The Efficiency of X-ray Stereoscopy as Influenced by the Method of Trip of the Tube
Paul M. Andrus, A. Hambleton · Radiology · 1934
IN a former publication (1) we have analyzed the relation of the position of the x-ray tube to the resultant sharpness of film images. It was shown in that study that for the attainment of a given degree of sharpness, a saving in energy of up to 22 per cent is possible by the correct positioning of the tube, as opposed to commonly used positions. The data there presented relate mainly to single exposures with the focal spot centered over the film. The correct method of placing and tripping the tube for stereoscopic exposures, from the standpoint of obtaining maximum sharpness, is, however, also shown, and is repeated for the convenience of the reader (B, Fig. 7). The matter of stereoscopic agreement of the images was, however, not dealt with in the previous paper, and it is with this phase of stereoscopy that the present study is concerned. This paper thus aims to be supplementary to the foregoing study in presenting the effects of tube position for the special case of stereoscopic exposures, with special reference to the agreement of the stereoscopic images. The question of the desirability or necessity of tilting the tube between stereoscopic exposures is also important to the practising roentgenologist. Some manufacturers offer, while others do not, facilities for such tilting. The roentgenologist must make a decision in the face of these conflicting recommendations, as to whether or not he should make expenditures for such a purpose. Nature of Stereoscopic Errors The nature of the errors in stereoscopy which are to be examined will be clear from Figures 1, 2, 3. The object shown in these figures is thin and parallel to the film, a condition chosen because it simplifies the point to be discussed. In Figure 1 we have the desirable ideal condition in which the x-rays proceed from a true point, and it will be seen that the images cast on the films from the two stereoscopic tube positions are identical. In the x-ray tubes in common use, however, the rays proceed from an appreciable area—the focal spot—which lies at an angle to the surface of the film. As a result of these two factors the images cast from the two stereoscopic tube positions are no longer identical. In Figure 2 it will be seen that the image (marked I) from one tube position is much larger than that from the other, i.e., the images undergo different degrees of distortion. The penumbral shadows (marked P) which determine the degree of sharpness of the images are also different from the two tube positions. Obviously, stereoscopic agreement can be no better than the geometric agreement of these two images.2 In Figure 3 the tube is so placed and so tilted during the trip that from each of the two positions the central ray of the tube travels toward the center of the object to be radiographed. It is clear from the figure that the images much more nearly approximate each other both in size and in degree of sharpness than they do under the conditions depicted in Figure 2.