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Accurate marker-free alignment with simultaneous geometry determination and reconstruction of tilt series in electron tomography

机译:精确的无标记对准,同时进行几何确定和电子断层扫描中的倾斜序列重建

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摘要

An image alignment method for electron tomography is presented which is based on cross-correlation techniques and which includes a simultaneous refinement of the tilt geometry. A coarsely aligned tilt series is iteratively refined with a procedure consisting of two steps for each cycle: area matching and subsequent geometry correction, The first step, area matching, brings into register equivalent specimen regions in all images of the tilt series. It determines four parameters of a linear two-dimensional transformation, not just translation and rotation as is done during the preceding coarse alignment with conventional methods. The refinement procedure also differs from earlier methods in that the alignment references are now computed from already aligned images by reprojection of a backprojected volume. The second step, geometry correction, refines the initially inaccurate estimates of the geometrical parameters, including the direction of the tilt axis, a tilt angle offset, and the inclination of the specimen with respect to the support film or specimen holder. The correction values serve as an indicator for the progress of the refinement. For each new iteration, the correction values are used to compute an updated set of geometry parameters by a least squares fit. Model calculations show that it is essential to refine the geometrical parameters as well as the accurate alignment of the images to obtain a faithful map of the original structure.
机译:提出了一种用于电子断层摄影的图像对准方法,该方法基于互相关技术,并且包括倾斜几何形状的同时细化。粗略对齐的倾斜序列通过每个循环包含两个步骤的过程进行迭代完善:面积匹配和后续的几何校正。第一步,面积匹配,将倾斜序列的所有图像中的等效试样区域对齐。它确定了线性二维变换的四个参数,而不仅仅是平移和旋转,这与之前使用常规方法进行的粗对准时一样。细化过程与早期方法的不同之处还在于,现在可以通过反投影体积的重新投影从已经对准的图像中计算对准参考。第二步,几何校正,改进了几何参数的最初不准确的估计值,包括倾斜轴的方向,倾斜角偏移以及样品相对于支撑膜或样品支架的倾斜度。校正值用作细化进度的指标。对于每个新的迭代,校正值用于通过最小二乘拟合来计算一组更新的几何参数。模型计算表明,优化几何参数以及图像的精确对齐以获得原始结构的真实映射至关重要。

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