Radiological technique for obtaining clear X-ray images of internal structures by focusing on a specific plane within the body to produce a cross-sectional image. It allows the examination of structures that are obscured by overlying organs and soft tissues and do not show up clearly on conventional X-ray images. Seealso computerized axial tomography.

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Imaging technique used in diagnosis and biomedical research. A chemical compound labeled with a radioactive isotope (see radioactivity) that emits positrons is injected into the body, and detectors measure their activity in the tissues as they combine with electrons and are annihilated. Computers analyze, integrate, and reconstruct the data to produce images of the organs scanned. PET is particularly useful for studying brain and heart functions.

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Tomography is imaging by sections or sectioning. A device used in tomography is called a tomograph, while the image produced is a tomogram. The method is used in medicine, 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 "a section", "a slice" or "a cutting". A tomography of several sections of the body is known as a polytomography.

Description

For example, 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, ideated in the 30's by the radiologist Alessandro Vallebona, proved useful in reducing the problem of superimposition of structures in projectional (shadow) radiography.

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. 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 tomograph
X-rays	CT
gamma rays	SPECT
electron-positron annihilation	PET
nuclear magnetic resonance	MRI
ultrasound	ultrasonography
electrons	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.

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

• Atom probe tomography (APT)
• Computed tomography (CT)
• Confocal laser scanning microscopy (LSCM)
• Cryo-electron tomography (Cryo-ET)
• Electrical capacitance tomography (ECT)
• Electrical resistivity tomography (ERT)
• Electrical impedance tomography (EIT)
• Functional magnetic resonance imaging (fMRI)
• Magnetic induction tomography (MIT)
• Magnetic resonance imaging (MRI), formerly known as magnetic resonance tomography (MRT) or nuclear magnetic resonance tomography
• Neutron tomography
• Optical coherence tomography (OCT)
• Optical projection tomography (OPT)
• Process tomography (PT)
• Positron emission tomography (PET)
• Positron emission tomography - computed tomography (PET-CT)
• Quantum tomography
• Single photon emission computed tomography (SPECT)
• Seismic tomography
• Ultrasound assisted optical tomography (UAOT)
• Ultrasound transmission tomography
• X-ray tomography (CT, CATScan)
• Photoacoustic tomography (PAT), also known as Optoacoustic Tomography (OAT) or Thermoacoustic Tomography (TAT)
• Zeeman-Doppler imaging, used to reconstruct the magnetic geometry of rotating stars.

See also

• Geophysical imaging
• Medical imaging
• Radon transform
• Tomographic reconstruction
• Network tomography
• Nonogram, a type of puzzle based on a discrete model of tomography
• MRI compared with CT

External links

• Neutron Radiography/Tomography
• International Journal of Tomography & Statistics (IJTS)