Patent GB-1468741-A (GB)
Title: Production of halftone images
IPC: G09G5/00
Filed: 1974-03-25
Abstract: 1468741 Half-tone image production BAIRD-ATOMIC Inc 25 March 1974 [9 April 1973] 13204/74 Heading H4F Halftone images are produced on a CRT screen in response to coded signals indicative of the spatial distribution of radiation (e.g. radioactive events) emanating from a body, like areas being illuminated to have different mean brightness by arranging for different numbers of like sub-regions of each like area to be illuminated with like brightness in dependence upon the mean brightness indicated by each coded signal A high resolution radio-activity distribution detection system is described in which radioactive events originating in a subject under diagnosis are detected at XY locations of an array of sensing devices i.e. photodetectors optically coupled via light guides to an XY array of scintillators associated with a multiplane focussed collimator. The detected events at each XY location are accumulated and stored in corresponding address locations in a computer, and then the number stored at each address location is normalized into a Gray scale coded signal as a function of the greatest number of detected events at any one address location. The normalized Gray coded signals are then applied to a random access memory from which they are selectively fed to a CRT monitor via a read-only memory including Gray field character generators which controls the number of dot picture elements per unit area at XY locations on the display. Thus the number of radio-active events detected at each XY location of the seintillator array are presented at corresponding XY locations on the display as a number of illuminated sub-regions constituting picture dot elements, the greatest number of detected events in one location of the array being represented by the greatest number of picture dot elements per unit area. In the embodiment described sixteen shades of gray are presented on the display, the coded signal indicative of the greatest number of events detected at an XY location being the number sixteen, and the least number being the number one. In Fig. 4 a picture area 192 which corresponds to one address location is defined by a four by four matrix 194 having sixteen like sub-regions 196, and one or more sub-regions illuminated to form dot picture elements 198 are presented at predetermined locations (as defined by the Gray field character generators) within matrix 194 for each address location as a function of the normalized Gray scale coded signal (1-16) for that address location. The height and width dimensions of each sub-region 196 and each dot picture element 198 are equal to the height dimension of a raster line. The pattern of illumination in picture areas 192 containing the same odd number of dot picture elements 198 which are immediately above or below each other are mirror images of each other. As shown the patterns 192 increase in means brightness from one to the next in the series, and in accordance with the invention there is a first set 1-7 of consecutive patterns of illumination of the series that are respectively complementary, when considered in increasing order of mean brightness, to a second set 15-9 of consecutive patterns of the series considered in decreasing order to mean brightness, the mean brightness of each pattern of the second set being greater than that of each pattern of the first set. The subject under diagnosis is located on a platform which is movable by stepping motors in XY directions so that different areas of the subject may be analysed by the stationary scintillator array.