阿德莱德Adelaide代写Essay:核磁共振成像

2020-01-10 13:18

核磁共振成像(MRI)在评估大脑结构中的作用在研究大脑复杂本质的神经科学家中越来越受欢迎。这种非侵入性工具能够在体内研究随着时间的推移所发生的空间变化模式，这些模式是正常发育和衰老的函数，以及在疾病存在时对这一轨迹的扰动。核磁共振的多模态特性使其能够评估大脑形态、连接和功能，而且这项技术的精确度只随着时间的推移而提高。此外，这些测量数据与遗传和认知指数之间的联系推动了我们对大脑结构和功能之间协同作用的理解。在这些实现中，有一种结构MRI，它允许量化大脑中灰质和白质的密度、体积和厚度。这些宏观的解剖学变化被认为是发生在神经元和突触水平的微观变化的结果，最终揭示了驱动疾病或功能的生物学过程(Tardif等，2016)。基于体素的形态测量是一种利用结构MRI的技术，在比较不同兴趣组之间的局部灰质浓度方面发挥了重要作用。下面描述的方法步骤已经在Ashburner等人2000年的文章中详细描述过了。总而言之，管道的第一步涉及到对图像进行预处理，以便将它们重新排列并规范化到一个标准模板空间，其中所有生成的图像将占用相同的3D坐标。这是通过估计最佳的12参数仿射变换来实现的，其中图像会相应地缩放、旋转、平移或剪切。大脑大小变化的先验知识被用来限制最大估计值。第二步考虑全局非线性形状差异，实现平滑空间基函数的线性组合，然后计算函数的系数，使模板与目标图像之间的差异最小，同时使变形的平稳性最大。
阿德莱德Adelaide代写Essay:核磁共振成像
 The role of magnetic resonance imaging (MRI) in assessing brain architecture has become increasingly popular among neuroscientists studying the complex nature of the brain. This non-invasive tool has enabled the in-vivo investigation of spatially varying patterns that occur over time as a function of normal development and aging as well as in the perturbation of this trajectory in the presence of disease. The multi-modal nature of MRI has allowed for the assessment of brain morphology, connectivity, and function and the precision of this technology has only increased with time. Moreover, the association of these measurements with genetic and cognitive indices has propelled our understanding of the synergistic interplay between brain structure and function. Among these implementations lies structural MRI, a modality that allows for the quantification of density, volume, and thickness of grey and white matter within the brain. These macroscopic anatomical changes are thought to be an outcome of the microscopic changes occurring at the neuronal and synaptic level ultimately shedding light on the biological processes driving a disease or function (Tardif et al., 2016). Voxel based morphometry, a technique that utilizes structural MRI, has been instrumental in comparing local gray matter concentrations between different groups of interest .Voxel-based morphometry(VBM) involves a number of steps in order to ensure that regional differences between groups are initially comparable in their spatial location. The methodological steps described below have been described at length in Ashburner et al., 2000. To summarize, the first step of the pipeline involves pre-processing the images so that they are realigned and normalized to a standard template space where all resulting images will occupy the same 3D coordinates. This is performed by estimating the optimum 12-parameter affine transformations where images are scaled, rotated, translated, or sheared accordingly. Prior knowledge of variability in brain size is used to constrain maximum estimates. A second step is taken to account for global nonlinear shape differences by implementing a linear combination of smooth spatial basis functions and then by calculating the coefficients of the functions that minimize the difference between the template and the subject image while maximizing the smoothness of the deformations. 

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