otherwise known as body section radiography, planigraphy,
laminography or stratigraphy, is the process of using motion of the
X-ray focal spot and image receptor (e.g. film) in generating
radiographic images where object detail from only one plane or
region remains in sharp focus
Fig. 1. a and b
tomograms of the temporal bone 6 mm apart, acquired with
hypocycloidal tomography. The more external tomogram (a) shows the
midpart of the temporomandibular joint, and the more medial
tomogram (b) shows an exostosis in the anterior part of the
external auditory canal (arrow).
The motion of
X-ray tube and film in linear tomography.
Details from other
planes in the object which would otherwise contribute confounding
detail to the image, are blurred and effectively removed from visual
consideration in the image. A variety of tomography techniques have
been developed, which differ primarily in the manner in which the
X-ray source and film move.
Linear tomography is one of the most basic techniques (Fig. 2). As
the tube and film move from the first position to the second, all
points in the focal plane project to the same position on X-ray
film. Thus, points a, b and c project to points a', b' and c' in the
first position and a", b" and c" in the second position. Points
above or below the focal plane do not project to the same film
positions and are blurred. By changing the relative motion of the
film and tube, the focal plane can be adjusted upward or downward.
In addition to linear tomography, other types of tube and film
motion have been used. These motions include circular, elliptical,
figure-8, hypocycloidal, trispiral (Fig. 3). Each of these motions
has advantages regarding the way in which out of plane structures
are blurred. For example, a linear structure which is aligned with
the linear motion of a linear tomograph, will not appear blurred,
except at the ends, whereas such a structure will be blurred by the
circular motion of a circular tomograph.
tomographic motions. circular
tomography and hypocycloidal tomography.
a tomographic method where the X-ray focus and the film cassette are
moved in circular patterns. The X ray tube and cassette holder are
mechanically connected and move in a pattern as demonstrated in
Fig.1. As can be seen from the figure, the film cassette does not
rotate along its path.When grids are used, the grid lines must
follow the rotation in order to prevent grid cut off. The advantage
of circular tomography is that a uniform body section thickness is
obtained in the image. The disadvantages are the long exposure time
and the complex design of the equipment.
tomography in which the X-ray tube and film move in a hypocycloidal
path. Conventional tomography can be made using several movement
patterns for the X-ray tube and the film. The common linear movement
is mechanically easy to produce but will give rise to rather thick
tomographic sections and a short blurring path (the length of the
tomographic section). If thinner sections and longer blurring paths
are required, more complex movements are needed. Circular motion
will for the same angulation of tube and film produce three times
longer blurring paths than the linear motion and thinner sections.
However, artefacts can be generated for circular-shaped objects in
the tomographic plane. Spiral movement, which is a combination of
circular and radial movement, will overcome the artefacts, but
requires that the tube (and film) speed decreases when the tube is
spiralling out from the centre of the spiral. This is difficult to
The hypocycloidal movement is also a combination of a circular and
radial movement (Fig. 1). The pattern can be produced by letting an
inner gearwheel rotate inside another gearwheel with teeth on the
inside. The proportion of "teeth" inner/outer wheel is 2/3. If the
tube and film support is connected to the centre of the inner wheel,
it will describe a hypocycloidal movement. This movement is fairly
easy to achieve mechanically and performs superior to all others. It
will produce thin sections with a blurring path five times longer
than for linear movement with the same angulation. No object should
present a hypocycloidal shape, so virtually no artefacts will be
produced. The only disadvantage is that the tomographic section
produced is extremely thin, which imposes the need for very high
precision with regard to the film position.
Notes - Zonography - a
form of tomography where the tomographic angle is small, on the
order of 10, resulting in a thick plane of focus. The technique is
sometimes used to better delineate suspected pathology.
- a special tomography technique where panoramic roentgenograms of
curved surfaces are obtained by rotating the X ray tube and
film-screen holder around the patient, who is usually in a sitting
position (Fig.1). The film holder, which is much longer than the
film, has a protective lead front with a narrow slit. The film is
exposed through this slit starting from one end. The film moves
across the slit as the X-ray tube and film holder rotate around the
patient. The result is a PA image of a curved surface, e.g. the
mandible, flattened out on the two-dimensional film. In dentistry
radiology, the technique is also called orthopantomography and is
there still in much use, while other conventional tomographic
techniques have been mostly replaced by computed tomography CT .