共查询到18条相似文献,搜索用时 156 毫秒
1.
通过限制球形卷积(constrained spherical deconvolution,CSD)和神经纤维追踪技术(tractography),以了解大脑左右半球的弓状束(arcuate fasciculus,AF)神经结构完整性与语言理解能力的相关性.该文使用磁共振成像(MRI)仪对14例大脑左半球罹患肿瘤的右利手患者进行了术前术后扫描(每次扫描均伴随一次波士顿失语症测试),并用分析软件DSI Studio计算术前术后左右半球弓状束的四项扩散指标,并进行了比较.结果发现左脑弓状束有两项指标在术前术后有非常显著的差异(p0.01),而右脑四项指标均无显著改变(p0.05).另外,该文还将扩散指标与患者相对应的语言测试所反映的语言理解能力进行了相关性分析,发现无论术前术后,左半球弓状束与语言理解能力相关系数r介于0.6~0.8之间,而右半球则仅介于0.3~0.4之间.上述结果表明,语言理解能力与左侧弓状束密切相关(p0.01),而与右侧弓状束相关性不大,此结果与以往研究者对大脑侧化的认知相符. 相似文献
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磁振灌流造影:对“流动敏感交互反转恢复”的评论(英文) 总被引:1,自引:1,他引:0
磁振造影在过去的数十年內取得了长足的进步,除了可提供生物解剖构造的资讯外,如今更可以进行组织灌流造影.磁振灌流造影主要可分成2种:动态磁感对比(Dynamic susceptibility Contrast)和动脉标记(Arterial Spin Labeling) .相较于动态磁感对比,动脉标记能非侵入性地观测灌流.动脉标记包括了数种技术:如CASL (continu-ous arterial spinlabeling) ,EPISTAR(echo planar imaging and signal targeting with al-ternating radiofrequency) ,PICORE(proxi mal inversion with a control for off-resonance effects)和FAIR(flow-sensitive alternating inversion recovery) .该文主要提供流动磁感交互反转恢复(FAIR)技术的综合介绍,包括其理论基础和实践,特别针对T1法在FAIR定量上的使用.定量上的困难亦将会在文章中被讨论.文章的最后总结FAIR之实际应用情形. 相似文献
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粒子模拟和实验研究发现磁绝缘线振荡器(MILO)的起振电压会随注入工作电压的增大而增大, 通过进一步的分析与研究, 找到了引起该现象的根本原因, 即随着注入工作电压的增大, 工作电压上升前沿的斜率必然增大, 上升前沿斜率的增大必然引起起振电压的增大, 当斜率无限大时, MILO的起振电压等于工作电压. 因此, 工作电压上升前沿斜率的变化是引起MILO器件起振点变化的根本原因; 当注入电压上升沿的斜率固定不变时, 同一MILO器件的起振点不变, 即对应的起振电压值不变; 当注入电压上升沿的斜率增大时, 同一MILO器件的起振点对应的起振电压值随之增大. 对MILO器件的自磁绝缘临界电流公式进行了部分修正.
关键词:
磁绝缘线振荡器
工作电压
起振电压
工作曲线 相似文献
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在二维正方绝缘铁磁系统基础上建立了一个磁振子-声子相互作用模型. 利用格林函数方法研究了磁振子-声子相互作用下的二维绝缘铁磁体的磁振子衰减(即-ImΣ*(1)(k)), 计算了布里渊区的主要对称点线上的-ImΣ*(1)(k).发现在布里渊区边界区域磁振子衰减很明显, 但小波矢区(kxa/π<0.22附近)磁振子衰减非常弱, 而且温度很低时磁振子衰减有极大值. 比较了纵向声子与横向声子对磁振子衰减的影响, 也讨论了各项参数的变化对磁振子衰减的影响. 根据关系式-Im*(1)(k)=/(2τ)可以对磁振子寿命进行判断.
关键词:
磁振子-声子相互作用
磁振子衰减
铁磁体
磁振子寿命 相似文献
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文章中,通过磁控溅射制备了界面处插入4d,5d元素薄层(包括Ru,Pd,Ag和Au)的Ta/NiFe/Ta多层膜,并对它们的磁输运和磁性以及微结构进行了测试和表征.结果显示,Pd和Pt一样界面效应显著,能有效地提高NiFe薄膜退火前后的AMR比值,并抑制磁性死层.表面能比较小、熔点相对低的插层材料Ag,Au在退火过程中容易通过晶界扩散,强烈破坏其AMR性能.对于熔点高、表面能比较大的插层材料如Ru,磁性死层同样得到了抑制,NiFe薄膜的温度稳定性也可以得到提高.结果表明界面插层从界面电子自旋-轨道散射、界面死层和界面原子扩散等方面深刻影响NiFe薄膜的AMR.
关键词:
各向异性磁电阻
界面效应
原子扩散 相似文献
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在真空介质中,由两种磁绝缘离子二极管产生的中性化离子束在横向磁场中B(?)400Gs传输了2m以上的距离 反磁信号以及极化电场测量证实:不论在低β0.01<β<1或在高β情况(1<β<300)下。束传输的机制都是由于的漂移结果。其中β=4πnm_1r_n~2·B~2。是束能量密度与横向磁场能量密度之比。在高β下,我们观察到磁场B向束中的快速扩散。本文对此进行了理论探讨 相似文献
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利用脉冲激光溅射(PLD)和分子束外延(MBE)方法制备了超薄膜系统 Co/Pd/Cu(100).脉冲激 光溅射生长的单原子Pd层呈现了很好的二维生长模式.在这个Pd表面上,分子束外延生长的C o层直至12个原子层都表现了层-层生长模式.利用俄歇电子谱(AES)和低能电子衍射(LEED)研 究了该系统的表面结构.利用低温磁光克效应(MOKE)研究了系统的磁学性质.结构研究表明, Co层由于面内晶格失配应力而具有一个四方正交结构;与对比样品Co/Cu(100)的比较研究说 明Pd层的存在强烈地改善了Co膜的起始生长模式和结构.磁光克效应测量表明,Pd层的存在 改变了Co层的磁学性质.
关键词:
薄膜的磁性质
组织与形貌
界面磁性 相似文献
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通过核磁共振扩散张量成像(DTI)得到的特定值域的扩散各向异性指数(DAI) 可用于揭示水分子扩散椭球的形态学特征, 定量反映被成像物体内部水分子扩散的优势方向和强度, 间接得到被成像物体内部的组织结构信息. DAI的可靠性直接影响对DTI数据的分析和理解. 本文基于扩散张量椭球的几何学信息, 提出利用扩散椭球几何比(EGR)定量描述水分子扩散的各向异性程度. 通过蒙特卡罗模拟实验和对人脑DTI数据进行分析, 并与当前广泛应用的水分子扩散各向异性分数(FA)和近期文献提出的扩散椭球面积比(EAR)进行对比. 实验发现EGR在不同级别噪声影响下的对比度效果和抗噪性都优于FA及EAR. 而且EGR 加入了体积修正, 增强了盘形扩散张量情况下的敏感性, 能够更好地鉴别神经纤维束交叉情况, 对于各向异性扩散程度较高的白质深层和相对均质的表层都有较好的量化区分结果.
关键词:
扩散系数
各向异性扩散
扩散张量成像
扩散椭球几何比 相似文献
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Jiaolong Qin Maobin Wei Haiyan Liu Jianhuai Chen Rui Yan Lingling Hua Ke Zhao Zhijian Yao Qing Lu 《Magnetic resonance imaging》2014
Previous studies had explored the diagnostic and prognostic value of the structural neuroimaging data of MDD and treated the whole brain voxels, the fractional anisotropy and the structural connectivity as classification features. To our best knowledge, no study examined the potential diagnostic value of the hubs of anatomical brain networks in MDD. The purpose of the current study was to provide an exploratory examination of the potential diagnostic and prognostic values of hubs of white matter brain networks in MDD discrimination and the corresponding impaired hub pattern via a multi-pattern analysis. We constructed white matter brain networks from 29 depressions and 30 healthy controls based on diffusion tensor imaging data, calculated nodal measures and identified hubs. Using these measures as features, two types of feature architectures were established, one only included hubs (HUB) and the other contained both hubs and non hubs. The support vector machine classifiers with Gaussian radial basis kernel were used after the feature selection. Moreover, the relative contribution of the features was estimated by means of the consensus features. Our results presented that the hubs (including the bilateral dorsolateral part of superior frontal gyrus, the left middle frontal gyrus, the bilateral middle temporal gyrus, and the bilateral inferior temporal gyrus) played an important role in distinguishing the depressions from healthy controls with the best accuracy of 83.05%. Moreover, most of the HUB consensus features located in the frontal-parieto circuit. These findings provided evidence that the hubs could be served as valuable potential diagnostic measure for MDD, and the hub-concentrated lesion distribution of MDD was primarily anchored within the frontal-parieto circuit. 相似文献
13.
扩散张量脑模板包含丰富的大脑白质组织信息,在空间标准化或者脑图谱创建中具有重要价值,然而基于扩散张量模型构建的脑模板精度不高,特别是在脑部复杂的神经元微观结构区域中应用受到限制.针对这一问题,研究者们提出了基于高分辨率扩散成像构建大脑模板的方法.本文对使用扩散张量成像方法进行脑模板构建的研究进展进行了综述,首先介绍了扩散张量脑模板构建的发展进程,阐述了脑模板构建中解决的技术问题及同时存在的局限性;接着详细论述了基于扩散频谱成像及高角度分辨率扩散成像构建脑模板的不同方法间的差异,并总结了这些研究方法取得的重要进展;最后通过分析目前研究进展提出该研究问题中存在的不足以及未来的发展趋势. 相似文献
14.
Cross-terms between imaging and diffusion gradients, unaccounted for during tensor calculations, can lead to erroneous estimation of diffusivity and fractional anisotropy (FA) in regions of isotropic and anisotropic diffusion. Cross-term of magnitude 136.8±1.6 s/mm(2), artificially introduced in the slice-encode direction, caused an increase in FA in isotropic phantom from 0.0546±0.0001 to 0.0996±0.0001, while the change in chimpanzee brain depended on the orientation of the white matter (WM). Mean diffusivity (MD) remained unchanged in isotropic phantom, but increased by ~20% in the WM due to cross-terms. A bias was observed in the principal eigenvectors in both phantom and chimpanzee brain, resulting in significant increase in midline crossing fibers along the bias than perpendicular to it in tractography in chimpanzee brain. Post hoc correction of these artifacts was achieved by estimating the cross-term factors using calibration scans on an isotropic phantom and modifying the b-matrix before tensor calculation. Upon correction, the FA and MD values closely resembled the values obtained from sequence without cross-terms, and the bias in principal eigenvectors was eliminated. Customized sequences involving large b-values, high-resolution imaging, or long diffusion or echo times should therefore be evaluated and any residual cross-terms corrected before implementation. 相似文献
15.
Shiou-Ping Lee Chien-Sheng Wu Li-Chun Hsieh Wing-Keung Cheung Ming-Chung Chou 《Magnetic resonance imaging》2014
Systemic lupus erythematosus (SLE) is an autoimmune disease frequently associated with neuropsychiatric manifestations. No follow-up case report has characterized white matter alterations in patients with neuropsychiatric lupus erythematosus (NPSLE) before and after treatment. In this study, a 16-year-old NPSLE patient with severe neuropsychological symptoms was treated with steroid pulse therapy, and was scanned with conventional magnetic resonance (MR) and diffusion tensor imaging (DTI) at onset and 17 months after treatment. Conventional MR images showed diffuse brain atrophy and focal vasogenic edema in the putamen, but they did not reveal abnormalities in the corpus callosum. Region-of-interest analysis of DTI images showed that fractional anisotropy and fiber tracts increased significantly, while axial diffusivity, radial, and mean diffusivity decreased significantly in the corpus callosum after treatment. The results indicated that the vasogenic edema was present in the corpus callosum at onset and was significantly reduced after treatment. These changes were generally compatible with the patient’s clinical manifestations. Hence, we concluded that MR-DTI and fiber tractography are helpful to reveal the relationship between white matter alterations and neurological dysfunctions in NPSLE patients. 相似文献
16.
PurposeIncreased Body-Mass-Index (BMI) has been associated with brain atrophy in both gray and white matter structures. However, little is known concerning the integrity of white matter tracts in obesity. The purpose of the study was to evaluate the pattern of changes in white matter microstructure in human adiposity.Material and methodsThe study included 268 participants (52 obese, 96 overweight and 120 normal-weight) that were retrospectively evaluated by Diffusion Tensor Imaging. The fractional anisotropy, axial, radial and mean diffusivity values were compared between the above groups using Tract Based Spatial Statistics.ResultsThe analysis revealed that the increased BMI was related with decreased fractional anisotropy in several white matter regions including the anterior and posterior thalamic radiation, the inferior fronto-occipital fasciculus, the inferior and superior longitudinal fasciculus, the corpus callosum (callosal body and forceps minor), the uncinate fasciculus, the internal capsule, the corticospinal tract and the cingulum (cingulate gyrus and hippocampus).ConclusionsAnisotropic diffusion of anatomic regions governing important brain circuits such as reward seeking inhibition, motivation/drive and learning/conditioning decreases with increasing BMI. 相似文献
17.
The nonhuman primate brain study provides important supplemental means for human brain exploration since the two species share close anatomical and functional similarities. MR diffusion tensor imaging (DTI) in human brain has revealed exquisite details of brain structures especially in the brain white matter. However, most previous monkey brain DTI results lack the spatial resolution in comparison to the conventional tracing and postmortem imaging methods, especially when it is acquired in commonly available human MRI scanners of field strength of 3 T or lower. To meet the increasing demands for nonhuman primate DTI studies, we proposed an in vivo high-resolution monkey DTI acquisition protocol that is practically feasible and combined it with an improved postprocessing procedure for a 3-T human scanner. The acquisition protocol, susceptibility distortion correction method with phase reversal acquisition, and postprocessing steps were proved to be effective in our study of rhesus monkeys. Results from diffusion tensor estimations and fiber tractography at 1 x 1 x 1 mm(3) resolution were found to be comparable to previous ex vivo DTI studies with much longer acquisition times. Effects of image resolution were evaluated and it was confirmed that the partial volume effect due to the larger voxel size in low-resolution data biased the diffusion tensor estimation and produced erroneous fiber tractography. Our results suggest that in vivo high-resolution monkey brain DTI can be achieved within practical time, which allows accurate diffusion tensor estimation and fiber tractography in monkey brains, so that the complex anatomical structures within many small but important anatomic structures can be delineated. 相似文献
18.
Martina Campanella Elisa Molinari Patrizia Baraldi Luca Nocetti Carlo A. Porro Daniel C. Alexander 《Magnetic resonance imaging》2013
The study of anatomical connectivity is essential for interpreting functional MRI data and for establishing how brain areas are linked together into networks to support higher-order functions. Diffusion-weighted MR images (DWI) and tractography provide a unique noninvasive tool to explore the connectional architecture of the brain. The identification of anatomical circuits associated with a specific function can be better accomplished by the joint application of diffusion and functional MRI. In this article, we propose a simple algorithm to identify the set of pathways between two regions of interest. The method is based upon running deterministic tractography from all possible starting positions in the brain and selecting trajectories that intersect both regions. We compare results from single-fiber tractography using diffusion tensor imaging and from multi-fiber tractography using reduced-encoding persistent angular structure (PAS) MRI on standard DWI datasets from healthy human volunteers. Our results show that, in comparison with single-fiber tractography, the multi-fiber technique reveals additional putative routes of connection. We demonstrate highly consistent results of the proposed technique over a cohort of 16 healthy subjects. 相似文献