### tomography

(observation of sections of a 3D object)

The term **tomography** refers to the reconstruction of
the three-dimensional structure of an object or objects from
data collected on slices (sections) of it.
(Sometimes the term *tomography* is used more generally
for "acquiring data on sections" and the term **tomographic reconstruction**
is used for the process of determining the three-dimensional
structure.)
The term is commonly used in medicine and is the "t"
of the term **CT scan** for **computerized tomography scan**
(or *CAT scan* for *computerized axial tomography scan*).
It uses data amounting to column densities from
different directions, i.e., multiple two-dimensional
maps, and uses computation to work out the structure
in three dimensions.

The computational techniques generally are based upon the
**Radon transform**, a mathematical device that calculates
a three dimensional object's column densities in any
given direction. They actually use the **inverse Radon transform**,
doing the opposite, which is imperfect because of the
potential for ambiguities and
uses heuristics regarding the objects' typical structure,
some information about likely shapes and scales.

Many astronomical observations, unfortunately, do not
give a practical opportunity to collect data from
different directions, but a few do. Some of the
calculations of interferometry are not quite
tomography but use related mathematical methods.
Tomography has also been used for adaptive optics, essentially
to develop more detail about the atmospheric
structures causing seeing issues.
Tomography variants
have also been used in studying accretion disks
associated with contact binaries, in which our viewed
radial velocities vary over the course of the
binary's orbit (**Doppler tomography**), and
data from different time points within the orbital period
have been used to work out structural details of the disk.
Variants have been used to work out Faraday rotation
details from **rotation measures** (**RMs**)
(**Faraday tomography**, rotation measure presents
a degeneracy because the EMR may have
passed through magnetic fields produce
rotation in opposite directions).
Tomography also has potential within the solar system,
such as planetary science and the study of the Sun.

(*science,physics*)
**Further reading:**

http://en.wikipedia.org/wiki/Tomography

http://en.wikipedia.org/wiki/Tomographic_reconstruction

http://en.wikipedia.org/wiki/Radon_transform

**Referenced by pages:**

ARGOS

Zeeman-Doppler imaging (ZDI)

Index