Electron tomography
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Electron tomography (ET) is a tomography technique for obtaining detailed 3D structures[1] of sub-cellular, macro-molecular, or materials specimens. Electron tomography is an extension of traditional transmission electron microscopy and uses a transmission electron microscope to collect the data. In the process, a beam of electrons is passed through the sample at incremental degrees of rotation around the center of the target sample. This information is collected and used to assemble a three-dimensional image of the target. For biological applications, the typical resolution of ET systems[2] are in the 5–20 nm range, suitable for examining supra-molecular multi-protein structures, although not the secondary and tertiary structure of an individual protein or polypeptide.[3][4] Recently, atomic resolution in 3D electron tomography reconstructions has been demonstrated.[5][6]
BF-TEM and ADF-STEM tomography
[edit]In the field of biology, bright-field transmission electron microscopy (BF-TEM) and high-resolution TEM (HRTEM) are the primary imaging methods for tomography tilt series acquisition. However, there are two issues associated with BF-TEM and HRTEM. First, acquiring an interpretable 3-D tomogram requires that the projected image intensities vary monotonically with material thickness. This condition is difficult to guarantee in BF/HRTEM, where image intensities are dominated by phase-contrast with the potential for multiple contrast reversals with thickness, making it difficult to distinguish voids from high-density inclusions.[7][8] Second, the contrast transfer function of BF-TEM is essentially a high-pass filter – information at low spatial frequencies is significantly suppressed – resulting in an exaggeration of sharp features. However, the technique of annular dark-field scanning transmission electron microscopy (ADF-STEM), which is typically used on material specimens,[9] more effectively suppresses phase and diffraction contrast, providing image intensities that vary with the projected mass-thickness of samples up to micrometres thick for materials with low atomic number. ADF-STEM also acts as a low-pass filter, eliminating the edge-enhancing artifacts common in BF/HRTEM. Thus, provided that the features can be resolved, ADF-STEM tomography can yield a reliable reconstruction of the underlying specimen which is extremely important for its application in materials science.[10] For 3D imaging, the resolution is traditionally described by the Crowther criterion. In 2010, a 3D resolution of 0.5±0.1×0.5±0.1×0.7±0.2 nm was achieved with a single-axis ADF-STEM tomography.[11]
Atomic Electron Tomography (AET)
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Atomic level resolution in 3D electron tomography reconstructions has been demonstrated. Reconstructions of crystal defects such as stacking faults, grain boundaries, dislocations, and twinning in structures have been achieved.[12] This method is relevant to the physical sciences, where cryo-EM techniques cannot always be used to locate the coordinates of individual atoms in disordered materials. AET reconstructions are achieved using the combination of an ADF-STEM tomographic tilt series and iterative algorithms for reconstruction. Currently, algorithms such as the real-space algebraic reconstruction technique (ART) and the fast Fourier transform equal slope tomography (EST) are used to address issues such as image noise, sample drift, and limited data.[13] ADF-STEM tomography has recently been used to directly visualize the atomic structure of screw dislocations in nanoparticles.[14][15][16][17] AET has also been used to find the 3D coordinates of 3,769 atoms in a tungsten needle with 19 pm precision[18] and 20,000 atoms in a multiply twinned palladium nanoparticle.[19] The combination of AET with electron energy loss spectroscopy (EELS) allows for investigation of electronic states in addition to 3D reconstruction.[20][21] Challenges to atomic level resolution from electron tomography include the need for better reconstruction algorithms and increased precision of tilt angle required to image defects in non-crystalline samples.
Different tilting methods
[edit]The most popular tilting methods are the single-axis and the dual-axis tilting methods. The geometry of most specimen holders and electron microscopes normally precludes tilting the specimen through a full 180° range, which can lead to artifacts in the 3D reconstruction of the target.[22][23] Standard single-tilt sample holders have a limited rotation of ±80°, leading to a missing wedge in the reconstruction. A solution is to use needle shaped-samples to allow for full rotation. By using dual-axis tilting, the reconstruction artifacts are reduced by a factor of compared to single-axis tilting. However, twice as many images need to be taken. Another method of obtaining a tilt-series is the so-called conical tomography method, in which the sample is tilted, and then rotated a complete turn.[24]
See also
[edit]- Tomography
- Tomographic reconstruction
- 3D reconstruction
- Cryo-electron tomography
- Positron emission tomography
- Crowther criterion
- X-ray computed tomography
- tomviz tomography software
- imod tomography software
- X-ray diffraction computed tomography
References
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- ^ R. A. Crowther; D. J. DeRosier; A. Klug (1970). "The Reconstruction of a Three-Dimensional Structure from Projections and its Application to Electron Microscopy". Proc. R. Soc. Lond. A. 317 (1530): 319–340. Bibcode:1970RSPSA.317..319C. doi:10.1098/rspa.1970.0119. S2CID 122980366.
- ^ Frank, Joachim (2006). Frank, Joachim (ed.). Electron Tomography. doi:10.1007/978-0-387-69008-7. ISBN 978-0-387-31234-7. S2CID 241282825.
- ^ Mastronarde, D. N. (1997). "Dual-Axis Tomography: An Approach with Alignment Methods That Preserve Resolution". Journal of Structural Biology. 120 (3): 343–352. doi:10.1006/jsbi.1997.3919. PMID 9441937.
- ^ Y. Yang; et al. (2017). "Deciphering chemical order/disorder and material properties at the single-atom level". Nature. 542 (7639): 75–79. arXiv:1607.02051. Bibcode:2017Natur.542...75Y. doi:10.1038/nature21042. PMID 28150758. S2CID 4464276.
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- ^ Bals, S.; Kisielowski, C. F.; Croitoru, M.; Tendeloo, G. V. (2005). "Annular Dark Field Tomography in TEM". Microscopy and Microanalysis. 11. doi:10.1017/S143192760550117X.
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- ^ Li, H.; Xin, H. L.; Muller, D. A.; Estroff, L. A. (2009). "Visualizing the 3D Internal Structure of Calcite Single Crystals Grown in Agarose Hydrogels". Science. 326 (5957): 1244–1247. Bibcode:2009Sci...326.1244L. doi:10.1126/science.1178583. PMID 19965470. S2CID 40526826.
- ^ Xu, Rui; Chen, Chien-Chun; Wu, Li; Scott, M. C.; Theis, W.; Ophus, Colin; Bartels, Matthias; Yang, Yongsoo; Ramezani-Dakhel, Hadi; Sawaya, Michael R.; Heinz, Hendrik; Marks, Laurence D.; Ercius, Peter; Miao, Jianwei (November 2015). "Three-dimensional coordinates of individual atoms in materials revealed by electron tomography". Nature Materials. 14 (11): 1099–1103. arXiv:1505.05938. doi:10.1038/nmat4426. PMID 26390325. S2CID 5455024.
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