High-Kilovoltage Radiography

LIKE THE OTHER factors involved in radiography, the kilovoltage factor has been the subject of almost continuous study. The equipment available has always had a profound influence on the voltage range over which routine work could be done. At no time has there been any universally accepted technic for any single part of the body. Personal preference and experience have in general been the dominant factors in determining the kilovoltage used in most laboratories. In its infancy radiography was limited, by the generators and tubes available, to relatively low voltages. With the advent of the alternating current transformer, mechanical rectifiers, and the Coolidge tube, the kilovoltage was increased. The universal type, fine-focus Coolidge tube was frequently used at 100 kv.p. for radiography of the lumbar spine and the chest in large patients. When the radiator-type tube became available, the voltage was limited to 85 kv.p., a limitation willingly accepted because of the improved definition made possible by the smaller focal spot. For some years, the voltage factor received little attention, as improved films and screens and increased tube current reduced the exposure time required. During this period, there was a general tendency toward higher contrast through the use of reduced voltage. There were some centers of influence, however, where increased latitude was preferred at the expense of contrast, at least for some regions of the body. In the last few years, the question of the use of higher kilovoltage has become the subject of considerable discussion. This would seem to be the result of a number of contributing influences. Notable are the works of Fuchs, Hodges, Morgan, and their associates. The recent works of Lamerton and of de Waard have contributed to a better understanding of scattered radiation, always of prime importance when the effect of voltage on the radiograph is to be considered. No small part of the interest in increased voltages can be traced to their use in mass radiography. Further impetus has come from renewed interest in cineradiography and fluoroscopic intensification. The work reported here had its inception in some work done by Files and his coworkers in 1944 and 1945 during their search for an optimum ratio grid. A dry spine and wire mesh suspended in a water phantom were used as test materials. It was soon evident that increased grid ratios (greater than 8:1) were feasible and that they made possible a marked reduction in the amount of scattered radiation reaching the films. Grids of ratios as high as 34:1 were studied. At the increased ratios, the exposure time became so great that, to reduce it to usable limits, higher voltage became necessary. As the voltage was increased, the exposure decreased, as had been anticipated, but at a rate considerably in excess of that expected. The higher ratio grids made possible a high degree of contrast even at much higher voltages than were in general use at the time.