Tomography is the method of imaging a single plane, or slice, of an object resulting in a
tomogram. There are several forms of tomography:
Linear tomography: This is the most basic form of tomography. The X-ray tube moved from
point "A" to point "B" above the patient, while the cassette holder (or "bucky") moves
simultaneously under the patient from point "B" to point "A." The fulcrum, or pivot point,
is set to the area of interest. In this manner, the points above and below the focal plane
are blurred out, just as the background is blurred when panning a camera during exposure. No
longer carried out and replaced by computed tomography. Poly tomography: This was a complex form of tomography. With this technique, a number of
geometrical movements were programmed, such as hypocycloidic, circular, figure 8, and
elliptical. Philips Medical Systems [1] produced one such device called the 'Polytome.' This
unit was still in use into the 1990's, as it's resulting imgaes for small or difficult
phsiology, such as the inner ear, was still difficult to image with CT's at that time. As
the resolution of CT's got better, this procedure was taken over by the CT. Zonography: This is a variant of linear tomography, where a limited arc of movement is used.
It is still used in some centres for visualising the kidney during an intravenous urogram
(IVU). Orthopantomography (OPT or OPG): The only common tomographic examination in use. This makes
use of a complex movement to allow the radiographic examination of the mandible, as if it
were a flat bone. It is often referred to as a "Panorex", but this is incorrect, as it is a
trademark of a specific company's equipment Computed Tomography (CT), or Computed Axial Tomography (CAT): A CT scan, also known as a CAT
scan, is a helical tomography (latest generation), which traditionally produces a 2D image
of the structures in a thin section of the body. It uses X-rays. It has a greater ionizing
radiation dose burden than projection radiography; repeated scans must be limited to avoid
health effects.