TEM-based Protein Tomography
Protein Tomography, which is based on advanced transmission electron microscopy (TEM), has the potential to bridge these gaps in investigative capabilities and support a more rational and, presumably, more productive approach. A TEM illuminates a thin sample with a broad beam of high energy electrons and focuses the transmitted electrons into an image. The optical path is analogous to that of a light microscope using transmitted light. In nonbiological materials, advanced TEMs have resolutions better than 0.1 nm. However, the lower beam exposure required for biological specimens reduces image contrast and limits resolution.
Protein Tomography mathematically combines a sequence of TEM images, each acquired from an incrementally different perspective, into a 3-D reconstruction of the specimen. The computation methods of tomography are well-known and used routinely in other medical imaging techniques.
The low signal-to-noise ratio of the biological images, however, requires an additional step to optimize performance. Sidec´s (www.sidec.com) Protein Tomography applies an algorithm that starts with an initial 3-D reconstruction, then iteratively maximizes the entropy of the model under the constraint of a chi squared fitting parameter, essentially producing the most featureless reconstruction that still fits the observed projection data.