Quantitative Imaging Group, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands.
Ultramicroscopy. 2011 Jul;111(8):1029-36. doi: 10.1016/j.ultramic.2011.03.001. Epub 2011 Mar 15.
Today, the resolution in phase-contrast cryo-electron tomography is for a significant part limited by the contrast transfer function (CTF) of the microscope. The CTF is a function of defocus and thus varies spatially as a result of the tilting of the specimen and the finite specimen thickness. Models that include spatial dependencies have not been adopted in daily practice because of their high computational complexity. Here we present an algorithm which reduces the processing time for computing the 'tilted' CTF by more than a factor 100. Our implementation of the full 3D CTF has a processing time on the order of a Radon transform of a full tilt-series. We derive and validate an expression for the damping envelope function describing the loss of resolution due to specimen thickness. Using simulations we quantify the effects of specimen thickness on the accuracy of various forward models. We study the influence of spatially varying CTF correction and subsequent tomographic reconstruction by simulation and present a new approach for space-variant phase-flipping. We show that our CTF correction strategies are successful in increasing the resolution after tomographic reconstruction.
今天，相位对比低温电子断层扫描的分辨率在很大程度上受到显微镜对比度传递函数（CTF）的限制。CTF 是离焦的函数，因此由于样品的倾斜和有限的样品厚度，会在空间上发生变化。由于其计算复杂性高，包含空间相关性的模型尚未在日常实践中采用。在这里，我们提出了一种算法，将计算“倾斜”CTF 的处理时间减少了 100 多倍。我们对完整的 3D CTF 的实现，其处理时间与全倾斜系列的 Radon 变换相当。我们推导出并验证了一个描述由于样品厚度导致分辨率损失的阻尼包络函数的表达式。通过模拟，我们量化了样品厚度对各种正向模型准确性的影响。我们通过模拟研究了空间变化的 CTF 校正和随后的断层重建的影响，并提出了一种新的用于空间变化相位翻转的方法。我们表明，我们的 CTF 校正策略在断层重建后提高分辨率方面是成功的。
