Axial transverse tomography of the chest.

Abstract

Axial transverse tomography is one of the more recent advances in roentgen diagnostic procedure. Conventional tomography enables one to show in detail roentgenographic images of the structures lying in a predetermined plane of tissue, while blurring or eliminating details of the structures in other planes. Any plane which is parallel to the long axis of the body may be chosen; but sections in the frontal and sagittal planes are most commonly made. Axial transverse tomography is a method for making roentgenograms of a thin slice of the body in a plane horizontal to its long axis. The fundamental principle in any form of tomography is that, of the three components participating in the production of a tomogram, i.e., the roentgen-ray source, subject, and film, two are moving synchronously during a continuous exposure and one is stationary. The motion is of such character that the roentgenographic image of a thin slice of the body at the predetermined level remains stationary on the film, while the images outside this slice have a continuous relative displacement and consequently are blurred. A theoretical solution for obtaining roentgenograms of cross sections of the body was first suggested by an American physicist, Kieffer (1), in 1938 but was not carried out practically. About the same time Watson (2) built the first practical apparatus in England. Only within the last few years, however, have the potentialities of the method been explored, mainly as a result of the great enthusiasm of Vallebona (3, 4, 5) in Italy. The method gained considerable popularity in Western Europe, where there is already a considerable volume of literature available on it. American contributions on the subject are practically nonexistent. As far as the fundamental principle is concerned, axial transverse tomography resembles conventional planigraphy. It is obvious, however, that, because of the relatively long vertical axis of the body the technical approach of conventional tomography is of little avail. There is a variety of equipment for axial transverse tomography on the European market (6). Most of it represents modifications of the original apparatus of Watson. Figure 1, a schematic drawing in perspective of the apparatus manufactured by Siemens, illustrates the general principle and practical aspect of axial transverse tomography. The cassette (C) is placed horizontally on a turntable (R′). The patient is immobilized in a vertical position on a turntable (R″). The x-ray beam from a rotating anode (O) emerges through a horizontal slit aperture, traverses a narrow section of the body at an angle of about 25° and strikes the film. The cassette and the tube can be moved up and down for the selection of different section levels. A spot light (L) attached to the tube carriage parallels the central ray (O-O′-O″), indicates the direction of the x-ray beam, and serves to co-ordinate the height of the tube with that of the cassette.