Standard and High-Scan-Line Television Systems

Abstract

STANDARD (525 horizontal lines per frame) and various high-definition (837, 875, and 945 horizontal lines per frame) television systems are being marketed by radiologic equipment manufacturers. Improved resolution is the theoretical advantage of a high-scan-line television system (5). However, is such a system of advantage to the practicing radiologist? Are there future developments in the total imaging process which might influence the choice of a particular scanning system? Will new uses of television by the radiologist, within the hospital complex, favor one system over another? The theoretical advantages of a high-scan-line television system are reviewed in this presentation. Rather unique standard and high-resolution television systems are described which utilize one inten-sifying tube, one Vidicon television camera, and one monitor. The performances of these two systems are compared, using a resolution test pattern, phantoms, and clinical material, and the results are presented. The significance of the results is discussed, and new television applications which might influence the choice of a particular scan-line system are noted. Theoretical Considerations The usable picture content of a television system is predominantly influenced by the number of horizontal lines in one frame and by the bandwidth of the system. Horizontal scan lines, which determine to a large part the vertical resolution capability of a television system, are different from horizontal lines resolved, which specifically identify the vertical resolution. It is necessary that the number of horizontal scan lines be in excess of the number of picture elements (1) to be resolved in the vertical direction. Vertical resolution can be determined by means of the following equation (2, 3): Thus, for a 525 horizontal scan-line system, vertical resolution is: VR = 0.7 × 490 = 343 horizontal lines. A 945 horizontal-scan-line system has a vertical resolution of about 618 horizontal lines. Resolution is the ability of an image-reproducing system to generate the micro-structure of a scanned object, and it can be determined by measuring how well the system reproduces the boundary or edge between a very dark and a very bright image. A second method of determining system-resolution is to observe from a specified distance the maximum number of ad-jacent parallel lines which can be identified. Horizontal resolution is governed by the number of picture elements (1) needed to be resolved on each line and is dependent on the bandwidth of the system. The picture elements along each line are reproduced by changes in the voltage applied to the electron gun in the picture tube. A determination of horizontal resolution can be expressed by: Thus, the number of vertical lines (horizontal resolution) that can be resolved is directly proportional to the bandwidth and inversely proportional to the number of scan lines in a frame. Once the scanning portion of an entire television system is standardized, the performance of the system is quite specifically limited. Scanning is a process by which optical information with functions of space and time is converted into electrical information expressed as a function of time and electrical signal amplitudes. In the scanning process, the amount of detail actually converted to useful information depends upon the percentage of picture area actually contacted by the electron beam.