Tomography involves the generation of a two-dimensional image representing a slice or section through a three-dimensional object. The result is a tomogram. (Note that the Greek word tomos conveys the meaning of "a section" or "a cutting".)

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 proved useful in reducing the problem of superimposition of structures in projectional (shadow) radiography.

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 2004 technology, tomograms are derived using several different physical phenomena including X-rays, gamma rays, positron electron annihilation reaction, nuclear magnetic resonance, and ultrasound. These yield CT, SPECT, PET, MRI, and ultrasonography images, respectively.

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.

See also:

• Radon transform
• tomographic reconstruction
• Medical imaging