Introduction: Principles of Electron Tomography

Tomography is a method for reconstructing the interior of an object from its projections. The word tomography literally means the visualization of slices, and is applicable, in the strict sense of the word, only in the narrow context of the single-axis tilt geometry: for instance, in medical computerized axial tomography (CAT-scan imaging), the detector-source arrangement is tilted relative to the patient around a single axis (Fig. 1a). In electron microscopy, where the beam direction is fixed, the specimen holder is tilted around a single axis (Fig. 1b). However, the usage of this term has recently become more liberal, encompassing arbitrary geometries, provided that the specimen is actively tilted into multiple angles. In line with this relaxed convention, we will use the term electron tomography for any technique that employs the transmission electron microscope to collect projections of an object that is tilted in multiple directions and uses these projections to reconstruct the object in its entirety. Excluded from this definition are ‘single-particle’ techniques that make use of multiple occurrences of the object in different orientations, with or without the additional aid of symmetry (Fig. 1c). These techniques are covered elsewhere (non-symmetric: Frank, 1996, 2006; symmetric: Glaeser et al., 2007).