Tomography


Tomography
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. The method is used in radiology, archaeology, biology, geophysics, oceanography, materials science, astrophysics and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction. The word was derived from the Greek word tomos which means "part" or "section", representing the idea of "a section", "a slice" or "a cutting". A tomography of several sections of the body is known as a polytomography.

Contents

Etymology

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

Description

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

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. With current (2005) technology, tomograms are derived using several different physical phenomena listed in the following table.

Physical phenomenon Type of tomogram
X-rays CT
gamma rays SPECT
radio-frequency waves MRI
electron-positron annihilation PET
electrons Electron tomography or 3D TEM
ions atom probe

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 subsume 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 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

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

Types of tomography

Name Source of data Abbreviation Year of introduction
Atom probe tomography Atom probe APT
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
Functional magnetic resonance imaging Magnetic resonance fMRI 1992
Magnetic induction tomography Magnetic induction MIT
Magnetic resonance imaging or nuclear magnetic resonance tomography Nuclear magnetic moment MRI or MRT
Neutron tomography Neutron
Ocean acoustic tomography Sonar
Optical coherence tomography Interferometry OCT
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.

See also

References

  1. ^ MeSH Tomography
  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. http://chestjournal.chestpubs.org/content/24/6/663.abstract. Retrieved July 10, 2011. 
  3. ^ Herman, G. T., Fundamentals of computerized tomography: Image reconstruction from projection, 2nd edition, Springer, 2009

External links


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