'Electron tomography' ('ET') is a tomography technique for obtaining detailed 3D structures of sub-cellular macro-molecular objects. 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. Current resolutions of ET systems 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.
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. 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) 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 material science. 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. Presently, the highest electron tomography resolution is around 2.4 angstrom as demonstrated by UCLA Miao group using a gold nanoparticle. This technique has recently been used to directly visualize the atomic structure of screw dislocations in nanoparticles.
Different tilting methods
The most popular tilting methods are the single-axis and the dual-axis tilting methods. By using dual-axis tilting, the elongation effect is reduced by a factor of however, twice as many images need to be taken. Another solution to obtain tilt-series is offered by the so-called conical tomography, during which the sample is tilted, and then rotated a complete turn.
- "Electron Tomography". 2006. doi:10.1007/978-0-387-69008-7. ISBN 978-0-387-31234-7.
- Mastronarde, D. N. (1997). "Dual-Axis Tomography: An Approach with Alignment Methods That Preserve Resolution". Journal of Structural Biology 120 (3): 343. doi:10.1006/jsbi.1997.3919.
- 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.
- Midgley, P. A.; Weyland, M. (2003). "3D electron microscopy in the physical sciences: The development of Z-contrast and EFTEM tomography". Ultramicroscopy 96 (3–4): 413. doi:10.1016/S0304-3991(03)00105-0.
- Xin, H. L.; Ercius, P.; Hughes, K. J.; Engstrom, J. R.; Muller, D. A. (2010). "Three-dimensional imaging of pore structures inside low-κ dielectrics". Applied Physics Letters 96 (22): 223108. doi:10.1063/1.3442496.
- Scott, M. C.; Chen, C. C.; Mecklenburg, M.; Zhu, C.; Xu, R.; Ercius, P.; Dahmen, U.; Regan, B. C.; Miao, J. (2012). "Electron tomography at 2.4-ångström resolution". Nature 483 (7390): 444. doi:10.1038/nature10934.
- Chen, C. C.; Zhu, C.; White, E. R.; Chiu, C. Y.; Scott, M. C.; Regan, B. C.; Marks, L. D.; Huang, Y.; Miao, J. (2013). "Three-dimensional imaging of dislocations in a nanoparticle at atomic resolution". Nature 496 (7443): 74. doi:10.1038/nature12009.
- Midgley, P. A.; Dunin-Borkowski, R. E. (2009). "Electron tomography and holography in materials science". Nature Materials 8 (4): 271. doi:10.1038/nmat2406.
- Ercius, P.; Weyland, M.; Muller, D. A.; Gignac, L. M. (2006). "Three-dimensional imaging of nanovoids in copper interconnects using incoherent bright field tomography". Applied Physics Letters 88 (24): 243116. doi:10.1063/1.2213185.
- 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. doi:10.1126/science.1178583.
- Zampighi, G. A.; Fain, N; Zampighi, L. M.; Cantele, F; Lanzavecchia, S; Wright, E. M. (2008). "Conical electron tomography of a chemical synapse: Polyhedral cages dock vesicles to the active zone". Journal of Neuroscience 28 (16): 4151–60. doi:10.1523/JNEUROSCI.4639-07.2008. PMC 3844767. PMID 18417694.