Experimental results with the panel x-ray amplifier.

Abstract

During the past decade, the field of solid-state physics has expanded greatly. One aspect of this growth is the advance made in the field of electroluminescent phosphors. With these materials it is possible to produce a pattern of emitted light from a flat sheet by direct application of voltages to the surface. Another aspect of this growth is in the field of photoconductivity. Not only have extremely sensitive materials been developed which respond to low-level x-rays, but such materials have been developed in powder form particularly suitable for fabricating thin large-area devices. About a year ago, an experimental x-ray amplifier was described (1) making use of electroluminescent and photoconductive materials. This amplifier demonstrated the feasibility of obtaining high brightness gains with a thin panel which could be directly viewed and whose amplifying action was not dependent on vacuum tube structures. Because of its potential applications some exploratory tests have been made recently as a means of evaluating it and as a guide in determining the direction of future developmental effort. Since the tests described below were made with early laboratory-constructed amplifiers, they should be considered only as indicative of the general possibilities of such panels rather than as exact criteria of their specific limitation. Problems in present fluoroscopy The need for increasing the brightness of a fluoroscopic image insofar as it concerns inconvenience in viewing needs no emphasis. However, the inability of the radiologist to perceive picture information on the screen because of the lowlevel image is of a more serious nature. This problem was thoroughly studied by Chamberlain (2) many years ago, and some of his conclusions can be briefly summarized. In clinical practice, the brightness of the typical fluoroscope screen is between 0.0001 and 0.01 millilambert. A thick abdomen reduces the screen brightness to a level which is less than 1/10,000 that of a sheet of paper in normal reading light. At the low brightness levels of fluoroscopic work we are usually concerned with rod vision, which limits both acuity and intensity discrimination. In reducing the brightness of an image from 10 millilamberts (typical of ordinary room illumination) to 0.001 millilambert (typical of an 18-cm. abdomen in fluoroscopy), the visual acuity of the eye is reduced about fifteen times, i.e., image details which can be separated by the eye must be fifteen times farther apart at the lower light level. At a viewing distance of 10 inches, for example, black and white contours separated by less than 1 mm. cannot be detected at 0.001 millilambert brightness. A comparable limitation is the reduced ability of the eye to distinguish gross details of low contrast at low light levels.