Tomography

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Basic principle of tomography: superposition free tomographic cross sections S1 and S2 compared with the projected image P

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, geophysics, oceanography, materials science, astrophysics, quantum Information, and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction.

Etymology[edit]

The word tomography is derived from the Greek tomē ("cut") or tomos ("part" or "section") and graphein ("to write").

Description[edit]

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.[1] 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.[2]

Modern tomography[edit]

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.[3] 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:[citation needed]

Physical phenomenonType of tomogram
X-raysCT
gamma raysSPECT
radio-frequency wavesMRI
electron-positron annihilationPET
electronsElectron tomography or 3D TEM
muonsMuon tomography
ionsatom probe
magnetic particlesmagnetic particle imaging

Some recent advances rely on using simultaneously integrated physical phenomena, e.g. X-rays for both CT and angiography, combined CT/MRI and combined CT/PET.

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.[citation needed]

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.[3]

Although MRI and ultrasound make cross sectional images they don't acquire data from different directions. In MRI spatial information is obtained by using magnetic fields. In ultrasound, spatial information is obtained simply by focusing and aiming a pulsed ultrasound beam.

Synchrotron X-ray tomographic microscopy[edit]

Recently a new technique called synchrotron X-ray tomographic microscopy (SRXTM) allows for detailed three dimensional scanning of fossils.[citation needed]

Types of tomography[edit]

NameSource of dataAbbreviationYear of introduction
Atom probe tomographyAtom probeAPT
Computed Tomography Imaging Spectrometer[4]Visible light spectral imagingCTIS
Confocal microscopy (Laser scanning confocal microscopy)Laser scanning confocal microscopyLSCM
Cryo-electron tomographyCryo-electron microscopyCryo-ET
Electrical capacitance tomographyElectrical capacitanceECT
Electrical resistivity tomographyElectrical resistivityERT
Electrical impedance tomographyElectrical impedanceEIT1984
Electron tomographyElectron attenuation/scatterET
Functional magnetic resonance imagingMagnetic resonancefMRI1992
Magnetic induction tomographyMagnetic inductionMIT
Magnetic resonance imaging or nuclear magnetic resonance tomographyNuclear magnetic momentMRI or MRT
Muon tomographymuons
Neutron tomographyNeutron
Ocean acoustic tomographySonar
Optical coherence tomographyInterferometryOCT
Optical diffusion tomographyAbsorption of lightODT
Optical projection tomographyOptical microscopeOPT
Photoacoustic imaging in biomedicinePhotoacoustic spectroscopyPAT
Positron emission tomographyPositron emissionPET
Positron emission tomography - computed tomographyPositron emission & X-rayPET-CT
Quantum tomographyQuantum state
Single photon emission computed tomographyGamma raySPECT
Seismic tomographySeismic waves
Thermoacoustic imagingPhotoacoustic spectroscopyTAT
Ultrasound-modulated optical tomographyUltrasoundUOT
Ultrasound transmission tomographyUltrasound
X-ray tomographyX-rayCT, CATScan1971
Zeeman-Doppler imagingZeeman effect

Discrete tomography and Geometric tomography, on the other hand, are research areas[citation needed] 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.

See also[edit]

Media related to Tomography at Wikimedia Commons

References[edit]

  1. ^ Tomography at the US National Library of Medicine Medical Subject Headings (MeSH)
  2. ^ Pollak, B. (December 1953). "Experiences with Planography". Chest (American College of Chest Physicians) 24 (6): 663–669. doi:10.1378/chest.24.6.663. ISSN 0012-3692. Retrieved July 10, 2011. 
  3. ^ a b Herman, G. T., Fundamentals of computerized tomography: Image reconstruction from projection, 2nd edition, Springer, 2009
  4. ^ Ralf Habel, Michael Kudenov, Michael Wimmer: Practical Spectral Photography

External links[edit]