Zhang, Peijun Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania.
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A powerful 3D imaging method using an electron microscope. Imaging biological objects with an electron microscope, in principle, is very similar to imaging with a light microscope. In electron tomography imaging, a high-energy (typically 100–400 kV) coherent electron beam is used as the illumination source, and the focusing lenses are magnetic lenses. The incident electron beam interacts with the specimen and carries information about the inner structure of the specimen as the images are formed through the objective lenses. These images are further magnified and recorded on photographic films or by charge-coupled-device (CCD) cameras as 2D projection images. The 2D projection images contain information through the thickness of the specimen collapsed onto a plane. Traditionally, this methodology has been used successfully to study cellular ultrastructure and analyze immunolabeling of ultrathin sections of biological specimens at resolutions that are 100 times better than that achieved with light microscopy. However, when thicker specimens are analyzed, due to the large depth of focus with the electron beam, 2D projection images contain superposition of features in the direction of the electron beam that obscures much of the information. Nevertheless, because these 2D projection images contain information from all heights of the specimen, it is therefore possible by combining different views of the projection images to obtain the 3D internal structure of the object from a set of 2D projection images using an imaging method such as electron tomography (ET). See also: Charge-coupled devices; Electron microscope; Optical microscope; Polarized light microscope
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