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Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. A device used in tomography is called a tomograph, while the image produced is a tomogram. Tomography as the computed tomographic (CT) scanner was invented by Sir Godfrey Hounsfield, and thereby made an exceptional contribution to medicine. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantum information, and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction.
The word tomography is derived from Ancient Greek τόμος tomos, "slice, section" and γράφω graphō, "to write".
In conventional medical X-ray tomography, clinical staff make a sectional image through a body by moving an X-ray source and the film in opposite directions during the exposure. Consequently, structures in the focal plane appear sharper, while structures in other planes appear blurred. By modifying the direction and extent of the movement, operators can select different focal planes which contain the structures of interest. Before the advent of more modern computer-assisted techniques, this technique, developed in the 1930s by the radiologist Alessandro Vallebona, proved useful in reducing the problem of superimposition of structures in projectional (shadow) radiography.
In a 1953 article in the medical journal Chest, B. Pollak of the Fort William Sanatorium described the use of planography, another term for tomography. A chapter in the American Roentgen Ray Society's 1996 book A History of the Radiological Sciences also provides a detailed history of the development of conventional tomography from its inception until being supplanted by computer assisted tomographic techniques starting in the mid to late-1970s.
More modern variations of tomography involve gathering projection data from multiple directions and feeding the data into a tomographic reconstruction software algorithm processed by a computer. Different types of signal acquisition can be used in similar calculation algorithms in order to create a tomographic image. Tomograms are derived using several different physical phenomena listed in the following table:
|Physical phenomenon||Type of tomogram|
|electrons||Electron tomography or 3D TEM|
|magnetic particles||magnetic particle imaging|
The term volume imaging might describe these technologies more accurately than the term tomography. However, in the majority of cases in clinical routine, staff request output from these procedures as 2-D slice images. As more and more clinical decisions come to depend on more advanced volume visualization techniques, the terms tomography/tomogram may go out of fashion.
Many different reconstruction algorithms exist. Most algorithms fall into one of two categories: filtered back projection (FBP) and iterative reconstruction (IR). These procedures give inexact results: they represent a compromise between accuracy and computation time required. FBP demands fewer computational resources, while IR generally produces fewer artifacts (errors in the reconstruction) at a higher computing cost.
Although MRI and ultrasound are transmission methods, they typically do not require movement of the transmitter to acquire data from different directions. In MRI, both projections and higher spatial harmonics are sampled by applying spatially-varying magnetic fields; no moving parts are necessary to generate an image. On the other hand, since ultrasound uses time-of-flight to spatially encode the received signal, it is not strictly a tomographic method and does not require multiple acquisitions at all.
|Name||Source of data||Abbreviation||Year of introduction|
|Atom probe tomography||Atom probe||APT|
|Computed Tomography Imaging Spectrometer||Visible light spectral imaging||CTIS|
|Confocal microscopy (Laser scanning confocal microscopy)||Laser scanning confocal microscopy||LSCM|
|Cryo-electron tomography||Cryo-electron microscopy||Cryo-ET|
|Electrical capacitance tomography||Electrical capacitance||ECT|
|Electrical resistivity tomography||Electrical resistivity||ERT|
|Electrical impedance tomography||Electrical impedance||EIT||1984|
|Electron tomography||Electron attenuation/scatter||ET|
|Functional magnetic resonance imaging||Magnetic resonance||fMRI||1992|
|Laser Ablation Tomography||Laser Ablation & Fluorescent Microscopy||LAT||2013|
|Magnetic induction tomography||Magnetic induction||MIT|
|Magnetic resonance imaging or nuclear magnetic resonance tomography||Nuclear magnetic moment||MRI or MRT|
|Ocean acoustic tomography||Sonar|
|Optical coherence tomography||Interferometry||OCT|
|Optical diffusion tomography||Absorption of light||ODT|
|Optical projection tomography||Optical microscope||OPT|
|Photoacoustic imaging in biomedicine||Photoacoustic spectroscopy||PAT|
|Positron emission tomography||Positron emission||PET|
|Positron emission tomography - computed tomography||Positron emission & X-ray||PET-CT|
|Quantum tomography||Quantum state|
|Single photon emission computed tomography||Gamma ray||SPECT|
|Seismic tomography||Seismic waves|
|Thermoacoustic imaging||Photoacoustic spectroscopy||TAT|
|Ultrasound-modulated optical tomography||Ultrasound||UOT|
|Ultrasound transmission tomography||Ultrasound|
|X-ray tomography||X-ray||CT, CATScan||1971|
|Zeeman-Doppler imaging||Zeeman effect|
Discrete tomography and Geometric tomography, on the other hand, are research areas that deal with the reconstruction of objects that are discrete (such as crystals) or homogeneous. They are concerned with reconstruction methods, and as such they are not restricted to any of the particular (experimental) tomography methods listed above.
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