Tomography

Tomography 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

Fig 2

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.

Fig 3

Circular 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.

Hypocycloidal tomography
Hypocycloidal tomography,
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 achieve mechanically.

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.

Pantomagraphy - 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 .

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