口内法X線撮影における最適X線エネルギーの研究

抄録

The optimum X-ray energy distribution in intra-oral radiography was investigated by computer analysis. In this investigation, a simplified mathematical model of oral tissues was used in order to make quantitative evaluation of the contrast producability of X-rays depending on photon energy.
This computerized model consists of three parts of oral tissues such as cortical bone, cancellous bone and soft tissues. The thickness of cortical and cancellous bone were variable in this model in order to correspond to any part of oral tissues. Radiographic cotrast produced by adding cancellous bone as a signal at each thickness of cortical bone with 3cm thick water was calculated under various X-ray energy and exposure. In this model, an available diagnostic area in which X-ray image had resonable contrast and density for interpretation was considered for the X-ray energyevaluation.
Practical optimization of X-ray energy distribution was also investigated using rare earth elements as a radiation filter.
The results were as follows:
1) This computerized model was useful not only for quantitative investigation of the optimum X-ray energy distribution but also for quantitative diagnosis.
2) Changes of the available diagnostic area depending on various X-ray energys were compared. In this comparison, these areas of 25, 35 and 40 keV monoenergetic X-rays were similar to those of conventional wide-range X-rays generated with 30, 60 and 90kVp tube voltage respectively.
3) The use of monoenergetic X-rays reduced the skin exposure by 50 percent and the total absorbed energy by 30 percent as compared with the use of conventional X-rays.
4) Photons in the 28 to 40keV range were best suited for intra-oral radiography from the view point of the image forming quality and the radiation protection.
5) X-rays distributed widely within this range were more suitable to increase the latitude than monoenergetic one.
6) In practical optimization of X-ray energy distribution, an aluminum filtration was effective for conventional X-rays generated with 50-55kVp tube voltage, but for more high kVp region, filtrations of rare earth elements were more effective.
7) Using a sheet of 0.25mm thick neodymium as a filter, reduced the skin exposure by 50 percent and slightly increased radiographic contrast as compared with using 2mm thick aluminum filter at 60kVp conventional X-rays.
8) It is desirable to increase the out put of conventional dental X-ray equipment about two or three times and to make reasonable filtration of the rare earth elements.