1988 Volume 30 Issue 2 Pages 134-140
At present, the cone diameter of usual intraoral X-ray machines is approximately 7.5 cm for an exposure field. On the other hand, the size of a dental X-ray film is 41mm×31mm, so that the ratio between the area of the exposure field and that of the dental film is about 3 to 1. It would thus be possible to reduce the exposure dose by reducing the exposure field area. Furthermore, the image sharpness and contrast can be improved by reducing the degree of X-ray scatter. However, creation of a small exposure field would produce other problems such as cone-cut and retaking. Therefore, some devices such as film holders and indicators have been designed, but there is difficulty in using them in patients with mouth-opening disorders or wisdom teeth, since the holder and beam aligning unit are both in one unit. We report in this article the development of a magnetic sensor for the prevention of cone-cut.
A rotary magnetic field is generated by coils installed at the end of the cone, so that the center of rotation and the central part of the X-ray beam are on the same axis. Secondly, this center of rotation is sensed by a pair of Hall ICs (integrated circuits) which are fitted to the back of the film, and the output from these ICs is transmitted to the location detector. The location of the Hall ICs is calculated, and the result is indicated by light-emitting diodes (LED) arranged in the shape of a cross.
By using this apparatus, it has become possible to confirm that the center of the X-ray beam is vertical to the center of the film when taking dental X-rays. Accordingly, this apparatus facilitates a considerable reduction of the exposure field and also prevents cone-cut.
