Visual perception in fluoroscopy and radiography. Annual oration in memory of John D. Reeves, Jr., M. D., 1924-1964.

Abstract

I am honored that your President has chosen me to address you on this, the occasion of the Annual Oration in memory of John D. Reeves, Jr. You have heard Stanley Wyman's eloquent tribute to Doctor Reeves. I should only like to add that I had the privilege of working with John when he was a member of the Board of Chancellors of the American College of Radiology. He was one of the most tireless workers I have known, always a devoted servant to radiology, a man fully worthy of the recognition given him today. I should like to speak to you on a subject which I am sure would have interested John Reeves, namely the place of physiological optics in diagnostic roentgenology. The importance of visual physiology to the radiologist was clearly outlined by W. Edward Chamberlain in his Carman Lecture to this Society in 1941 (1). Many things have happened since that time. New concepts have been proposed, new methods of measurement developed. A review of these matters seems in order. All visual systems consist basically of four elements: (1) a source of radiant energy which, in the case of radiography and fluoroscopy, is the x-ray tube and its associated apparatus; (2) the structure under observation which, when the source's radiant energy interacts with it, generates a radiation signal; (3) a series of processing devices, including the observer's eyes, which modify the signal and present it in a form suitable for analysis; and (4) the observer's brain with which the signal is analyzed and interpreted. It will be noted that the term, radiation signal, has been used to depict the output product of the energy-structure interaction. The more commonly used term, radiation image, could have been used. However, this term does not express clearly the fact that the energy-structure interaction yields information or intelligence about the structure's composition. Because visual processes are primarily concerned with the acquisition of information by the observer, the term signal seems more appropriate. In both radiography and fluoroscopy, the radiation signals emerging from a structure under examination require considerable processing before they may be interpreted. Several stages are needed to convert the x-ray information to neural signals on which the brain may operate for analysis and interpretation. In conventional fluoroscopy, the radiation signal undergoes relatively less processing than in most other radiological systems (Fig. 1). First, there is a grid for the removal of scattered radiation. This is followed by a fluoroscopic screen to convert the x-ray signal into a light signal which is then projected on the retina of the observer's eye and converted into a neural signal acceptable to the brain.