歯科 X 線撮影時における散乱線に関する研究 : 被写体内部およびその周辺における散乱線について

Abstract

Using a dental X-ray apparatus, an investigation was conducted to analyze a variety of factors that will influence the scattered X-rays from the exposed object and those distributed within the object. The factors involved included physico-chemical nature of object, thickness of object, the tube voltage and the field area of exposed X-rays. As preliminary experiments, water and wax phantoms were exposed with a dental X-ray apparatus and the primary and scattered X-rays distributing within each object were respectively measured. In the next step, the surface scatter doses of each object were measured and the relation between their distribution and the above factors which would influence upon them were studied. The scatter distribution doses were measured mainly by the X-ray film density method, and the quality of scattered X-rays was measured by the half value layer method with aluminium. The study showed that the maximum scatter distribution dose was found to lie around the depth of half value layer of the object radiated which gradually decreased past that point toware the transmission surface. The scattered X-rays generated in the object were dispersed in the whole body toward every directions, of which the back scatters were more abundant near the exposure surface and the front scatters more abundant elsewhere, but the latter decreased toward the transmission surface. When the soft matters such as human tissues were radiated, the less the X-ray absorption coefficient of the object, the more increased all surface scatter doses. The dispersion rate at the exposure surface and transmission surface increased gradually with the increase of any one of the factors such as thickness of the object, field area and tube voltage of the X-rays until it reached saturation. With increase of thickness, the back scatter doses radiated from the object increased gradually towrd saturation, while the transmission scatter doses decreased inversely when the thickness of the object surpassed its maximum transmission scatter value. Every surface scatter doses of the object increased with the increase of tube voltage or field area of X-rays. When it deals with hard matters which are difficult to transmit for dental X-rays, only very small amount of back scatter doses could be observed. The metalic plates such as Pb, Fe and Cu showed only less than 5% back scatter doses of that produced by water phantom. Correspondingly the back scatter doses from these hard objects were hardly affected by changes of thickness of the object and tube voltage apllied within the limit of dental X-ray. Tha quality of surface scatters differed with respective radiating direction from the obiect irradiated. The transmission scatters were essentially identical with the primary transmission X-rays in quality. The mean wave length of back scatters, however, became a little shorter with increasing of thickness of the object, but it never reached the mean wave length of the primary X-rays. Since the mean wave length of lateral scatters became shorter only by the photo-electric absorption effect of surrounding tissues outside of the exposure field, it was often found that the mean wave length of lateral scatters became shorter than the primary X-rays according to the nature and thickness of the surrounding tissues. In the actual case of intraoral roentogenography, the scatter doses measured on the cheak surface of patient were about 13% and the same measured within oral cavity were about 9% of the total surface X-ray doses, and about one third of the intraoral scatter doses was consisted of back scatter doses from the intraoral surrounding tissues. It was confirmed that 30 to 60% of the total film density of the intraoral roentgenogram was caused by intraoral scatters of which 30 to 50% was responsible to back scatters of intraoral surrounding tissues.