazard parameter is recommended, in order to refine further the results of the ECD analysis, which presently conservatively assumes the MCE to range between M=6 in the near-field of Hong Kong to M=8 in the very far-field, at distances greater than 280 km from Hong Kong. 相似文献
17.
Depositional ages and characteristics of Middle–Upper Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia 
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下载免费PDF全文 Hitoshi Hasegawa Hisao Ando Noriko Hasebe Niiden Ichinnorov Tohru Ohta Takashi Hasegawa Masanobu Yamamoto Gang Li Bat‐Orshikh Erdenetsogt Ulrich Heimhofer Takayuki Murata Hironori Shinya G. Enerel G. Oyunjargal O. Munkhtsetseg Noriyuki Suzuki Tomohisa Irino Koshi Yamamoto 《Island Arc》2018,27(3)
Lower Cretaceous lacustrine oil shales are widely distributed in southeastern Mongolia. Due to the high organic carbon content of oil shale, many geochemical studies and petroleum exploration have been conducted. Although most of the oil shales are considered to be Early Cretaceous in age, a recent study reveals that some were deposited in the Middle Jurassic. The present study aims at establishing depositional ages and characteristics of the Jurassic and Cretaceous lacustrine deposits in Mongolia. The Lower Cretaceous Shinekhudag Formation is about 250 m thick and composed of alternating beds of shale and dolomite. The Middle Jurassic Eedemt Formation is about 150 m thick and composed of alternating beds of shale, dolomitic marl, and siltstone. The alternations of shale and dolomite in both formations were formed by lake level changes, reflecting precipitation changes. Shales were deposited in the center of a deep lake during highstand, while dolomites were formed by primary precipitation during lowstand. Based on the radiometric age dating, the Shinekhudag Formation was deposited between 123.8 ±2.0 Ma and 118.5 ±0.9 Ma of the early Aptian. The Eedemt Formation was deposited at around 165–158 Ma of Callovian–Oxfordian. The calculated sedimentation rate of the Shinekhudag Formation is between 4.7 ±2.6 cm/ky and 10.0 ±7.6 cm/ky. Shales in the Shinekhudag Formation show micrometer‐scale lamination, consisting of algal organic matter and detrital clay mineral couplets. Given the average thickness of micro‐laminae and calculated sedimentation rate, the micro‐lamination is most likely of varve origin. Both Middle–Upper Jurassic and Lower Cretaceous lacustrine oil shales were deposited in intracontinental basins in the paleo‐Asian continent. Tectonic processes and basin evolution basically controlled the deposition of these oil shales. In addition, enhanced precipitation under humid climate during the early Aptian and the Callovian–Oxfordian was another key factor inducing the widespread oil shale deposition in Mongolia. 相似文献
18.
A. C. LIAKOPOULOS Dr. Eng. 《水文科学杂志》2013,58(1):69-110
ABSTRACT The one-dimensional transient downward entry of water in unsaturated soils is investigated theoretically. The mathematical equation describing the infiltration process is derived by combining Darcy's dynamic equation of motion with the continuity and thermodynamic state equations adjusted for the unsaturated flow conditions. The resulting equation together with the corresponding initial and boundary conditions constitues a mathematical initial boundary value problem requiring the solution of a nonlinear partial differential equation of the parabolic type. The volumetric water content is taken as the dependent variable and the time and the position along the vertical direction are taken as the independent variables. The governing equation is of such nature that a solution exists for t > 0 and is uniquely determined if two relationships are defined, together with the specified state of the system, at the initial time t = 0 and at the two boundaries. The two required relations are those of pressure versus permeability and pressure versus volumetric water content. Since the partial differential equation has strong non-linear terms, a discrete solution is obtained by approximating the derivatives with finite-differences at discrete mesh points in the solution domain and integrated for the corresponding initial and boundary conditions. The use of an implicit difference scheme is employed in order to generate a system of simultaneous non-linear equations that has to be solved for each time increment. For n mesh points the two boundary conditions provide two equations and the repetition of the recurrence formula provides n—2 equations, the total being n equations for each time increment. The solution of the system is obtained by matrix inversion and particularly with a back-substitution technique. The FORTRAN statements used for obtaining the solution with an electronic digital computer (IBM 704) are presented together with the input data. Analysis of the errors involved in the numerical solution is made and the stability and convergence of the solution of the approximate difference equation to that of the differential equation is investigated. The method applied is that of making a Fourier series expansion of a whole line of errors and then following the progress of the general term of the series expansion and also the behavior of each constituent harmonic. The errors (forming a continuous function of points in an abstract Banach space) are represented by vectors with the Fourier coefficients constituting a second Banach space. The amplification factor of the difference equation is shown to be always less than unity which guarantees the stability of the employed implicit recurrence scheme. Experiments conducted on a vertical column packed uniformly with very fine sand, show a satisfactory agreement between the theoretically and experimentally obtained values. Many experimental results are shown in an attempt to explain the infiltration phenomenon with emphasis on the shape and movement of the wet front, and the effects of the degree of compaction, initial water content and deaired water on the infiltration rate. 相似文献
19.
Church repair swarms and earthquakes in Rhodes Island, Greece 总被引：1，自引：0，他引：1
20.
GHODRAT TORABI 《Island Arc》2012,21(3):215-229
Late Permian trondhjemites in the Anarak area occur as stocks and dykes, which cross cut the Anarak ophiolite and its overlying metasedimentary rocks, and are exposed along the northern Anarak east–west main faults. These leucocratic intrusive bodies have enclaves of all ophiolitic units and metamorphic rocks. They are composed of amphibole, plagioclase (oligoclase), quartz, zircon and muscovite. Secondary minerals are chlorite (pycnochlorite), epidote, albite, magnetite and calcite. Whole‐rock major‐ and trace‐element analyses reveal that they are characterized by high SiO 2 (67.8–71.0 wt%), Al 2 O 3 (14.9–17.1 wt%) and Na 2 O (5.3–8.6 wt%), low K 2 O (0.1–1.5 wt%; average: 0.8 wt%), low Rb/Sr ratio (0.01–0.40; average: 0.09), low Y (3–6 ppm), negative Ti, Nb and Ta anomalies, slightly negative or positive Eu anomaly, LREE enrichment and fractionated HREE. These rocks present 2 to 40 times enrichment in inclined chondrite‐normalized REE patterns. Geochemical characteristics of the Anarak trondhjemites all reflect melting of a mafic protolith at more than 10 kbar. The field evidence and whole‐rock chemistry reveal that these rocks have been crystallized from magmas derived from melting of subducted Anarak oceanic crust. This study reveals that melting of garnet amphibolite was an important element of continent formation in the study area. 相似文献