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. 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. 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.

A somewhat more complicated technique known as multidirectional tomography produces an even sharper image by moving the film and X-ray tube in a circular or elliptical pattern. As long as both tube and film move in synchrony, a clear image of objects in the focal plane can be produced. These tomographic approaches have been used to study the kidneys and other abdominal structures that are surrounded by tissues of nearly the same density and so cannot be differentiated by conventional X-ray techniques. They have also been employed to examine the small bones and other structures of the ear, which are surrounded by relatively dense temporal bone.