Roentgen-stereoscopy

Abstract

A DISCUSSION of roentgen-stereoscopy and some of its phases may seem rather unnecessary; no American roentgenologist doubts the advantages to be derived from the study of stereo-roentgenograms. A vast literature covers the subject from many angles; to the well-informed observer it is even evident that frequently great over-emphasis is placed upon the esthetic satisfaction of the tridimensional impression, especially by many a clinician using roentgenograms rather casually to the point of refusal of any non-stereoscopic projection, no matter how instructive it may be. Nevertheless, the student of this subject is all too often impressed by the lack of knowledge of the basic physical principles of stereoscopy and particularly of roentgenstereoscopy on the part of radiographers and physicians, especially the younger ones. In consequence, shortcomings of the method, which cannot be denied, are entirely overlooked and, consequently, erroneous results are obtained, which reflect badly on the system and the diagnostician who, without being aware of its fallacies, employs this method. The objects of this dissertation shall be: (1) A brief résumé of the differences between visual or photographic and roentgen images and corresponding stereoscopy, leading to the conceptions of subjective and objective roentgen-stereoscopy ; (2) a discussion of the mistakes most frequently encountered in technical procedures and the resulting visual errors; (3) technical suggestions. It is well known that quite a few individuals are unable actually to see stereoscopically. Pordes emphasized the importance of this fact for the study of roentgen-stereograms. Emmetropic or isometropic persons normally use binocular vision. With increasing myopia monocular vision becomes the rule, at least for relatively close distances, and the non-participating eye becomes somewhat amblyopic. With increasing anisometry (difference in the refraction of both eyes), the incidence of monocular vision increases considerably as does also amblyopic degeneration of the eye which is physiologically and optically inferior. However, we may eliminate detailed discussion of these facts from closer consideration in this paper, merely keeping in mind that one may frequently find persons unaware of their more or less exclusively monocular vision, who are easily deceived by attempted observation of stereo-roentgenograms. Recognition of tridimensional relationship is based upon primary and secondary factors. Primary Factors: Accommodation and Convergence Accommodation is a function of each individual eye which by active variations of the curvatures of the refracting lens, especially its anterior surface, varies the focussing distance of this “camera”; thus it produces a sharp image of objects at varying distances and conveys the knowledge of these differences to our mental conception. Wundt, in 1862, published the following data concerning the rôle of accommodation for the recognition of depth: