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The term tomography refers to the reconstruction of the three-dimensional structure of an object or objects from data collected on slices (sections) of it. (Sometimes the term tomography is used more generally for "acquiring data on sections" and the term tomographic reconstruction is used for the process of determining the three-dimensional structure.) The term is commonly used in medicine and is the "t" of the term CT scan for computerized tomography scan (or CAT scan for computerized axial tomography scan). It uses data amounting to column densities from different directions, i.e., multiple two-dimensional maps, and uses computation to work out the structure in three dimensions.
The computational techniques generally are based upon the Radon transform, a mathematical device that calculates a three dimensional object's column densities in any given direction. They actually use the inverse Radon transform, doing the opposite, which is imperfect because of the potential for ambiguities and uses heuristics regarding the objects' typical structure, some information about likely shapes and scales.
Many astronomical observations, unfortunately, do not give a practical opportunity to collect data from different directions, but a few do. Some of the calculations of interferometry are not quite tomography but use related mathematical methods. Tomography has also been used for adaptive optics, essentially to develop more detail about the atmospheric structures causing seeing issues. Tomography variants have also been used in studying accretion disks associated with contact binaries, in which our viewed radial velocities vary over the course of the binary's orbit (Doppler tomography), and data from different time points within the orbital period have been used to work out structural details of the disk. Variants have been used to work out Faraday rotation details from rotation measures (RMs) (Faraday tomography, rotation measure presents a degeneracy because the EMR may have passed through magnetic fields produce rotation in opposite directions). Tomography also has potential within the solar system, such as planetary science and the study of the Sun.